PCM or PPM
Taylor Jones
I wanted to know the group's thoughts about PCM vs. PPM. I am going to buy
an Airtronics Stylus and wanted to know if I should spend the money to get
it PCM. From what
I know I do not need PCM right now but wanted to see if anyone has reasons
for me getting it PCM over PPM.
Thanks,
Taylor Jones
WFTEX
Ken
A.T.
refer to the following sites from
Alan's Hobby Web Links
http://homepages.ihug.co.nz/~atong/ - check before putting "odd" suggestions
to the test.
Frequencies - Intl lists. PCM Vs PPM. FCC Regs.
http://www.silvertone.com.au/maa.htm
http://www.bmfa.org/handbook/hbook_13.html
http://www.ecmc.com/frame_index.html
http://www.geocities.com/roger_forgues/Frequency.html
http://www.bekkoame.ne.jp/~way/freque.html
http://members.tripod.com/~NZMAA/freqs_01.txt
http://www.med.govt.nz/rsm/publications/pibs.html
http://www.torreypinesgulls.org/Radios.htm
http://www.perconcorp.com/datafinder/index.html
http://www.rcaerosport.com/
http://www.phoenix-mfc.freeserve.co.uk/
http://ultrahot.com/RCfreq/
http://www.access.gpo.gov/nara/cfr/waisidx_00/47cfr95_00.html
--
Alan's Hobby Web Links
http://homepages.ihug.co.nz/~atong/
..............................................................
Taylor Jones <mooc...@hawaii.rr.com> wrote in message
news:mphM7.102610$OW.33...@typhoon.hawaii.rr.com...
John R. Agnew
PCM is much more expensive, and for ordinary sport flying at the
average model field, you don't need to spend it.
Yvan Grondin
PCM = Pulse coded Modulation
PPM is mostly analogic while PCM is digital
One of the best article i have seen is at the web site and everybody should
read it .
http://www.torreypinesgulls.org/Radios.htm
yvan grondin
"Taylor Jones" <mooc...@hawaii.rr.com> a écrit dans le message de news:
mphM7.102610$OW.33...@typhoon.hawaii.rr.com...
MacGyver
>PCM is much more expensive, and for ordinary sport flying at the
>average model field, you don't need to spend it.
Did I say I totally agree with you?
Tom C
aircraft.
It is not a matter of being cheap or lack of knowledge. It is PPM is an
excellent system in our area.
"WFTEX" <wf...@aol.com> wrote in message
news:20011125235527...@mb-cc.aol.com...
RedFred1
What is your source for this nugget of wisdom?
FRed
Doucoursey
> PCM is now the most preferred.
What is your source for this nugget of wisdom?
FRed
>>
just go to any TOC type contest and see what they use, almost 100% PCM. DOUG
C.
daytripper
But that's a rather rarified altitude to be taking surveys, don't you think?
After all, many if not most if not all of those guys get their gear from their
sponsors...
I suspect you'd find a very different result if you surveyed more mortal
pilots ;-)
/daytripper (Unencoded FM all the way, baby! ;-)
RedFred1
>After all, many if not most if not all of those guys get their gear from
>their
>sponsors...
>
>I suspect you'd find a very different result if you surveyed more mortal
>pilots ;-)
>
>/daytripper (Unencoded FM all the way, baby! ;-)
>
>
>
find a much different result. The big buck high end radios are around, but the
majority of folks do not fly them. I will not go as far as to try and say what
is most preferred because even that changes from field to field and even by
region.
FRed
Paul Nesbitt
aerobatic (3D) planes, big warbirds and all of my helicopters. Why,
because I believe the PCM recievers have better noise rejection
capabilities and each of these cases represents high vibration and/or
potentially high noise environments (ie...gas engines on the big planes,
lots of moving metal parts on the helicopters). On my sport, combat,
fun-fly planes I use PPM receivers. Not that I don't care about the
plane, but I don't care as much. Plus it doesn't seem to make sense to
put a $200 RX in an airframe that cost $100 more or less.
In addition, I have been to many IMAA (Giant Scale)events, Warbird
events, IMAC and Pattern Competition. In all cases, the majority of the
pilots use PCM receivers, but keep in mind that in almost all of these
cases, we are talking about real expensive airplanes. Do they need PCM?
Who knows. All I know is anytime I spend $2-5000 on a flying maching, I
won't flinch at putting the best RX (or two) I can in it.
Paul
Steve
So, then, is it fair to say that 150CC gas engines are the most preferred
for powering model aircraft?
How would that fit with the fact that 90% of model engines sold are small
glow engines? Or that the vast majority of radios sold are not PCM?
I guess I don't see this logic...
Steve
medi...@mindspring.com
RC web Site http://webpages.charter.net/mediashop/rc-giant-scale/
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Steve
>Personally, I use both PCM and PPM. I use PCM in my Pattern planes, big
>aerobatic (3D) planes, big warbirds and all of my helicopters. Why,
>because I believe the PCM recievers have better noise rejection
>capabilities and each of these cases represents high vibration and/or
>potentially high noise environments (ie...gas engines on the big planes,
>lots of moving metal parts on the helicopters). On my sport, combat,
>fun-fly planes I use PPM receivers. Not that I don't care about the
>plane, but I don't care as much. Plus it doesn't seem to make sense to
>put a $200 RX in an airframe that cost $100 more or less.
>
>In addition, I have been to many IMAA (Giant Scale)events, Warbird
>events, IMAC and Pattern Competition. In all cases, the majority of the
>pilots use PCM receivers, but keep in mind that in almost all of these
>cases, we are talking about real expensive airplanes. Do they need PCM?
>Who knows. All I know is anytime I spend $2-5000 on a flying maching, I
>won't flinch at putting the best RX (or two) I can in it.
>
>Paul
>
>
As another poster mentioned, it even changes from region to region. In the
Northeast, I've actually found that at most IMAA events, PPM is the most
common, while at IMAC events, PCM seems to be fairly even with PPM.
Depending on the number of larger planes entered in the IMAC event, PPM or
PCM could be the most common. In most 35% or larger planes, it seems that
PCM is the overwhelmingly preferred choice for single or dual receivers,
and for 35% or smaller, PPM seems to be the normal choice.
Even though I understand the advantages and disadvantage of the two
systems, I haven't asked enough people to understand the motivating factors
behind their decisions. I have, however, kept a 'head tally' every time
I've looked inside someone's plane, and never miss an opportunity to peek
inside. I keep a similar tally of electronic components, servos, and
mechanical systems. I just like to know what everyone is using, and am
constantly on the lookout for new or different setups.
Doucoursey
> just go to any TOC type contest and see what they use, almost 100% PCM.
DOUG
>C.
So, then, is it fair to say that 150CC gas engines are the most preferred
for powering model aircraft?
How would that fit with the fact that 90% of model engines sold are small
glow engines? Or that the vast majority of radios sold are not PCM?
I guess I don't see this logic...>>>
Steve
medi...@mindspring.com
RC web Site
the point is, these guys have mega bucks in these planes, fly them very low to
the ground,very close to themselves. they dont want radio problems. their
choice is PCM, beit JR or FUTABA. dont say they use PCM because they get the
stuff for free, because they could get PPM recievers for free also. DOUG C.
daytripper
>
> the point is, these guys have mega bucks in these planes, fly them very low to
>the ground,very close to themselves. they dont want radio problems. their
>choice is PCM, beit JR or FUTABA. dont say they use PCM because they get the
>stuff for free, because they could get PPM recievers for free also. DOUG C.
The first part of your argument has merit (if we assume that PCM really offers
additional "asset protection") but I respectfully disagree with that last
statement.
Human Nature 101: if *I* was able to get gear for free, *of course* I'd go for
the premier products from my sponsor - even if I was unconvinced that there
was any up-side offered wrt uncommanded behavior of my aircraft...
/daytripper (in need of a Sugar Daddy ;-)
Doucoursey
Human Nature 101: if *I* was able to get gear for free, *of course* I'd go for
the premier products from my sponsor - even if I was unconvinced that there
was any up-side offered wrt uncommanded behavior of my aircraft...
/daytripper (in need of a Sugar Daddy ;-)
>>
so your saying that PCM is either better because it cost more or it is better
because it offers better perfofmance?
asuming that premier means better..
DOUG C.
matthew
--
Matthew Orme
Microsoft
"Where do you want to go today?
It doesn't matter, you're coming with us."
For answers to your electric questions, sign up for the eflight list at http://www.ezonemag.com
"Doucoursey" <douco...@aol.com> wrote in message news:20011128151409...@mb-fn.aol.com...
Doucoursey
What he said was that the person will always choose the more expensive =
(retail) product if he has a choice, and the cost is the same to him =
(in this case zero) since he will eventually sell it anyway
--=20
Matthew Orme=20
>>
what he said was PREMIER, that is different. just because you pay more
for something doesn't make it better.
matthew
--
Matthew Orme
Microsoft
"Where do you want to go today?
It doesn't matter, you're coming with us."
For answers to your electric questions, sign up for the eflight list at http://www.ezonemag.com
"Doucoursey" <douco...@aol.com> wrote in message news:20011128162902...@mb-fg.aol.com...
daytripper
Premier in this context clearly means "top of the line".
"Top of the line" usually means "most expensive".
If Futaba was offering to GIVE you your pick of radio systems for free, would
you really ask for an 8U and a set of PPM receivers, instead of a 9Z and a set
of their biggest, baddest PCM receivers??
Again, this has nothing to do with the PPM vs PCM question, but rather is a
simple illustration that polling TOC finalists (for instance) on their choice
of gear is a pretty shakey method of determining what's "best"...
/daytripper
Jimmy Huang
>read it .
>
>http://www.torreypinesgulls.org/Radios.htm
This was an interesting read, and it should definitely be one to be
kept for the history books. However, having an EE background, I beg to
differ.
From what it says, it's basically saying that
a) AM PPM is better than
b) FM PPM
futher
c) FM PCM is the worst
when it comes to noise immunity.
It first argues that AM PPM is more like PM (pulse modulation) and
therefore theoretically superior to FM PPM which is more like FSK.
What??? That above is true, but since when was morse code
theoretically superior to FSK?
Then it goes on to tell us how PCM is almost purely digital and so
prone to noise interference. Hmmm. Further the only advantage of PCM
is the "precision" of 255 positions. Huh? I'd rather go with analog
anyday than settle for only 255 positions. Anyways, there's PCM1024
now.
Everyone I know in the R/C community usually switch over to PCM radios
when they start using gas engines -> specifically because PCM is less
prone to this kind of interference. And those gasers who have had
problems with PPM receivers usually switch to PCM and problems go
away.
The article is dated. Back in 1995 it would have been a great read.
Things have changed. Kind of reminds me of when I bought the Feynman's
Lectures of Physics books. These books were about "modern" physics,
copyrighted 1961. Got around to reading it this year, Man things have
changed!!!
My order of preference:
FM PPM -> cheaper and almost as reliable, use this if you don't use
gas engines.
FM PCM -> the best, but uses proprietary standards, Futaba has their
own and so does JR, not interchangable.
AM PPM -> no longer exists. I have a post 1991 AM radio and receiver,
did a range check, and my Hitec Focus 4 outranges my Futaba Attack 4
by about 25 ft antenna down. (threw radio away early in 2001).
Jimmy
P.S. There *used* to be a problem when a JR radio was using Ch 35 and
a Futaba radio was using Ch 36, due to JR using positive FSK and
Futaba using negative FSK, thusly they interfered with each other.
However, this has nothing to do with the inherent weakness of FSK
versus PM. And those problems have been solved.
I have no experience with single conversion receivers. However, based
on the ramblings on this newsgroup, I'd have to say, continue to stay
away from these things!
Doucoursey
>One of the best article i have seen is at the web site and everybody should
>read it .
>
>http://www.torreypinesgulls.org/Radios.htm
That article is definately full of shit. AM was horrible,FM was a big
improvment.
a...@a.com
"From this we can see that R/C "AM" is not fundamentally inferior to
R/C "FM" - it just uses one frequency instead of FM's two. Indeed a
properly tuned AM radio is theoretically superior to its FM
counterpart because it can only be glitched on ONE frequency whereas a
glitch on either FM frequency will knock the FM receiver out."
This part is really in left field...far left field. Where did he get
garbage like this.
JEFGEO123
>From: douco...@aol.com (Doucoursey)
>>One of the best article i have seen is at the web site and everybody should
>>read it
>That article is definately full of shit. AM was horrible,FM was a big
>improvment.
I beg to differ with a uninformed source, but i have flown my old futaba am
systems at points where the 80 inch span plane was a very small dot and still
had complete control while the fm systems were putting them in the ground at a
much shorter range!
there may have been differences in the antenna placement in the craft or state
of charge on the batteries, but if it works nobody in this world is going to
convince me that i need to fix what works fine!
am will gradually get glitchy as you get to the end of its range, but fm will
just turn off like a light switch!
pcm may add a little bit to the range of fm, but will still do the light switch
trick!
if a system is properly set up it will control an aircraft as far as the un
aided human eye can see, but people think that fm or pcm will fix all the
problems that an improper (read that as sloppy) setup can present!
the antenna needs to be away from all other wiring in the aircraft, so that it
does not receive extranious signals.
if am is soo bad why can i listen to am stations 2000 miles away and not hear
fm stations 200 miles away, when they are running the same amount of power?
if i get my fm antenna up about 25 feet i can get a good solid stereo light
from stations 300 miles away, but if i put my am antenna up that same 25 feet i
can get stations from 8-9000 miles away.
the problem is not the type of modulation, but how good is your antenna, and
it's installation, versus the transmitted power and the orientation of both
antennas?
George
Natural Philosopher
Doucoursey wrote:
>
> <<
> Human Nature 101: if *I* was able to get gear for free, *of course* I'd go for
> the premier products from my sponsor - even if I was unconvinced that there
> was any up-side offered wrt uncommanded behavior of my aircraft...
>
> /daytripper (in need of a Sugar Daddy ;-)
> >>
>
> so your saying that PCM is either better because it cost more or it is better
> because it offers better perfofmance?
PCM is better in that it is less likely to do more than 'go all neutral'
under interference. PPM can, and does, occasionally cause massive
amounts of servo flutter IN INTERFERENCE CONDITIONS, and, if the battery
can't cope with all that drain, you can get an 'all servos to end stops'
situation.
Its just a question of whether that slight extra security is worth all
the extra cost.
All my accidents are pilot error, not interference. What's the point in
risking and expensive RX when I can't even fly the cheap ones properly
yet :-)
Personally, I would rather go dual conversion than PCM.
Obviously, experienced pilots whose accidents are seldom and who are
putting megabucks of whatever into the air, have a different
perspective...
> asuming that premier means better..
Indeed :-)
> DOUG C.
--
The real question is, does the mortar hold the bricks apart, or does it
stick them together?
Gareth Glaccum+the paperweight
signal is effectively a square wave with bits missing) is sent out, and
interference causes the nice sharp edges to be eroded. The square edges are
more prone to eroding. BUT. BUT. BUT the receiver is expecting a square
wave, and can guess at what the signal is, or if it falls within a dead
zone, it ignores that little bit of information.
Gareth
--
NitroClicks
http://www.nitroclicks.com/join.phtml?refered=penguin
Or try:
http://www.goingplatinum.com/member/verycoldpenguin
"Jimmy Huang" <jimm...@kuentos.guam.net> wrote in message
news:k4s90u46g3vrh1nsc...@4ax.com...
Jerel
Here's another aspect; which receivers are the manufacturers spending
more of their design and development dollars on? I would venture it's
the higher dollar PCM versions. I've also seen (not always) a bit more
quality in the higher dollar receivers. It's more obvious in servos of
course, that you get more and better as the price goes up.
I've got JR receivers from my Galaxy system that came out in the mid 80's
and they're still rock solid. So take that extra $50-$100 and amortize it
over 5-15 years and it comes out pretty cheap.
I went to the Nat's in '90 and looked at the radios used for pattern,
pylon and scale. It was ironic that the people who spent the most time
building also spent (on average) the least on their radio equipment;
pattern was mostly top of the line systems, pylon was the midrange radios
and scale was everything else (cheap and old discontinued
systems)....again, on average.
If your time is worth anything and you have a significant investment in
your plane and engine, I'd recommend buying the best possible radio you
can afford.
my $.02.
Jerel
The Natural Philosopher
a...@a.com wrote:
>
> That article wrote:
>
> "From this we can see that R/C "AM" is not fundamentally inferior to
> R/C "FM" - it just uses one frequency instead of FM's two. Indeed a
> properly tuned AM radio is theoretically superior to its FM
> counterpart because it can only be glitched on ONE frequency whereas a
> glitch on either FM frequency will knock the FM receiver out."
>
> This part is really in left field...far left field. Where did he get
> garbage like this.
Lord knows. Just listen to any FM radio, and any AM radio and make up
your own mind..
FM is definitely better at rejecting broadband interference, and PCM is
much bteer PROVIDED THE INTERFERENCE IS LOW. At high interference
levels, PCM dies completely, and FM may be swamped. I doubt that AM
works much better tho.
The rationale behind FM radio was to get a high quality signal in good
reception areas. AM is still good at punching through heavy noise and
interference. The rationale behind digital communications is that the
encoding is so complex that interfernec won't mimic it, so
- AM degrades smoothly under intereference, and with morse/vopice, may
be ultimately the best for really long ranges. This is not an issue with
R/C. Its very simple and cheap to make.
- FM is less sensitive to low level interference, as it tends to 'lock
on' to the strongets signal around. Of course if there is a stronger
signal than your TX......:-) it degrades very quickly, BUT most
interference doesn't mimic FM, so the tendency is not to lose signals
under interference, not to get spurious respones, though this is not an
inalienable fact...
- PCM is either working 100%, or probably intermittently, or not at all.
Depending on the level of interference. ISTR (not having used such) that
it has a failsafe mode if the encoding is lost. I think FM/PPM stays
where it was, servo wise, but PCM goes to whatever the rx thinks neutral
is, and low throttle.
No doubt those who use it can illuminate more than I can.
IMHO, dual conversion is probably more useful than PCM in eliminating
interference, but of courtse it depends on how well designed it is.
The Natural Philosopher
JEFGEO123 wrote:
>
> >Subject: Re: PCM or PPM
> >From: douco...@aol.com (Doucoursey)
> >>One of the best article i have seen is at the web site and everybody should
> >>read it
> >>http://www.torreypinesgulls.org/Radios.htm
>
> >That article is definately full of shit. AM was horrible,FM was a big
> >improvment.
>
> I beg to differ with a uninformed source, but i have flown my old futaba am
> systems at points where the 80 inch span plane was a very small dot and still
> had complete control while the fm systems were putting them in the ground at a
> much shorter range!
That is not necessarily an indication that FM is worse/better tho.
For one thing, yoer old AM set may have been proucing a lot more power
than the modern FM sets do. Most modern sets are not that powerful,
although they should all be capable of 'limits of sight' operation.
I built an AM set that transmitted an awesome estimated 5W at 27Mhz.
Hams can get round the world on less :-)
Secondly, FM does tend to be there, or not there. AM degrades more
slowly, but is more prone to iteference anyway, so its 6 of one...
Thirdly, the FM sets - early ones - may not have been as sesnsitive as
the AM sets.
Its really hard to compare like with like. And test reports in magazines
never really get down to these sorts of issues. Its much easier to say
'has elevon rate flaperon engine control grobbulator function' that get
out a spectrium analyser and say 'this bastard TX will probabaly run a
plane on channel 80 when a channle 60- Xtal is installed, due to crap RF
output filtering or 'This RX seems capable of responding to every TV
station in the county as well as occasionally responding to the supplied
transmitter'
Just anecdotally, because I have time on my hands today...I bought a
really old Pioneer FM tuner in a car boot sale. Now when they cam out I
was impressed by the reviews, and they allegedly were as sensitive as
you could get - within a dB or two of the theoretical limits.
When my CD player dies, I got a sony, and realised that I could control
a Sony tuner of the same remote, so I got one...and BOY was it ever so
much better on weak signals than that 'state of the art' pioneer. And
yet, the 'test results' matched the Sony's specs to a 'T'.
So, all is not equal where radios are concerned. I have flown planes to
the limit of my eyesight (48" planes) on a crap old futaba system (fm)
and never had a range problem. I have experienced strange lapses in
communicatiosn with my newer futaba system, but since I replaced the TX
xtal and aerial they have not, over a couple of flights, re-occurred....
The response of a reciever, to various conditions, is a function of
front end sensitivity, selectivity, AGC range and control, IF gain,
phase shift (with FM), and selectivity, and encoding type and decoding
quality. This thread picks on merely one thing - encoding type, and
ignores every single other factor determining interference rejection.
Its a credit to the manufacturers that radio is as good as it is, but
its a heck of a challenge to do it all at 10gm weight for $30....My
*GUESS* is that PCM RX's are not only PCM, but also carry more
components, and better design in them than simple FM PPM RX's.
All things being equal, my shoddy futaba PPM's are up to the job I want
of them. They are just a tad heavy and a bit prone to interference from
my electric motors, but careful use of suppression capacitors, and
careful siting of the ESC's gets me enough reliable range for what I
want....
...so what is the best lightweight 500meter plus range PPM RX to use
guys?
Bill Vail
Bill Vail
"Doucoursey" <douco...@aol.com> wrote in message
news:20011128211440...@mb-fb.aol.com...
Mike Slaughter
double conversion heterodyning is done to avoid image frequency problems and
has nothing to do with either modulation (PPM) and coding (PCM). The
modulation scheme for PPM and PCM are the same (FM). The difference is that
PCM modulates a piece information represented in a digital word (many bits),
while PPM has a single digital bit encoded in its analog position in a pulse
stream. Both have good and undesired attributes, but it is clear that in
our low data rate systems, PCM offers a huge gain in received signal
fidelity. But, as said here quite often, PPM provides excellent service for
a good price. Many people choose PCM to gain that extra SNR when flying
highly expensive models where the cost of the PCM Rx is small compared to
the cost of the entire system. Some others like 'cadillac' systems, and
others plan for growth.
Many Tx's are switchable between PCM and PPM. That is because both systems
are exactly the same except for the coding. I have never met a switched Rx.
ALL PPM and PCM Rx's (that I have ever seen in a modern system) are dual
conversion unless so stated, as is done in the small flyer Rx's where weight
is critical. Fewer chips, smaller circuit, lighter receiver.
This is not a very simple subject, and very difficult to convey when the
terminology is not fully understood.
"Natural Philosopher" <infi...@nospam.everywhere.net> wrote in message
news:3C06117B...@nospam.everywhere.net...
Mike Slaughter
and gains in the digital word encoding used in PCM. It is an ignorant old
wives tale perpetuated by people not understanding what they were observing.
The truth is that in a noisy environment, at the point where a PCM Rx is
just beginning to be affected, the PPM Rx has been rendered completely
unusuable for a very long time. PCM provides 12-14 Db improvement in SNR
over PPM. It is a given that is inherent in the hardware. What people
observe is that the PCM locks servos rather than allow them to react to the
noise, because it recognizes the noise. The small glitches that herald the
onset of overwhelming noise are still present, but can not be easily
observed if the flier is not attentive. However, if the flier had been on
PPM at that time, his plane would long have gone out of control and crashed.
That is the result signal processing done after the signal is demodulated
and decoded in the Rx.
I have proved this many times in the past on this ng, posting EE textbook
technical sources to be checked out to show what I can't express in a ng
message. Any source that has a comprehensive comparison of different comm
systems at baseband will show the improvement. No one has yet to post a
credited technical source that supports anything to the contrary. If you
have some, please reference them here, I would enjoy the opportunity to
learn something new.
Mike
"The Natural Philosopher" <Now...@there.org> wrote in message
news:3C06433B...@there.org...
The Natural Philosopher
Mike Slaughter wrote:
>
> You are confusing coding with modulation with de-heterodyning.
No, I am not. What I was saying is that as =far as I am concerned, I
would rather tackle potential interfernec problems with dual conversion
than a change in the encoding system.
You are talking to a fully qualified electronic engineer here, who has
designed and built a few things, including mil spec radios, in his time.
Just not recently :-)
Dual or
> double conversion heterodyning is done to avoid image frequency problems and
> has nothing to do with either modulation (PPM) and coding (PCM).
No, that is entirely correct. I never said it had, BUT teh whole desihgn
pupose of a reciever is to collect the signals you want and discard the
ones you down't. Dual conversion is a better way to blot out the
unwanted - chiefly co-channel - but also a lot of out-of-band stuff.
> The
> modulation scheme for PPM and PCM are the same (FM). The difference is that
> PCM modulates a piece information represented in a digital word (many bits),
> while PPM has a single digital bit encoded in its analog position in a pulse
> stream. Both have good and undesired attributes, but it is clear that in
> our low data rate systems, PCM offers a huge gain in received signal
> fidelity.
Well digital systems with parity either get it 100% accurately, or don't
get it at all :-)
PPM, like all analogue systems, has a range where it gets it sort of
mostly most of the time - you know - the servos start to chatter at
extreme range, but still sort of follow the sticks :-)
I suspect that teh behaviour under low signal/strong interfernec is
radically different. I would expect the PCM to just neutral the servos,
whilst teh PPM would make them twitch and hunt about. I haven't flown a
PCM system, or done range checks on ione, so I'll bow to those who have.
But, as said here quite often, PPM provides excellent service for
> a good price. Many people choose PCM to gain that extra SNR when flying
> highly expensive models where the cost of the PCM Rx is small compared to
> the cost of the entire system. Some others like 'cadillac' systems, and
> others plan for growth.
I don't think S/N ratio is quite the term you mean. All digital systems
work fine till you get to a critical S/N ratio - then they pack up
completely. Analogue systems degrade more slowly. They are to an extent
worse earlier, but better later.
If what I think I have read about PCM is correct, I would buy it for its
ability to 'go neutral under a burst of interference, rather than 'go
wild'.
But nothing will cope with a strong jamming signal that turns the RX agc
down. The only solution is a shielded Rx (so it doesn't pick up signals
except via the aerial) and a really good front end that gets rid of
unwanrted frequencies before they can overload it, or turn the RX gain
down via AGC.
The best yoi can hope for is that it doesn';t throw the model into an
irrecoverable position. In this respect PCM would seem to be better, not
beacuse its PCM, so much as because if it reckons its lost it, it drives
the servos to center, rather than letting nature take itts course...
> Many Tx's are switchable between PCM and PPM. That is because both systems
> are exactly the same except for the coding.
I know. I have just such a one.
> I have never met a switched Rx.
Too expensive and heavy to carry around all that stuiff. Besides, you
have one tranny for many rxes, so it makes sense to make it verstaile.
> ALL PPM and PCM Rx's (that I have ever seen in a modern system) are dual
> conversion unless so stated,
I would say that all PPM Rxs are single conversion unless so marked.
Certainly my Futaba kit is pretty typical in this respect. Its single
conversion. Dual RX's are available, as are PCM, but they represent a
cost that I don't personally see is justified. I will take it on trust
that most if not all PCM RX's are dual. It would be sensible to make
sure that your top of the line RX is as good as you can make it eh?
Looking at the kit for sale in the local hobby store. 99% of it is
single conversion PPM. Thats in the UK, on a fairly un cluttered 35MHz.
PCM and dual PPM are vailable, but cost a fair bit more and weigh more.
For example the cheapest Futaba PCM RX is £85 here, and the D/C PPM is
£50, but I can get a good S/C PPM RX for under £20.
> as is done in the small flyer Rx's where weight
> is critical. Fewer chips, smaller circuit, lighter receiver.
Thats me! :-)
Cheaper too. And *good enough* for where I fly.
> This is not a very simple subject, and very difficult to convey when the
> terminology is not fully understood.
Which it is by me at any rate ;-)
The Natural Philosopher
Mike Slaughter wrote:
>
> The notion that PCM dies completely in interference ignores the difference
> and gains in the digital word encoding used in PCM. It is an ignorant old
> wives tale perpetuated by people not understanding what they were observing.
> The truth is that in a noisy environment, at the point where a PCM Rx is
> just beginning to be affected, the PPM Rx has been rendered completely
> unusuable for a very long time. PCM provides 12-14 Db improvement in SNR
> over PPM.
I am willing to grant that, but would like more than a bald assertion
to be sure....
And to be sure the gains were in fact due to the encoding technique,
rather than the fact that a given PCM RX is just overall better built
with better interference rejection.
> It is a given that is inherent in the hardware. What people
> observe is that the PCM locks servos rather than allow them to react to the
> noise, because it recognizes the noise. The small glitches that herald the
> onset of overwhelming noise are still present, but can not be easily
> observed if the flier is not attentive. However, if the flier had been on
> PPM at that time, his plane would long have gone out of control and crashed.
> That is the result signal processing done after the signal is demodulated
> and decoded in the Rx.
Yes, but in general, what would be a twitchiness on PPM, is a sort of
freeziness on PCM. The question is at what S/N ratio the plane becomes
unflyable. My guess is that its not radically different. I would
certainly expect a $100 plus RX to outperform a $20 one, and teh fact
that PPM is a simple-ish encoding technique IS reflected in that price,
BUT I reckon that a lot of other things are too - like as you said,
implicit dual conversion, and maybe a lot better front end design.
In the end, its aerials down, and range checks at dawn. Back to back,
keep walking away till the servos are either unrespionsive or chattering
like mad...and see who has walked the furthest.
> I have proved this many times in the past on this ng, posting EE textbook
> technical sources to be checked out to show what I can't express in a ng
> message.
In the end, its down to detecting some form of FM change, and
interpreting what to do with it. There are lots of ways to tweak the
edges, but in the end Nyquist (I think it was Nyquist) rules. You
cannot recover signals buiried in noise beyond a certain point. I agree
that the more complex the encoding, the better you are able to do it,
because to an extent you know what you are looking for, and its not
noise.
However, is this the real issue? Is ultimate range the issue? Or is it
more likley that you have all the range you could ever realistically use
UNTIL SOME INTERFERENCE COMES ALONG. THEn the issue is, what does that
interference do to your radio?
If you have a good front end, not much. If you haven't, maybe it floods
it, and reduces your effective sensitivity to next to bugger all. Or it
gets into the IF striop directly, and turns the AGC down, having teh
same effect. Or it bypasses the lot and gets into the actual detectors.
My reason for choosing PCM would be to make sure that when one of the
above happens, something less drastic than 'servos to the end stops'
happens.
Flying electric models, I have had several layouts where motor
interference made teh servos twich alarmingly at range. As well as the
motor hunting as it drove itself into slow throttel etc. Whether PCM
would have given more range, and whether it would have been *BEACUSE*
its PCM rather than because its simply a beytter piece of radio before
the computer is stuck on teh back is a moot point. My experiences with
modems show that they packed up long before the lines on which they were
running became unsuitable for voice :-) Again, its down to subtle
issues. And DSP modems are now much better than the old ones.
> Any source that has a comprehensive comparison of different comm
> systems at baseband will show the improvement. No one has yet to post a
> credited technical source that supports anything to the contrary. If you
> have some, please reference them here, I would enjoy the opportunity to
> learn something new.
I am not supporting the contrary position, merely cautioneing that
painting PCM on teh casing of a piuece of crap electronics, doesn't make
it necessarily better. If there are good electronics designers out
there, and they have done the tests, and decided hat whatever is
actually in a Futaba PCM RX is so much better than waht is in a S/C PPM
RX that they are justified in a 2.5 times price hike, then fair enough.
But I bet its more than just the PCM that makes em good.
jim.archer
<SNIP>
> I am willing to grant that, but would like more than a bald assertion
> to be sure....
> In the end, its aerials down, and range checks at dawn. Back to back,
> keep walking away till the servos are either unrespionsive or chattering
> like mad...and see who has walked the furthest.
Why don't you try doing just that - rather than continuing to further
propogate this foolish arguement tha PPM is 'as good as or better than' PCM.
While you are at it, try adajacent channel rejection (suggest the BMFA
technique - models and pilots side by side, both 4 paces away from the
model, each extends then retracts aerial/antenna in turn)
I think you will find:-
1. Many PPM receivers will fail this test, all PCM receivers will pass
2. The ground range test (aerial down) will be _considerably_ further than
that achieved with PPM.
--
Best regards
Jim Archer
Norwich. UK
The Natural Philosopher
"jim.archer" wrote:
>
> "The Natural Philosopher" <Now...@there.org> wrote in message
> news:3C06740E...@there.org...
> <SNIP>
> > I am willing to grant that, but would like more than a bald assertion
> > to be sure....
> <SNIP>
> > In the end, its aerials down, and range checks at dawn. Back to back,
> > keep walking away till the servos are either unrespionsive or chattering
> > like mad...and see who has walked the furthest.
>
> Why don't you try doing just that - rather than continuing to further
> propogate this foolish arguement tha PPM is 'as good as or better than' PCM.
I am not doing that. I am merely pointing out that to be fair and
banlanced, one needs to get wto identical rtecievers, one PPM, one PCM
and eliminate all the other variables.
Because others are loudly proclaiming that PCM is better in all respects
than PPM because its PCM.
I don't buy that either.
> While you are at it, try adajacent channel rejection (suggest the BMFA
> technique - models and pilots side by side, both 4 paces away from the
> model, each extends then retracts aerial/antenna in turn)
>
> I think you will find:-
> 1. Many PPM receivers will fail this test, all PCM receivers will pass
Now that wouldn't be because all PCM RX's are dual conversion, and only
some PPM's are would it?
:-)
> 2. The ground range test (aerial down) will be _considerably_ further than
> that achieved with PPM.
Now that wouldn't be because all PCM rx's are dual convesrion, and
therefore able to carry more gain than PPM rx's and are anyway built to
a higher standards as reflects their 'top of teh range' image.
And, what is good anyway?
For me, flying what I do, where I do, my highest priorities are
- low cost
- reasonable range (500 meters is totally adequate)
- light weight.
I don't have interference problems. I am not flying 50kg scale models at
1 mile ranges.
So for me, PPM is definitely best. For me.
Its cheap.
Its light.
It has adequate range.
I can solve the electrical motor interference probblems I have by using
my knowledge of electronics and lots of anecdotes from the guys who run
RC cars, to suppress it at source.
What this means is that I can get a whole plane into the air for the
cost of a PCM RX.
I am no particular advocate of anything. I merely take exception to
people who get religious, and spout things that are inconsistent with my
knowledge of communications theory. I am fully prepared to believe that
e.g. Futabas PCM RX at £100 outperforms their £39.99 Single conversion
PPM offering. I wold be disgiusted if it didn't.
Why don't you be helpful and tell me where I can get a 500 meter plus RX
weighing less than 10gm for less than £20?
I don't give a flying acrowot4 whether its PPM or PCM, as long as it
works in the field out the back of my house :-) With a Futaba FF6
tranny.
jim.archer
>
>
<SNIP>
> > I think you will find:-
> > 1. Many PPM receivers will fail this test, all PCM receivers will pass
>
>
> Now that wouldn't be because all PCM RX's are dual conversion, and only
> some PPM's are would it?
>
> :-)
>
> > 2. The ground range test (aerial down) will be _considerably_ further
than
> > that achieved with PPM.
>
> Now that wouldn't be because all PCM rx's are dual convesrion, and
> therefore able to carry more gain than PPM rx's and are anyway built to
> a higher standards as reflects their 'top of teh range' image.
No - when I tried this it was with a Futaba 138DP and a 138DF. Both dual
conversion, same plane, same channel, same Tx. The 'only' difference was
that one Rx was PCM.
I did this because I was getting minor 'glitches' at the site wher I fly.
The PCM Rx gave me at least 10 times the ground range (got fed up with
walking), was rock solid on adjacent frequency test (PPM was not) and the
glitches became a thing of the past.
So I did the only sensible thing and used PCM from then on.
> And, what is good anyway?
> For me, flying what I do, where I do, my highest priorities are
> - low cost
> - reasonable range (500 meters is totally adequate)
> - light weight.
Fine - use a Jeti 7 - single conversion but every one I have used (in e
gliders) has been 'rock solid'.
Dont knock PCM just because you cannot afford one.
I do fly fairly large and expensive planes, but almost all of my smaller
planes have PCM receivers and dual nicads/switch harnesses - small planes
can kill people too.
Jim
The Natural Philosopher
"jim.archer" wrote:
>
> "The Natural Philosopher" <Now...@there.org> wrote in message
> news:3C0682F8...@there.org...
> >
> >
> <SNIP>
> > > I think you will find:-
> > > 1. Many PPM receivers will fail this test, all PCM receivers will pass
> >
> >
> > Now that wouldn't be because all PCM RX's are dual conversion, and only
> > some PPM's are would it?
> >
> > :-)
> >
> > > 2. The ground range test (aerial down) will be _considerably_ further
> than
> > > that achieved with PPM.
> >
> > Now that wouldn't be because all PCM rx's are dual convesrion, and
> > therefore able to carry more gain than PPM rx's and are anyway built to
> > a higher standards as reflects their 'top of teh range' image.
>
> No - when I tried this it was with a Futaba 138DP and a 138DF. Both dual
> conversion, same plane, same channel, same Tx. The 'only' difference was
> that one Rx was PCM.
>
> I did this because I was getting minor 'glitches' at the site wher I fly.
> The PCM Rx gave me at least 10 times the ground range (got fed up with
> walking), was rock solid on adjacent frequency test (PPM was not) and the
> glitches became a thing of the past.
> So I did the only sensible thing and used PCM from then on.
At last, some real evidence that counts for something!
I am glad you used the futaba gear as well...because it probly means
that the RX's were very similar...
>
> > And, what is good anyway?
> > For me, flying what I do, where I do, my highest priorities are
> > - low cost
> > - reasonable range (500 meters is totally adequate)
> > - light weight.
>
> Fine - use a Jeti 7 - single conversion but every one I have used (in e
> gliders) has been 'rock solid'.
I have heard poor stories about those in terms of electric motor
interference. But if they work, then I will risk the money and try one.
>
> Dont knock PCM just because you cannot afford one.
>
Oh I can, much more than you imagine ;-)
But I hate to waste money on unnecessary things. That is one of the
secrets of making, and keeping, lots of it :-)
Also, it saddens me to see others encouraged to do so. All the best gear
in the world is no subsitute for a decent beginners plane, a reliable
engine, and some helpful people at the flying field, so you can get
FLYING TIME.
> I do fly fairly large and expensive planes, but almost all of my smaller
> planes have PCM receivers and dual nicads/switch harnesses - small planes
> can kill people too.
Well in my book a large plane is over 36" wingspan :-) No thats a medium
plane. Small planes are 24" and under, and 48 and over is HUGE. :-)
I doubt any of my planes could kill. Knock an eye out yes. But in
general I fly on a completely empty field, and there nearest I got to
killing anything was a deliberate attempt to dive bomb a heron. It
proved more than a match for my plane and skills tho :-)
> Jim
Jack Goff
"JEFGEO123" <jefg...@aol.com> wrote in message
news:20011129065700...@mb-mh.aol.com...
<snip>
> if am is soo bad why can i listen to am stations 2000 miles away and not
hear
> fm stations 200 miles away, when they are running the same amount of
power?
> if i get my fm antenna up about 25 feet i can get a good solid stereo
light
> from stations 300 miles away, but if i put my am antenna up that same 25
feet i
> can get stations from 8-9000 miles away.
What you are experiencing is not the difference in modulation schemes
(AM-vs-FM), but rather the frequency of the transmitted signal.
AM stations broadcast on about 500-1700 kHz, which is low frequency for RF.
It has the characteristic of bouncing off atmospheric layers, or "skipping",
so very low power signals will propagate for very long distances, around the
curvature of the earth.
FM stations broadcast on about 88-108 MHz, a much higher frequency band.
These signal rarely skip, rather the atmosphere typically acts like a big
sponge to soak them up. They are pretty much line-of-sight signals, and the
higher the frequency, the more severe the effect.
Ever notice how on those AM stations, you can hear clicking and popping in
even a fairly strong signal? Say when you turn on a light switch, the
refrigerator kicks in, or you get too close to an electric motor? That's
because AM is Amplitude Modulation. It's a steady carrier frequency with
the desired signal superimposed on it. Impulse noise looks just like the
desired signal to the receiver, which passes it on to your speaker or servo.
FM is Frequency Modulation, meaning that the carrier frequency of the signal
is moved up and down to represent the desired signal. Up to a point,
impulse noise on top of the FM signal means nothing to the receiver, since
it only cares about the frequency of the signal, not the noise component
riding on top. That's the noise rejection advantage FM enjoys over AM.
The sudden loss of FM signals you referred to is because of receiver
"capture". When you're riding down the road listening to a weak FM station
and it suddenly goes away, being replaced by another, the previous signal is
still there. It's just that modern FM receivers capture the strongest
signal, and track it until it is finally overtaken by a stronger signal on
the same "channel". Another advantage for FM, since it effectively ignores
very weak signals in favor of a strong one.
The AM receiver, on the other hand, is happy decoding two interfering
signals at the same time. Ever heard two AM radio stations at the same time
while tuning through the dial?
Having said all of that (whew!), AM will work just fine in most of our
applications. Just don't fool yourself into thinking it has any real
advantage over FM for us. There's a reason it's called "Ancient
Modulation". :-)
Jack
Mike Slaughter
PPM/PCM radio that is on the verge of uncontrollability due to noise, and
switch RX's to PCM and switch TX mode, you gain ~12-14 dB snr. That's a
lot, and it is inherent in the coding mode, as seen in baseband comparisons.
Since all else in the tx is the same, the gain is there. I think the added
feature of post decoding processing that can detect a noisy signal and place
servos in the last valid received signal is very good. Rather than flailing
about randomly, the system helps you detect the problem and land before the
signal goes away entirely. My claim for dB improvements are provable in any
reference that has baseband snr comparisons. Or by the math, if you love
punishment.
If you have $3000 in a plane, wouldn't you want the extra protection?
If you have $100 in a plane and no plans to get expensive, I agree that a
pcm system would not be cost efficient.
But if you are going to make the decision, why not make it based on relative
facts rather than old rumors?
Too many people in the US, because of a poor education system, cannot
discern a fact from a feeling (emotion).
One thing I have not seen is whether our PCM radios employ error correcting
coding. A 7/4 Hamming code would be easy to employ, and has a huge snr
gain. It is pretty much standard in many chips manufactured for
communications. Manufacturers are (rightfully, imo) very tight on specs
such as these. All you can get is what the ad marketers write for general
consumption. Multiplex has a new system they claim is very good. From
their description, all I get are a bunch of tech words combined by admen to
sound cool to consumers. any info on that?news:3C06740E...@there.org...
Steve
>
>
>"jim.archer" wrote:
>>
>> "The Natural Philosopher" <Now...@there.org> wrote in message
>> news:3C06740E...@there.org...
>> <SNIP>
>> > I am willing to grant that, but would like more than a bald assertion
>> > to be sure....
>> <SNIP>
>> > In the end, its aerials down, and range checks at dawn. Back to back,
>> > keep walking away till the servos are either unrespionsive or chattering
>> > like mad...and see who has walked the furthest.
>>
>> Why don't you try doing just that - rather than continuing to further
>> propogate this foolish arguement tha PPM is 'as good as or better than' PCM.
>
>I am not doing that. I am merely pointing out that to be fair and
>banlanced, one needs to get wto identical rtecievers, one PPM, one PCM
>and eliminate all the other variables.
>
I went through this exact exercise a while ago, using two receivers, a
Futaba dual conversion PPM, and a Futaba PCM receiver. Same plane, a 35%
Cap 232 with a gas engine and electronic ignition, same servos, antenna run
in the same location. The only thing that changed was the receivers. The
transmitter was a Futaba 9ZWCII. The difference in the antenna collapsed
ground range check was very significant. Well over three times the range
check of the PPM receiver. It was unexpected.
With that said, I should also point out that I still use both PPM and PCM
receivers. To me, both are very reliable rock solid means to control a
plane, and I find the tradeoffs mainly between whether or not I want
failsafe, and the ability of the PCM system to completely mask
interference, which I find a very real disadvantage. Enough so, that with
every PCM receiver I purchase I end up spending another $60 for a glitch
counter to 'unmask' the 'masked' glitches. The cost of the airplane doesn't
play into the decision, as I have both PPM and PCM in my largest and most
expensive planes.
Probably my main motivation for going with PCM, is the ability to use the
failsafe to enhance the safety for spectators. I fly at lots of various
events throughout New England. Some have only 40 or 50 people attending,
others have pulled in just over a thousand spectators over a weekend
event. And then there is Joe Nall, with over 500 pilots alone, not counting
the spectators. The thought of having a plane get away from me and crash
into a crowd, pits, or crowd, isn't pleasant. The failsafe capability with
most all PCM receivers/transmitters allows me to configure the plane to
eliminate this under most circumstances. It gives me the ability to control
the outcome somewhat.
The planes that fly at these events, are all running PCM. The planes that I
usually fly only at our home field, in the wide open spaces with few on the
ground, are typically PPM. The deciding factor was the safety aspects of
using failsafe. In my helicopters, I ran PCM with glitch detectors, to aid
in controlling interference issues noted with these noisy machines.
On another side note, somewhat related, is the difference between the
Futaba 9Z and JR's 10X. I purchased a plane, a 42% Giles 300, and it came
with a JR10X and dual PCM receivers. This gave me the opportunity to do a
side by side comparison between the 9ZWCII and the 10X, and the respective
PCM receivers for each. The only odd difference, was the plane with the JR
receivers, had dual receivers installed in it. The plane with the Futaba
receiver, only had one. I also extended the antenna to the first segment on
the Futaba, which made similar to the extended antenna on the JR. Both
transmitters and receivers gave similar range checks in their respective
planes.
I eventually switched from the Futaba 9Z to the JR 10x, because it gave me
two extra channels over the 9Z. This was after a careful examination of the
features of each radio, and their use in my aircraft (I would have stuck
with the 9Z if I was still flying helicopters). I couldn't find much of
anything that I couldn't do on the JR that I could with the Futaba. The
Futaba has more functions, but they can all be replicated by using mixing
on the JR if need be. And after years of flying high performance aerobatic,
even with complex mixing required on some with ganged servos, 3D setups,
odd mixing requirements, etc, I found that most of the additional features
on the Futaba were going unused anyway. And the JR and Futaba were
identical with regard to the most often used features and functions. I
decided that if I wasn't using the additional features on the 9Z after all
this time, I figured I wouldn't mind too much if I had to actually build on
in a mixer, rather than have it as a built in function.
But the real benefit to the JR is the additional two channels. This allowed
great flexibility in the setup when ganging up dual or triple servos on a
control surface. Something that you start to struggle with on the 9Z with
only 8 usable channels. It gets you into a lot of custom programming, or
matching of servos for mechanical setups. When I changed over one of my
planes, the additional two channels allowed me to reduce the complexity in
the overall programming, freeing up three mixers, and not having to do a
lot of custom programming. But, just to make sure, I did go through the
exercise of recreating the complex programming on the JR just to be sure I
could, so that I could see if I was really giving up the power of the 9Z.
It was just as hard on the 10X as it was on the 9Z, but both radios could
easily handle it.
Anyway, I digress. I wanted to mention that in the switch over to JR, I was
able to do a comparison test in the same aircraft, with everything
identical. The plane is the same 35% Cap232 mentioned above. It was
initially configured and flow for a few seasons on Futaba PPM, then later
switched to PCM,which is when I did the above mentioned Futaba PPM/PCM
test. Being switched from Futaba PCM to the JR, with its comparable PCM
receiver, the only thing that changed was the transmitter and receiver. And
the result was very similar range checks. Both receivers/transmitters far
exceeded the PPM test, and the manufacturers recommendations.
It kind of makes any differences between JR and Futaba rather moot as far
as reliable methods of control. Both far exceeded PPM, which is working
fine our application. Kind of like comparing the top speed difference
between a Mercury Sable wagon, and a Mustang. When you don't drive over 65
to begin with, it doesn't matter all that much that one can go faster than
the other. They can both deliver on goods at 65mph.
IMHO, PCM is better for reasons other than max range and reliability. But,
does the price difference warrant the use of PCM for all applications? Or
the failsafe vs non-failsafe? Is the hold mode an advantage in your
application, or just indifferent? Is interference masking a positive or
negative benefit? In my mind, these are the more important questions than
range or electronic theory. PPM and PCM are both quite capable of
performing the task we ask of them, and have been doing so for a long time.
Steve
medi...@mindspring.com
RC web Site http://webpages.charter.net/mediashop/rc-giant-scale/
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Matthew Orme
My comments are based on direct observations of others, and personal
experience.
> The truth is that in a noisy environment, at the point where a PCM Rx is
> just beginning to be affected, the PPM Rx has been rendered completely
> unusable for a very long time.
True. PCM masks a noisy environment, OR poor/noisy instillation, or other
equipment problems..
>However, if the flier had been on
> PPM at that time, his plane would long have gone out of control and
crashed.
Not true. With sailplanes, which fly much farther away from the transmitter
than almost anything flown at a gas field (thousands of feet), the PPM pilot
notices the noise/glitches in the plane, turns the plane around (or actually
runs across the 20 acre field to get closer to the plane), and try's to
troubleshoot the problem. The PCM pilot sees everything working perfectly,
until the signal gets so bad that the receiver "locks out", and the plane
keeps flying away, and is lost. Even intermittent control is better than
none.
For instance. I had a transmitter with a broken lead to the antenna (the
circuit board it screwed into was cracked), and the signal was intermittent
as the antenna wiggled as I moved around. With my PPM planes, it was
unflyable. I changed to a PCM plane, and it worked fine. At the time, since
we were flying at a working airport that had shut down one runway for us, I
assumed I was getting interference from the radar, control tower radio
traffic, etc.
I did not even look to see if it was my equipment, which it was. It was not
until I got home that I was able to troubleshoot the problem, since I was
getting the same behavior in a noise free environment.
i.e., you can have serious equipment problems with PCM, and not know it.
Whether the signal to noise ratio is better is not relevant. PCM can mask
problems. I fly PPM systems at a slant range of at least 2500-3000 feet,
with no problems. (I also have PCM receivers, but they came with the radios,
and all new purchases are Hitec 8 ch micro receivers)
In F5B, the pilots used to climb to about 600-700 feet, power down the
motor, and dive straight down to gain speed to about 200 mph. (this task was
eliminated by the FAI this year) At about 50 meters (the minimum distance
allowed) from a sighting device, they level off at about 2M off the ground,
and fly at very high speed (power off in a glide at 180-200 mph) to pass
under the sighting device. (the device is at 3M). There is enough retained
energy after this "limbo" task to climb to about 600 feet without power, and
do a 5 minute thermal duration glide, with out starting the motor (which
subtracts points if run)
These guys no longer use PCM, because a half second lockout will put the
plane into the ground. There is always noise at these events, and I have
seen my own planes take an elevator hit, bounce down then back up (looks
really scary), and then continue on it's way. It is just their experience
that PCM costs them planes, and you can all tell them that they are wrong if
you like, but the planes are $2500 each, and they expect to loose one during
the event, and they are built so light that they are not intended to last
longer than a season.
Jimmy Huang
>One thing I have not seen is whether our PCM radios employ error correcting
>coding. A 7/4 Hamming code would be easy to employ, and has a huge snr
>gain. It is pretty much standard in many chips manufactured for
>communications.
Mike,
I am pretty damned convinced that Futaba PCM and Futaba PCM1024 both
use some sort of Forward Error Correction Scheme. There is no way they
can get the kind of performance they are currently getting simply by
using parity checks.
Having said that, I have no clue what they are using (it could just be
a better tuning ciruit with parity checks). I don't have the
equipment to see what they are sending. Sorry to be of no use here.
[Someone else sez]
>> Yes, but in general, what would be a twitchiness on PPM, is a sort of
>> freeziness on PCM. The question is at what S/N ratio the plane becomes
>> unflyable. My guess is that its not radically different. I would
I don't use PCM. However, I have personally witnessed 2 people at our
field, who had reception problems running PPM with gas engines. Their
trasmitters were of the high-end Futaba, so all they did was change
their receiver, and problems went away. No antennae reconfiguration
(ran antenna through the same tube), not a different plane, just a
different receiver.
I'm not saying that PCM is superior to PPM. I'm saying that the
technology that people are putting into PCM is superior to the
technology that they put into their PPM receivers.
>> In the end, its down to detecting some form of FM change, and
>> interpreting what to do with it. There are lots of ways to tweak the
>> edges, but in the end Nyquist (I think it was Nyquist) rules. You
>> cannot recover signals buiried in noise beyond a certain point. I agree
>> that the more complex the encoding, the better you are able to do it,
>> because to an extent you know what you are looking for, and its not
>> noise.
When Nyquist was making his theorems, Forward Error Correction wasn't
invented yet! Seriously, put in enough forward error correction, and
you can literally pull the digital signal out of the noise! S/N's of 0
dB!!! The signal and the noise is totally indistinguishable looking at
it from a spectrum analyzer.
Better yet how about pulling signal out from up to 20-40 dB BELOW the
noise floor? Where I work we have equipment that does this. However,
it does not use FEC, and it's not used for communications
transmission. We use it to monitor the undersea repeaters w/out
affecting its traffic. If we were to use this for transmission, I
think it's effective transmission rate would be around 300 bps, and
this is on a 2.5 Gbps line.
If you remember your physics classes, you'll remember "IN THE END, RF
signals never truly die, they just get smaller and smaller at an
exponential rate (when going through the air that is.)" If you know
what you are looking for, you will find it. Today, what matters is
energy per bit. What's the minimum energy per bit on the receiver do I
need for a successful transmission? Receivers are getting more and
more sensitive day by day as the tech improves. The FEC makes it so
that the receiver doesn't have to receive all the energy for that bit
in one shot, but rather spread out in multiple tries.
Jimmy
Steve
>Not true. With sailplanes, which fly much farther away from the transmitter
>than almost anything flown at a gas field (thousands of feet), the PPM pilot
>notices the noise/glitches in the plane, turns the plane around (or actually
>runs across the 20 acre field to get closer to the plane), and try's to
>troubleshoot the problem. The PCM pilot sees everything working perfectly,
>until the signal gets so bad that the receiver "locks out", and the plane
>keeps flying away, and is lost. Even intermittent control is better than
>none.
>
If the same pilot sees a lockout with PCM, he can also run across the 20
acre field just as the PPM pilot could, and regain control. Remember that
the lockout is only as long as the interference, just as is the case with
PPM interference. 1/2 second of interference causes a 1/2 second of
lockout, no more, no less. The old wives tale suggests that it is longer
than the interference event, and some even believe once it goes into
lockout, it doesn't come back.
Also, don't forget that with PCM comes failsafe. Failsafe will guarantee
that the plane won't fly away, and will almost always contain the aircraft
within the general flying area. The same can not be said for PPM. And its
not PPM vs PCM that guarantees this, but failsafe vs no-failsafe.
>For instance. I had a transmitter with a broken lead to the antenna (the
>circuit board it screwed into was cracked), and the signal was intermittent
>as the antenna wiggled as I moved around. With my PPM planes, it was
>unflyable. I changed to a PCM plane, and it worked fine. At the time, since
>we were flying at a working airport that had shut down one runway for us, I
>assumed I was getting interference from the radar, control tower radio
>traffic, etc.
>
>I did not even look to see if it was my equipment, which it was. It was not
>until I got home that I was able to troubleshoot the problem, since I was
>getting the same behavior in a noise free environment.
>
>i.e., you can have serious equipment problems with PCM, and not know it.
>
Exactly why I recommend glitch detectors with all PCM receivers. If you
had a glitch detector in your PCM plane, you would have noticed the problem
even though you still had full control. It would have given you the same
indication of a problem like your PPM plane, but would still afford you
full control.
I check mine glitch detector prior to take off, at the same time I do the
rest of my checks, and I check them when I land. They are capable of
counting glitches as small as a single frame of data, something that would
go completely unnoticed while flying in either PCM or PPM. This completely
'unmasks' the issue, and brings it front and center.
Secondly, failsafe would give you an indication that you received a hit, as
your control surfaces would change state during the transition to their
failsafe conditions. It appears very similar to a hit on a PPM system in
the air. A momentary control input change that was not done by the pilot on
the sticks. It does stand out, and the pilot is aware.
Your example also demonstrates the advantages of PCM over PPM, in its
ability to continue to operate under conditions that would send PPM
receivers into a frenzy. In the situation you just mentioned, which would
you rather have in the air, the PPM that failed completely, or the PCM that
you still had full control over?
Consider that you had just started your plane in the pitts, when the
interference kit. With PPM, there is a chance it could get away from you.
However, in your example, your PCM system would still have full control.
What about if it hit during take off? With your PPM plane, it would be
destroyed. With the PCM system, you might still have full control. If not,
failsafe would kick in and it would also be destroyed.
It really all depends on the circumstances on which system would be better
in any given situation. We could argue for days on all kinds of different
scenarios, and still get nowhere. Which says on thing to me. They are both
very good, and each has an advantage in particular circumstances. And with
the dynamic nature of our planes, the hobby, and where we fly, these
circumstances change daily.
>Whether the signal to noise ratio is better is not relevant. PCM can mask
>problems. I fly PPM systems at a slant range of at least 2500-3000 feet,
>with no problems. (I also have PCM receivers, but they came with the radios,
>and all new purchases are Hitec 8 ch micro receivers)
>
>In F5B, the pilots used to climb to about 600-700 feet, power down the
>motor, and dive straight down to gain speed to about 200 mph. (this task was
>eliminated by the FAI this year) At about 50 meters (the minimum distance
>allowed) from a sighting device, they level off at about 2M off the ground,
>and fly at very high speed (power off in a glide at 180-200 mph) to pass
>under the sighting device. (the device is at 3M). There is enough retained
>energy after this "limbo" task to climb to about 600 feet without power, and
>do a 5 minute thermal duration glide, with out starting the motor (which
>subtracts points if run)
>
>These guys no longer use PCM, because a half second lockout will put the
>plane into the ground. There is always noise at these events, and I have
>seen my own planes take an elevator hit, bounce down then back up (looks
>really scary), and then continue on it's way. It is just their experience
>that PCM costs them planes, and you can all tell them that they are wrong if
>you like, but the planes are $2500 each, and they expect to loose one during
>the event, and they are built so light that they are not intended to last
>longer than a season.
>
I don't blame them for not using PCM. In fact, I wouldn't blame them for
not using PPM either. A half second interference in PPM would be just as
ugly as a 1/2 second with PCM. There is little margin of error for either
system under those conditions.
If aircraft cost was an issue, then all the TOC pilots flying their 40% and
up aerobatic machines, costing from $5,000 to $8,000 each, would all be
using PPM receivers. I'm sure they don't want to loose any aircraft either.
Considering the low level antics they put these things through, from
terminators (similar to what you described above in F5B), torque rolls on
the deck with the rudder touching, low level rolling circles, etc. Image a
1/2 second interference event on PPM or PCM for these guys. Instant
rekitting. Yet, they have found quite the opposite, and prefer PCM
overwhelmingly over PPM. TOC is almost predominately PCM, with IMAC right
behind them.
Neither system is perfect, and each has its advantages and disadvantages.
Both are very reliable, and a pilot can feel comfortable with either
system. In some cases, a PPM system might have saved a plane or two. Yet,
in others, the PCM system would have saved a plane or two. We could all
spend an entire day arguing various instances and what ifs. Fact is, a
plane is dynamic in the air, and on the field. There will be many times
throughout a day, where the pilot will have his plane in situations where
one second a PPM system might have an advantage, only to find in the next
second the PCM system would have the advantage. Its pointless to argue each
and every instance.
Doucoursey
>Not true. With sailplanes, which fly much farther away from the transmitter
>than almost anything flown at a gas field (thousands of feet), the PPM pilot
>notices the noise/glitches in the plane, turns the plane around (or actually
>runs across the 20 acre field to get closer to the plane), and try's to
>troubleshoot the problem. The PCM pilot sees everything working perfectly,
>until the signal gets so bad that the receiver "locks out", and the plane
>keeps flying away, and is lost. Even intermittent control is better than
>none.>>>>
i beg to differ on how far away pcm can work. i have seen AIRPLANES and
helicopters flown untill they are just a dot in the sky, i think curtis has put
a heli 1500 feet up. DOUG
Steve
Doucoursey,
Ah, your story reminds me of an event I attended last year. I was at a
helicopter fly in at the Crow Island field in Stow MA. Some of the guys
were strapping an altimeter watch onto their helicopters, and taking them
up. The highest reading was around 3800 feet AGL, or 3300AGL. In either
case, it was over 3,000 feet. The winner, almost lost the helicopter, as
everyone lost sight of it, and couldn't reacquire it until it was almost in
the trees. It was fun to watch them all checking the sky frantically, then
even more fun to watch the screams when they found it low and over the
trees to their left. I don't know if they were flying PPM or PCM. Its
fairly evenly split in that group if memory serves. I have it on video.
Regardless if PCM or PPM can outdistance each other, they both can far
outdistance our normal eyesight by a long shot. Kind of like arguing if
your Corvette can out run my Porsche. They are both limited by the speed
limit, and even when exceeded, rarely if ever travel at their top
potential.
MacGyver
>Neither system is perfect, and each has its advantages and disadvantages.
>Both are very reliable, and a pilot can feel comfortable with either
>system. In some cases, a PPM system might have saved a plane or two. Yet,
>in others, the PCM system would have saved a plane or two. We could all
>spend an entire day arguing various instances and what ifs. Fact is, a
>plane is dynamic in the air, and on the field. There will be many times
>throughout a day, where the pilot will have his plane in situations where
>one second a PPM system might have an advantage, only to find in the next
>second the PCM system would have the advantage. Its pointless to argue each
>and every instance.
I am wondering what people think of
the IPD (intelligent pulse decoding) systems?
Check out the IPD receivers:
http://www.multiplexrc.com/ipd_info.htm
They are PPM compatible with futaba,JR,hitec etc. and they have
failsafe.
Anyone have any experience with those?
Mac.
http://egnatia.ee.auth.gr/~kosmour/index1.htm
Mike Slaughter
Hamming code. He worked at Bell labs in the late 30's and 40's when
information theory was developed. IIRC, one of the names he consistently
dropped was Nyquist, along with Shannon. I think that the digital ec code
was being developed simultaneously with the theories of communications.
Hamming was one of the most intelligent and egotistical engineers I ever
met. He earned it, though. His book is a great read.
Mikenews:6nef0u899k5i8nd9a...@4ax.com...
Mike Slaughter
better than PPM. I the noise level is just high enough to knock a PPM Rx
off the air, you still have 12 dB to work with in a PCM Rx. You can fly the
pcm and get no hits. As the noise increases, errors are manifested in the
same way as ppm. Your plane does not respond to your inputs while receiving
too much noise. The ppm plane will have the servos jump all over the place
as it forces noise into a servo position. The pcm will recognize the loss
of the data stream and put your servos in the last good signal. By
programming, it may put your throttle to idle. Either way, an alert flier
can notice it and land if control is regained. Most of of noise is of the
short duration blanking that occurs when other comm systems walk on our
signals. When a burst at level X occurs, it may knock the ppm plane out of
the sky. It takes 10 time X to even affect the PCM Rx. Now add the post
detection programming. What do you want your system to do when overcome by
noise?
a. servos jerk randomly
b. servos parked at last good input, and throttle dropped to idle
I think the programming is a great addition, and the best feature of pcm. I
don't see how the signal can be 'masked.' That is a subjective call based
on an undefined term, and passed along by Luddites who long for escapements.
Mike
"Matthew Orme" <Fred.Fl...@orme.org> wrote in message
news:9DON7.192$jh1....@paloalto-snr1.gtei.net...
Matthew Orme
Not at all, I just disagree.
If the servos twitch, there is another problem somewhere that needs
investigating. It is not just a "poor PPM receiver". I prefer to reduce the
noise/interference, rather than use a receiver that masks the problem (like
a broken antenna connection in my Vision). As I said. I fly at 2500-3000 ft
(slant range) ALL THE TIME. With a proper setup, there are no problems. If
your servos twitch, there is something else going on, and you prefer to hide
behind a PCM receiver, while I prefer to troubleshoot, and correct the
problem. (without the added weight/complexity of a glitch counter)
In some cases, I have had to add ferrite beads to all 3 servo leads in wing
servos in 100" sailplanes, or inductors + caps as noise filters.
BTW, the default "failsafe" for Futaba radio throttle channels is the
reverse of everyone else's. If you are using a speed controller in an
electric plane, the default is for the channel to go to 2 microseconds. this
is full on for speed controls. every other brand has off as 1 microsecond,
and full throttle as 2 microseconds. i.e., if you just plug-and-fly with a
Futaba (as nearly all consumers do), an electric plane will "failsafe" to
full throttle. (I don't kneed to know that you should program your radio,
any more than I need to know that you should wear your seatbelt.)
"Mike Slaughter" <MikeSl...@cinci.rr.com> wrote in message
news:Es7O7.83626$z55.10...@typhoon.neo.rr.com...
Bob Burrill
>
> Mike Slaughter wrote:
> >
> > ALL PPM and PCM Rx's (that I have ever seen in a modern system) are dual
> > conversion unless so stated,
>
> I would say that all PPM Rxs are single conversion unless so marked.
> Certainly my Futaba kit is pretty typical in this respect. Its single
> conversion. Dual RX's are available, as are PCM, but they represent a
> cost that I don't personally see is justified. I will take it on trust
> that most if not all PCM RX's are dual. It would be sensible to make
> sure that your top of the line RX is as good as you can make it eh?
>
> Looking at the kit for sale in the local hobby store. 99% of it is
> single conversion PPM. Thats in the UK, on a fairly un cluttered 35MHz.
> PCM and dual PPM are vailable, but cost a fair bit more and weigh more.
>
I think I see the problem here - the UK!. Here is the US nearly all
aircraft PPM and PCM Rxs are indeed Dual Conversion. The only legal single
conversion Rxs I have seen are the older Futaba 5 channel ones and they are
PCM. Dual conversion is almost required to get around the 23 channel
seperation problem that came up as a result of cutting the bandwith in half
and adding the odd channels. So single/dual conversion issues aren't
normally a problem unless someone is using the older (and no longer legal)
equipment.
BTW, SNR is usually better with the PCM Rxs which probably accounts for the
much higher price more than just the PCM decoding. In other words, it might
not really be the coding difference that gives the better SNR. I'll leave
that decision up "a fully qualified electronic engineer here, who has
designed and built a few things, including mil spec radios". My 30+ years
as an electronic engineer have been primarily in digital systems.
Thanks,
Bob
Doucoursey
PCM doesn't mask anything, it just does what it is suppose to do and only
listen to the coded signal from the transmitter. there are noise's
that a PPM reciever will listen to and react to in an incorect signal sent to
the servo's so the plane jerks around and is usually uncontrolable.
you might have control on some of the control surfaces but not all of them.
PCM is more tollerant to noise than PPM is and the same noise that
interferes with the signal on PPM might not effect PCM. there are all kinds
of noise in the air, PCM is more selective than PPM.
this is the reason people with high dollar equipment usually choose PCM
instaed of PPM. thats the reason i do.
DOUG C>
Matthew Orme
Really? Then please explain why my Vision radio, with a broken antenna
connection (making an intermittent contact, and therefore an intermittent
signal from the transmitter) behaved like nothing was wrong with a PCM
receiver, and was unflyable with a PPM receiver. I would not consider an
intermittent signal from the transmitter as noise.
It was not a weaker signal, but a random one.
then again, we may define "mask" differently.
Steve
Its probably how you two are defining "mask". I also think that PCM masks
the problem, which is why I recommend a glitch detector when using PCM.
However, technically, its isn't really masked, but ignored as Doug
suggests. The PCM receiver will only respond to valid control inputs, and
not invalid ones. This is why your PCM receiver appeared to be working
fine. But, I think you would also find that many of the frames the receiver
'heard', were just discarded as invalid. Thus, you were only presented with
what was left, which was a reduced set of valid frames.
But that was enough to allow you to effectively control your airplane, and
exactly why a PCM receiver is preferred by many. While your PPM system
simply shut down, the PCM system was able to get enough valid data, and had
the smarts to only act on those, and ignore the invalid data. Hence, your
PCM had control, where the PPM system did not. A very appealing alternative
to no control at all. And if masking is an issue, a glitch detector will
resolve that.
Doucoursey
well then let me ask you this, if you were hovering your $3000 airplane 5
feet off the ground with the broken antena, which reciever would you want?
the PCM that worked or the PPM one that didn't??
DOUG C.
Matthew Orme
Aliens2.
What I prefer is a system with no faults, not one with faults and a receiver
that hides them.
"Doucoursey" <douco...@aol.com> wrote in message
news:20011202083906...@mb-ck.aol.com...
Mike Slaughter
differences in design along with frequency use and restrictions.
You can find demodulation chips that contain all the local oscillators,
mixers, filters and amplifiers needed for both double and single conversion
systems. They are nearly identical in size and circuit board configuration.
The double costs just a few bucks more that the single, so why manufacture a
line with the single conversion systems? Might it have to do with the UK
equivalent of the US' FCC? Do they have a gov't org that is omnipotent in
its field, makes all kinds of unneccesary and restrictive rules, and only
responds to the masses of large manufacturers and special interests who have
megabucks, or pounds, to send their way?
Mike
"Bob Burrill" <bbur...@atinucleus.com> wrote in message
news:GC8O7.12206$lu5.286...@twister1.starband.net...
Steve
>Apparently I am not making myself clear. I sorta feel like Ripley in
>Aliens2.
>
>What I prefer is a system with no faults, not one with faults and a receiver
>that hides them.
>
>
Matthew,
You're not going to get a system with no faults. There will always be
interference, transmitter problems, and so on. The closest you are going to
get with todays equipment, would be a PCM receiver, with an installed
glitch detector. You would have had full control, yet it would have alerted
you to the problem as well.
Unfortunately, the price difference from a PPM system is substantial.
Probably why you don't see them in everyday models, by are in the vast
majority of high end planes.
Mike Slaughter
from some other problem than a signal overridden by noise, as you state,
then it doesn't matter what rx you use. My point of view is that if you are
operating in a noisy rf environment I would use the extra 12 dB snr gained
from pcm to prevent any problem occuring. The pcm does not hide glitches,
it prevents them, up to 12 dB gain in snr, and that is a huge gain. If two
planes are flying, one pcm and one ppm, and the ppm starts to receive
glitches caused by noise, the pcm rx is not even affected by noise. It is
not masking glitches, it does not have glitches. If the noise continues to
increase in amplitude, the ppm plane crashes because the pilot has no
control. Later, the pcm plane starts to receive glitches from noise, and
the programming may prevent the pilot from seeing this onset clearly, but
the point is that this onset of glitches is at a much, much higher noise
level.
I think that if your area's noise is very erratic, sometimes pulsing up to
levels much greater than average, the number of planes prevented from
crashing because they had more noise resistant rx's is much greater than the
number lost because the pilot did not recognize that his plane was
responding to interference.
If you had a pcm rx that had no post detection processing, that plane would
act exactly like a ppm when noise hits it, only the level of noise it takes
to affect it is much, much higher than the ppm rx.
You know what? I think we are arguing because we are pushing our point and
not agreeing with the other's correct views. I will admit that the feature
of the pcm rx that programs the servos to maintain the last ggod nput may
'mask' a noise problem.' Do you agree that pcm has an advantage in snr? I
hope so because that is eminently provable. If so, then we are just stuck
on an opinion whether the masking problem outweighs the gain in snr. I say
'no way,' you say 'absolutely.'
Do I read this correctly?
Mike
"Matthew Orme" <Fred.Fl...@orme.org> wrote in message
news:RX7O7.322$rL4.2...@paloalto-snr1.gtei.net...
> > I think you are missing my point, Matthew.
>
> Not at all, I just disagree.
Steve
Mike, this is an excellent point that I had forgot about. The extra s/n
difference does indeed make a difference, and your example conveys this
easily. Just another step in making the whole system far more reliable, as
it takes far more interference before its even seen by the receiver.
Steve
medi...@mindspring.com
RC web Site http://webpages.charter.net/mediashop/rc-giant-scale/
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Dan Thompson
<medi...@mindspring.com> writes:
>The closest you are going to
>get with todays equipment, would be a PCM receiver, with an installed
>glitch detector.
Steve,
What glitch detector do you use with a PCM receiver and how does it work with
PCM?
Dan Thompson (AMA, EAA , WB4GUK)
Remove POST in email address
Steve
I've used the BC6 from YNT, Inc. Its advertised mostly in the helicopter
magazines. I believe they have a web site. Currently, I use a custom built
unit that a friend of mine built. Its a combination PCM glitch detector and
fiber optic kill switch in one unit. It works on both PPM and PCM systems,
and includes a visual display that signals short, medium, and long
glitches, as well as a audible beeper.
René
<MikeSl...@cinci.rr.com> wrote:
(may be partially quoted)
>I don't consider using a pcm rx as hiding from a problem. If your glitch is
>from some other problem than a signal overridden by noise, as you state,
>then it doesn't matter what rx you use. My point of view is that if you are
>operating in a noisy rf environment I would use the extra 12 dB snr gained
>from pcm to prevent any problem occuring. The pcm does not hide glitches,
>it prevents them, up to 12 dB gain in snr, and that is a huge gain.
Can somebody please explain how PCM can generate up to 12 dB coding
gain? Did somebody introduce FEC schemes nowadays?
In the situations I find on my (European) flying field, most of the
PCM gain seems to be coming from the fact that PCM is actually used in
better & more expensive (top range) quality receivers (i.e. with
better RF characteristics due to better parts / alignment)
I can relate to some advantages pertaining to pre-programmed fail save
settings - but not the coding gain.
My deja-vue is hurting again :-)
--
- René
Courseyauto
Date: 12/2/2001 10:16 AM Eastern Standard Time
Message-id: <gBrO7.1286$C14.3...@dfiatx1-snr1.gtei.net>
Apparently I am not making myself clear. I sorta feel like Ripley in
Aliens2.
What I prefer is a system with no faults, not one with faults and a receiver
that hides them.
>>.
i see, you dont want too live in the real world. lotsa luck. DOUG C.
The Natural Philosopher
Nyquist still rules: All you are doing with complex error correction is
effectively narrowing the bandwidth of the reciever. You may not think
it - because lots of clever technology and maths disguises it - but that
is what you *are* doing in effect. My point was that sure PCM may be
doing all this, but its not *inherent* to PCM. It may be just that PCM
is teh easiest way to do it. Conventional tuned circuits and PPM start
to get unwieldy beyind a certain point.
I am fully prepared to accept at face value the statements that 'in
general PCM X's work better under interference and at longer range than
PPM'. As you say, there are ways to pull signals out that are buried in
noise *if you know what they are supposed to look like and you can
average over a few frames to do it*. Aperture synthesis on large radio
telescopes is a typical application of techniques like that. BUT in the
end, there is some sort of trade off - in teh case of aperture synthesis
it takes *WEEKS* to simulate a dish the size of the earths orbit :-)
My guess is that if you threw all the sticks into a corner with a PCM
system, itr would be a bit slower to respondf - but hey, the servos
aren't instant, so its probly a good tradeoff anyway.
>
> Better yet how about pulling signal out from up to 20-40 dB BELOW the
> noise floor? Where I work we have equipment that does this. However,
> it does not use FEC, and it's not used for communications
> transmission. We use it to monitor the undersea repeaters w/out
> affecting its traffic. If we were to use this for transmission, I
> think it's effective transmission rate would be around 300 bps, and
> this is on a 2.5 Gbps line.
>
Yes, thats exactly my point. White noise averages out on a root Hz
basis. The less Hz, the less noise.
Complex encoding gets you much closer to the theoretical limits, but
can't break em!
I 0once worked on a military laser rangefinder project. We fiddled for
ages until some boffin came down from on high, measured teh laser oulse
out put, applied inverse square law to it, adjusted for target
reflectifity, and looked at the noise level of teh avalnche detector. A
small bit of pencol sucking followed and then 'theoretical on that is
about 1.2miles - what are youi getting?' ' Er about 1.3 miles actually
sometimes'. Right. What's the design call for '5 miles' Forget it'. And
we did. IF we could have modulated teh laser beam rather than whacking
out a single pulse, and had tuined circuits in teh detector....we could
have got more ...
> If you remember your physics classes, you'll remember "IN THE END, RF
> signals never truly die, they just get smaller and smaller at an
> exponential rate (when going through the air that is.)" If you know
> what you are looking for, you will find it. Today, what matters is
> energy per bit. What's the minimum energy per bit on the receiver do I
> need for a successful transmission? Receivers are getting more and
> more sensitive day by day as the tech improves. The FEC makes it so
> that the receiver doesn't have to receive all the energy for that bit
> in one shot, but rather spread out in multiple tries.
Noise exists as background anyway (from a multitide of sources like
interference to cosmic backghround and the sun setc), and thermal noise
in the front end of the RX plus background noise plus whatever inherent
shot or carrier noise exists in teh input stage transistor sets a
theoretical limit - which as I said will be in teh form of watts per
root Hz - that is teh power goes up as teh square root of the bandwidth
you are trying to recieve. So *very* slow data rates, like morse, can
get through at S/N ratios that other things cannot hope to succeed at. 8
channels of 8 bits each repeating every 50ms is 20x64 bps = 3.2kbps.
That is a fairly low bandwidth really. If the error correction can get
the effective recieved bandwidth down to approaching that, and a dual
conversion RX achives maybe 100khz effective bandwidth (what is channel
spacing annyway?) you ought to get about 5 times more senistivity with
PCM. That equates to maybe double the range and a bit. . For a given
front end RX design and transmitter power.
Just speculating really, because I don't know what the actual encoding
techniques are, or indeed what is inside a standard RX anyway these
days.
But the jump in performance in modems from 2400BPS to 56kbps happened
mostly as a result of using fancy DSP technology and fancy encodings.
But you still can't break the theoretical limit of 56 (or 64) kbps used
on standard telephone circuits. The wirse will do it, but the equipment
at the exchange wont, until you replace it with DSL technology :-)
Oh well. Roll on teh day when we can get full blown RX's with ten times
the range and interference immunity, keyed to our own transmitters, for
$19.95...
>
> Jimmy
The Natural Philosopher
Mike Slaughter wrote:
>
> I had a communication theory class from Dr. Richard Hamming, author of the
> Hamming code. He worked at Bell labs in the late 30's and 40's when
> information theory was developed. IIRC, one of the names he consistently
> dropped was Nyquist, along with Shannon. I think that the digital ec code
> was being developed simultaneously with the theories of communications.
> Hamming was one of the most intelligent and egotistical engineers I ever
> met. He earned it, though. His book is a great read.
Yep. Thems the guys. Nyquist is about sampling rates, and hamming did
the basic theoretical work on digital encoding. Its a long time since
college :-(
(1970 in my case...)
The Natural Philosopher
This is becoming a very useful and educational thread, as far as I am
concerned. Thank you.
The Natural Philosopher
Steve wrote:
>
Kind of like arguing if
> your Corvette can out run my Porsche. They are both limited by the speed
> limit, and even when exceeded, rarely if ever travel at their top
> potential.
My jaguar has a failsafe mode as well. It 'says 'performance restricted'
and won't do more than 50mph until something gets reset when you switch
the engine off...:-)
The Natural Philosopher
"René" wrote:
>
> On Sun, 02 Dec 2001 16:33:05 GMT, "Mike Slaughter"
> <MikeSl...@cinci.rr.com> wrote:
> (may be partially quoted)
> >I don't consider using a pcm rx as hiding from a problem. If your glitch is
> >from some other problem than a signal overridden by noise, as you state,
> >then it doesn't matter what rx you use. My point of view is that if you are
> >operating in a noisy rf environment I would use the extra 12 dB snr gained
> >from pcm to prevent any problem occuring. The pcm does not hide glitches,
> >it prevents them, up to 12 dB gain in snr, and that is a huge gain.
>
> Can somebody please explain how PCM can generate up to 12 dB coding
> gain? Did somebody introduce FEC schemes nowadays?
>
> In the situations I find on my (European) flying field, most of the
> PCM gain seems to be coming from the fact that PCM is actually used in
> better & more expensive (top range) quality receivers (i.e. with
> better RF characteristics due to better parts / alignment)
>
Effctivly it must, if what these guys are saying, be reducing the
effective bandwidth of the reciever. This *does* make sense, in that an
analogue system is pretty crude, and will be using IF coils or quartz
resonators alone in the IF strip to shape the response. Using PCM would
be akin to using tones - do you remeber the old 4 tone proportionals? -
which enables you to build in selectivty after the IF sage, in the
decoder. PPM systems are really crude after all.
If you like, a PCM system is sort of like a triple conversion system :-)
There is another piece of selectivity going on...
This probably allows the whole thing to be tuned for more gain as well,
without the system going into oscillations. We used to have a rule of
thumb on radios at any given frequency of '20db per inch' being about
all you could get before parasitic coupling turned an amplifier into an
oscillator.
There was no theoretcical justifictaion for that figure, but as a rule
of thumb it worked extremely well. Or was it 40db per inch? I forget :-)
The Natural Philosopher
Matthew Orme wrote:
>
> > PCM doesn't mask anything,
>
> Really? Then please explain why my Vision radio, with a broken antenna
> connection (making an intermittent contact, and therefore an intermittent
> signal from the transmitter) behaved like nothing was wrong with a PCM
> receiver, and was unflyable with a PPM receiver. I would not consider an
> intermittent signal from the transmitter as noise.
Its precisely analogous to a strong intermittent signal shutting down
the AGC on your RX actually.
Noise sort of implies random, but not always.
The Natural Philosopher
Mike Slaughter wrote:
>
> You hit the nail on the head, I didn't think there would be international
> differences in design along with frequency use and restrictions.
> You can find demodulation chips that contain all the local oscillators,
> mixers, filters and amplifiers needed for both double and single conversion
> systems. They are nearly identical in size and circuit board configuration.
> The double costs just a few bucks more that the single, so why manufacture a
> line with the single conversion systems? Might it have to do with the UK
> equivalent of the US' FCC? Do they have a gov't org that is omnipotent in
> its field, makes all kinds of unneccesary and restrictive rules, and only
> responds to the masses of large manufacturers and special interests who have
> megabucks, or pounds, to send their way?
I don't think its a government thing here in the UK. We run on 35Mhz for
air exclusively as far as I know.
I don't know the channel spacings, but its ceretainly not uncommon to
use 'only odds' or 'only evens' at a given site so that people flying
the single conversions stuff don't get shoot-downs from adjacent
channels.
However single convesrion stuff is much cheaper than D/C. and PCM is
even more xepensive - no idea why really. I am surprised that the whole
thing including filters comes on a chip tho. The techniques for SAW
filters were not, when last I looked, amenable to slicon wafer
integration.
Again, I am well out of touch, but would love to hear what goes on
inside a futaba.
Steve
>
>
>Steve wrote:
>>
> Kind of like arguing if
>> your Corvette can out run my Porsche. They are both limited by the speed
>> limit, and even when exceeded, rarely if ever travel at their top
>> potential.
>
>My jaguar has a failsafe mode as well. It 'says 'performance restricted'
>and won't do more than 50mph until something gets reset when you switch
>the engine off...:-)
>
Well, of course. Its only a Jaguar :-)
Matthew Orme
millions of users of PPM receivers. From the talk here, one would think that
if you don't use a PCM receiver, you will loose your plane. It apparently
does not apply that way in the real world.
Unless you are a sailplane pilot, I guarantee that I fly much farther from
my transmitter on most occasions than you. Probably 5 times as far. "range"
is not an issue no matter what receiver. When I am in a group of other like
minded pilots, you will see 10-15 in the air at a time (on a few ocassions
nearly every frequency is in use), and the pilots all standing around in
little groups having conversations as they fly. The 'only' restrictions at
the clubs I like are that your frequency must be clear.
Not a "clean" environment by any means. The most common receivers are
aftermarket Hitec, which by definition must be PPM for all the Airtronics,
Futaba, and JR folks. I have yet to come across any receiver related
failure.
Then again, we go to great lengths to reduce any noise problems, as I
related earlier. The only reason some use PCM (usually they have 1-2 PCM
receivers and 20-30 PPM ones) is because the receiver came with the radio.
"Courseyauto" <cours...@aol.com> wrote in message
news:20011202174942...@mb-mm.aol.com...
The Natural Philosopher
Steve wrote:
>
> The Natural Philosopher <Now...@there.org> wrote:
>
> >
> >
> >Steve wrote:
> >>
> > Kind of like arguing if
> >> your Corvette can out run my Porsche. They are both limited by the speed
> >> limit, and even when exceeded, rarely if ever travel at their top
> >> potential.
> >
> >My jaguar has a failsafe mode as well. It 'says 'performance restricted'
> >and won't do more than 50mph until something gets reset when you switch
> >the engine off...:-)
> >
>
> Well, of course. Its only a Jaguar :-)
>
My days of treating teh roads like a race track are over.
This is luxury to get you long distances.
Actually, they are enormously improved these days :-)
Jimmy Huang
Hey Fred Flintstone,
I dunno if you realize this or not, but your arguments work just as
good, in an AM vs. FM comparison. But based on your arguments, I
think you should go back to the good ole' AM Tx/Rx sets, where any
small glitch will show up in an instant, then you can go and land your
glider. .
Me? I'd rather be up in the air, even with a broken antennae. Isn't
anyone amazed that a PCM receiver will work even with a broken
antenna?
That's like having 8 tires on your car. You can have up to 4 tires pop
while driving and never know the difference until you stop and take a
look! But then, there's those guys who keep driving the car and never
notice until the 5th tire pops, and then blame the equipment. Hmm.
In all truthfulness though, I use FM PPM, because using a PCM setup on
a $50 dollar plane is like having 8 tires on your car! It's overkill
for me. But for some people it's worth it. For those who use it cuz
it's worth it, don't blame the equipment for your own idiocy. Learn to
take advantage of the hi-techiness of your equipment.
There's no argument here. PCM is better than plain PPM. However, you
have to use it to your advantage the right way. And of course, there's
always room for improvement. In future, company's like Futaba will be
improving how their PCM equipment works. I can't say the same thing
for PPM. Guys like you need to give them the proper feedback so we can
all benefit.
>These guys no longer use PCM, because a half second lockout will put the
>plane into the ground. There is always noise at these events, and I have
>seen my own planes take an elevator hit, bounce down then back up (looks
>really scary), and then continue on it's way. It is just their experience
>that PCM costs them planes, and you can all tell them that they are wrong if
>you like, but the planes are $2500 each, and they expect to loose one during
>the event, and they are built so light that they are not intended to last
>longer than a season.
Wow. Okay, now *that* was very compelling. It explains why at least,
Hitec hasn't jumped into the PCM bandwagon and has a PCM solution of
their own...
So the kind of interference you guys are getting at these events are
STRONG enough to knock out both PCM and PPM receivers at the same
time, and lasts for very short bursts. Result, PPM glitches due to
this interference and you see it, but disaster doesn't strike.
However, PCM locks out and re-acquires and takes longer to re-acquire
the signal than PPM, and the glider is lost (met mother earth), due to
that 1/2 second delay compared to PPM...
Wow, if that's on every channel and that F5B event is always in the
same place, and it happens often enough. I'd say, stick with PPM!
Sheesh! Looks like PCM needs some improving... Hopefully someone from
Futaba or JR is reading this thread. This should be fixable, actually,
that is a design flaw.
FYI, we actually have the same problem with some digital comm
equipment here. Stuff that uses Reed Solomon encoding scheme works
great in noisy environs, everything gets cleaned up! However, when
running 2 modems in an clean environ one with Reed and one without
Reed, and you get a quick hit, you can tell with a test set that the
hit was 10 ms. The regular modem will take 50 ms to re-sync, but the
one with Reed takes sometimes up to 200 ms to re-sync. There is a fix
to this, it depends how they want to handle the overflows.
Jimmy
Steve
>Thanks. It's the world I am living in now, and apparently so are the other
>millions of users of PPM receivers. From the talk here, one would think that
>if you don't use a PCM receiver, you will loose your plane. It apparently
>does not apply that way in the real world.
>
>Unless you are a sailplane pilot, I guarantee that I fly much farther from
>my transmitter on most occasions than you. Probably 5 times as far. "range"
>is not an issue no matter what receiver. When I am in a group of other like
>minded pilots, you will see 10-15 in the air at a time (on a few ocassions
>nearly every frequency is in use), and the pilots all standing around in
>little groups having conversations as they fly. The 'only' restrictions at
>the clubs I like are that your frequency must be clear.
>
I think most responses here acknowledge that either receiver type is quite
acceptable for everyday RC use, I know mine do. I think the argument went
technical, in which case PCM is noted to be superior. But, that doesn;t
mean that PPM isn't acceptable for RC use, as we all know by the evidence
of vast number of PPM receivers operating along happily every day.
As mentioned before, both PPM and PCM exceed the requirements of our normal
operation, for all modes of typical RC, from large gas, small glow,
sailplanes, helicopters, and so on. The normal range PPM and PCM receivers
can operate far exceed the eyesight of the majority of people. Sailplane
operation is no different than other types of RC. You are still within the
normal operating range of PPM or PCM receivers, even if you are 5 times
further out.
If you argue which is better at noise reduction, has better range, is
better at rejecting and not responding to invalid control inputs, and so
on, PCM has been proven by theory, electrical engineering, and practical
tests to be superior to PPM.
If you argue whether PPM or PCM is reliable for our applications, you
would hear most people say either is fine, and one shouldn't feel unsafe
flying either method. That's always been my position, and proven as I fly
both in my high dollar planes without reservation.
If you argue which is better, based on the users application and needs
regarding error correction, failsafes, and comfort levels, you will get
widely differing opinions, mostly right for their application, but not
others. Some people tend to argue emotions, and assume that what's right
for them, is right for everyone else.
Fact remains, PCM is superior to PPM in most ways. Yet, like cars, an
escort is a very reliable method of transportation, and no better or worse
for its intended task as a BMW would be. One may offer some advantages in
speed and handling over the other, but if you are driving down the hi way
at 55mph, either is more than adequate at doing just that.
If it was me, I wouldn't feel uneasy hovering a $6,000 airplane just over
the runway with a PPM receiver. If I could hover low over the runway, I
could prove it :-) Given a choice though, I would prefer PCM, but mainly
for its failsafe features as they relate to safety.
gary gaugler
receivers. The issue is the update or frame rate at which the PPM
pulse is sent to the servo. PCM is going to update much faster than a
PPM RX. With CRC-32 error correction and reasonable bit dead band,
there are many bits available to control a servo.
JR makes only single conversion RXs except for one model. The dual
conversion RX is PCM-S. Their single conversion RXs can be had as
PCM-S, PCM-Z or PPM.
Gary Gaugler, Ph.D.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Modern surfers use PC boards....you can too at
http://photoweb.net
E-mail: gary@gaugler dot com
Steve
>
>
>Steve wrote:
>>
>> The Natural Philosopher <Now...@there.org> wrote:
>>
>> >
>> >
>> >Steve wrote:
>> >>
>> > Kind of like arguing if
>> >> your Corvette can out run my Porsche. They are both limited by the speed
>> >> limit, and even when exceeded, rarely if ever travel at their top
>> >> potential.
>> >
>> >My jaguar has a failsafe mode as well. It 'says 'performance restricted'
>> >and won't do more than 50mph until something gets reset when you switch
>> >the engine off...:-)
>> >
>>
>> Well, of course. Its only a Jaguar :-)
>>
>
>My days of treating teh roads like a race track are over.
>
>This is luxury to get you long distances.
>
>Actually, they are enormously improved these days :-)
>
I know they are. Very nice cars in fact. I was just teasing....
I sold the Porsche for the same reason. I was driving very risky, and it
would only be a matter of time before it caught up with me. In the years
prior to me buying one, I rarely traveled over 75 or 80 tops. After I
purchased the Porsche three years ago, I had seen 130+mph on the speedo far
too many times to count. The adreline rush for the acceleration and
handling was all to appealing.
So, sanity advised me to sell it, and I did. I bought a motorhome for
traveling around to RC events, and a Seville for my wife. We also looked at
Jag's, but she liked the Seville. Either way, I'm sure it increased my
lifespan considerably, since the speedo again rarely see's anything over 80
these days.
The Natural Philosopher
Yeah. I do better on tyres on my XKR than my old XK8. Somehow, sitting
in all that luxury, with climate control, top quality CD, smelling the
leather, who's in a rush anyway? :-)
Can't understand why the seville tho. Go for a nice XJR. The nicest
saloon I have *ever* driven - only one problem - like all trhe current
big cats, it cocks a rear leg on tight corners and loses traction :-(
The Natural Philosopher
Steve wrote:
Yet, like cars, an
> escort is a very reliable method of transportation,
Oh yeah? Apart from this, I agree with everything you said :-)
and no better or worse
> for its intended task as a BMW would be. One may offer some advantages in
> speed and handling over the other, but if you are driving down the hi way
> at 55mph, either is more than adequate at doing just that.
Dunno about that, if you had said Nissan, Toyota, Fiat, well Ok. but
FORD?
:-)
>
> If it was me, I wouldn't feel uneasy hovering a $6,000 airplane just over
> the runway with a PPM receiver. If I could hover low over the runway, I
> could prove it :-) Given a choice though, I would prefer PCM, but mainly
> for its failsafe features as they relate to safety.
And apparently, as pointed out in my BMFA journal that arrived today, a
nasty habit of resetting the failsafe everytime the RX battery is
disconnected....or something.
Interestingly, they are talking about TX's that cannot be powered up
until their channles are clear as 'default kit' they would like to see
in teh UK shops...interesting.
I want to say another thing - and that is that having just come into
this group, and neing naturally chatty, it seems to me that there are
some very basic differences in teh hobby, between 'sport'modellers, who
often fly alone, don't spend a fortune, and 'social' modellers, who club
toghether, and who are to an extent competitive - wheher this be in
flying standards or the size of their propellors...:-)
I have to declare myself firmly in the former group. I find committed
modellers often narrow and sad - at least some of the ones I have met at
our club are. The electric and glider lot seem much more my style - in
it for the pure pleasure of doing something highly unnatural with a few
sticks of balsa- i.e. making it fly, and delighting in it.
Ther is an element of 'seriousness' that seems to take out some of the
fun sometimes. I guess PCM is a bit 'serious' for me. It belongs with
$1000 dollar engines and 97% nitro fuel in the ranks of people who seem
to want to derive pleasure from spending enormous sums of money, rather
than doing what I originally wnated from R/C - to stop the bloody thing
flying away and getting lost :-)
The Natural Philosopher
Jimmy Huang wrote:
>
> Hey Fred Flintstone,
>
> I dunno if you realize this or not, but your arguments work just as
> good, in an AM vs. FM comparison. But based on your arguments, I
> think you should go back to the good ole' AM Tx/Rx sets, where any
> small glitch will show up in an instant, then you can go and land your
> glider. .
>
> Me? I'd rather be up in the air, even with a broken antennae. Isn't
> anyone amazed that a PCM receiver will work even with a broken
> antenna?
No, because any set will, with reduced range.
>
> That's like having 8 tires on your car. You can have up to 4 tires pop
> while driving and never know the difference until you stop and take a
> look! But then, there's those guys who keep driving the car and never
> notice until the 5th tire pops, and then blame the equipment. Hmm.
>
So having the 8 tires is the PCM? Or the PPM?
> In all truthfulness though, I use FM PPM, because using a PCM setup on
> a $50 dollar plane is like having 8 tires on your car! It's overkill
> for me. But for some people it's worth it. For those who use it cuz
> it's worth it, don't blame the equipment for your own idiocy. Learn to
> take advantage of the hi-techiness of your equipment.
>
> There's no argument here. PCM is better than plain PPM.
Technically. Not in overall cost benefit analysis. Depending on what
benefit YOU derive from your flying. I just want the model to come back
to me. Preferably in one piece, without spending a fortune. If I wreck a
PPM RX it is only the cost of a tank of gas. A PCM RX is at least two
tanks of gas...
> However, you
> have to use it to your advantage the right way. And of course, there's
> always room for improvement. In future, company's like Futaba will be
> improving how their PCM equipment works. I can't say the same thing
> for PPM. Guys like you need to give them the proper feedback so we can
> all benefit.
I don't actually give a monkeys how it does it, except out of
ex-professional curiousity. It would be as possible to build an AM style
set as good as any PCM, but given that no-one does things that way, the
chips would be horrendously expensive. PPM was introduced to get good
results on the very cheap, taking advantages of the digital stuff that
was then coming into use. This was the late 60's. PCM presumably takes
advantage of cheapo chips that are available in the 80's and 90's.
All engineering is about achieving a desired result at a sensible cost.
PCM achieves a higher spec result at currently a higher spec cost. Its
up to you if that spec is worth paying the extra for. For me, right now,
it isn't.
If you narrow the bandwidth, to eliminate interference, the penalty is
slower responses...
Delta PCM is an interesting concept. Only transmit *movement* of the
sticks. Very low data rate :-)
>
> Jimmy
Steve
I can. She liked it, and she drives it. I wouldn't have chosen it for me,
since my 42% Giles won't fit in it :-)
I got a new van instead. But, does it count that its got heated leather
seats, a great sound system, even though it cocks all legs in corners :-)
Steve
>
>
>Steve wrote:
> Yet, like cars, an
>> escort is a very reliable method of transportation,
>
>Oh yeah? Apart from this, I agree with everything you said :-)
>
>
> and no better or worse
>> for its intended task as a BMW would be. One may offer some advantages in
>> speed and handling over the other, but if you are driving down the hi way
>> at 55mph, either is more than adequate at doing just that.
>
>Dunno about that, if you had said Nissan, Toyota, Fiat, well Ok. but
>FORD?
>:-)
>
Yea, bad analogy. Ok, Nissan and BMW. Better?
Both entirely capable of driving 0-65 reliability day after day.
>>
>> If it was me, I wouldn't feel uneasy hovering a $6,000 airplane just over
>> the runway with a PPM receiver. If I could hover low over the runway, I
>> could prove it :-) Given a choice though, I would prefer PCM, but mainly
>> for its failsafe features as they relate to safety.
>
>And apparently, as pointed out in my BMFA journal that arrived today, a
>nasty habit of resetting the failsafe everytime the RX battery is
>disconnected....or something.
I don't get what you are saying here? PCM resetting the failsafe? Whose
receiver, and what actually happens? Inquiring minds need to know. Sounds
innocuous though, unless one turns off then on during flight. And if a bad
battery connection does this, which it might, what would the failsafe be
set to? A default, or the last set failsafe settings?
>
>
>Interestingly, they are talking about TX's that cannot be powered up
>until their channles are clear as 'default kit' they would like to see
>in teh UK shops...interesting.
>
Yes, I like this.
>
>I want to say another thing - and that is that having just come into
>this group, and neing naturally chatty, it seems to me that there are
>some very basic differences in teh hobby, between 'sport'modellers, who
>often fly alone, don't spend a fortune, and 'social' modellers, who club
>toghether, and who are to an extent competitive - wheher this be in
>flying standards or the size of their propellors...:-)
>
>I have to declare myself firmly in the former group. I find committed
>modellers often narrow and sad - at least some of the ones I have met at
>our club are. The electric and glider lot seem much more my style - in
>it for the pure pleasure of doing something highly unnatural with a few
>sticks of balsa- i.e. making it fly, and delighting in it.
>
>Ther is an element of 'seriousness' that seems to take out some of the
>fun sometimes. I guess PCM is a bit 'serious' for me. It belongs with
>$1000 dollar engines and 97% nitro fuel in the ranks of people who seem
>to want to derive pleasure from spending enormous sums of money, rather
>than doing what I originally wnated from R/C - to stop the bloody thing
>flying away and getting lost :-)
>
I tend to agree with you on this as well. One of the reason that I don't
compete. Some take it very serious, and the focus and competition turns
some people into machines, hell bent on winning at all costs. Friends,
comrades, or spectators be damned.
Yet, in fairness, I often see the exact opposite as well. Take the TOC.
Prestige, a winners purse in the tens of thousands of dollars, and fierce
competition at a world class level. Yet, when a competitor looses his
plane, he has offers up and down the flight line from his direct
competitors offering to let him fly their planes. He could then continue to
compete against them, with their own planes.
I believe I fit in the middle somewhere. I like the social aspects of the
hobby. I like camping at two and three day events, since there is a lot of
social activities going on in the evening. Some days, I only fly once or
twice. Others, I'm in the air all day long.
But, the flying aspect is what captures me the most. I love watching one of
the big planes in high alpha flight, or with smoke on in a elevator or
harrier, or a nice low high alpha knife edge pass. Then coming in for a
nice slow gentle landing. Its memorizing, and i get lost it in. The
addiction is the way these larger planes can handle the high alpha and slow
flight with grace. Something the smaller ones just can't do because of
physics.
I know you know the feeling. The smell of leather. Pleasing sounds through
your ears. The feel of a nice S curve through the countryside. The soft
feel of the interior....
Steve
>Technically. Not in overall cost benefit analysis. Depending on what
>benefit YOU derive from your flying. I just want the model to come back
>to me. Preferably in one piece, without spending a fortune. If I wreck a
>PPM RX it is only the cost of a tank of gas. A PCM RX is at least two
>tanks of gas...
>
If it took me 100 hours to build a $50 plane, it might be worth the $150 to
protect my investment. But, if I bought the plane ready to fly for $50,
then its not.
It takes me around 250 to 300 hours to build a giant scaler, and at a cost
of $6,000 or so. The $150, plus another $60 for a glitch detector is
peanuts in the scheme of things.
300 hours is 300 hours that I didn't share with my family, flying, or being
paid for work. You can't recoup that time. Once spent it gone. If you need
to put a dollar value to it, even at minimum wage its almost $2,000
>
>> However, you
>> have to use it to your advantage the right way. And of course, there's
>> always room for improvement. In future, company's like Futaba will be
>> improving how their PCM equipment works. I can't say the same thing
>> for PPM. Guys like you need to give them the proper feedback so we can
>> all benefit.
>
<snipped>
>
>>
>> >These guys no longer use PCM, because a half second lockout will put the
>> >plane into the ground. There is always noise at these events, and I have
>> >seen my own planes take an elevator hit, bounce down then back up (looks
>> >really scary), and then continue on it's way. It is just their experience
>> >that PCM costs them planes, and you can all tell them that they are wrong if
>> >you like, but the planes are $2500 each, and they expect to loose one during
>> >the event, and they are built so light that they are not intended to last
>> >longer than a season.
If they got a 1/2 second delay, it was because they received a 1/2 second
of interference. There is no inherent delay built into the PCM encoding.
The hold, or lockout, only lasts as long as the interference. A 1/10th of a
second interference hit will result in a hold or lockout of 1/10th of a
second. (Read further on in this reply)
Heading straight down, and the difference between making it and loosing it
at only 1/2 second, and I don't care what scheme you are running, an
interference event of 1/2 second will cause the demise of either aircraft.
Say One thousand one. Now say it twice as fast. That's all you got to
figure out what happened, what the attitude of the aircraft is, what you
need to do to recover, then to implement it. You're out of altitude, out of
ideas, and out of time.
Quick now, you have a half second. Recover.
Rolling left, pushing to the wheels, throttle decreased, yawing right.
Times up...
>>
>> Wow. Okay, now *that* was very compelling. It explains why at least,
>> Hitec hasn't jumped into the PCM bandwagon and has a PCM solution of
>> their own...
>>
>> So the kind of interference you guys are getting at these events are
>> STRONG enough to knock out both PCM and PPM receivers at the same
>> time, and lasts for very short bursts. Result, PPM glitches due to
>> this interference and you see it, but disaster doesn't strike.
>> However, PCM locks out and re-acquires and takes longer to re-acquire
>> the signal than PPM, and the glider is lost (met mother earth), due to
>> that 1/2 second delay compared to PPM...
But, this isn't true in my understanding of how PCM works. This is what I
understand, in a simplified form;
Example 1 - interference lasting 1/10 of a second.
The PCM receiver will detect the first invalid frame. It will then enter
hold, and leave all control surfaces as they are while ignoring the invalid
frame. It will keep all the control surfaces in this position, while
watching for the first valid frame to appear. 1/10 of a second later, the
interference stops, and valid frames are restored. Once the first valid
frame is decoded, the receiver will then act upon that frame, or the next
frame. Thus, full control is returned approximately 1/10th of a second
later. Total time the pilot didn't have control, as 1/10th of a second,
give or take a couple of frames.
Example 2 - interference lasting 1 second.
The PCM receiver will detect the first invalid frame. It will then enter
hold, and leave all control surfaces as they are while ignoring the invalid
frame. It will keep the controls as is, while still looking for the first
valid frame. If no valid frames are received by the 1/2 second point from
the onset of interference, it will then go into failsafe mode, and position
the controls appropriately. While in failsafe, it will continue to look for
valid frames. Once it sees the first valid frame, it will act upon it, or
the next one. Thus, full control is returned approximately 1 second after
the initial onset of the interference. Total time the pilot didn't have
control, would be 1 second, give or take a couple of frames.
This is true of Futaba, as explained a while back on one of the giant scale
or Futaba 9Z list. It came from an engineer within Futaba. I believe it
works similar JR's receivers/transmitters, but I am unclear as to how
Airtronics works.
The bottom line, if a pilot flying a PCM receiver gets a hold, or also
referred to as a lockout, for any length of time, its because the
interference lasted the same amount of time. There maybe a one or two frame
delay, but the length of time could be measured in milliseconds, not 1/2
second increments. This is part of the old wives tale regarding lockouts,
hold mode, and PCM. It just isn't true, as least with Futaba and JR.
>>
>> Wow, if that's on every channel and that F5B event is always in the
>> same place, and it happens often enough. I'd say, stick with PPM!
>> Sheesh! Looks like PCM needs some improving... Hopefully someone from
>> Futaba or JR is reading this thread. This should be fixable, actually,
>> that is a design flaw.
Its a misunderstanding on how the PCM system works. Futaba can, and has,
confirmed this. I have heard the same about JR, but I have no independent
confirmation of it, but I do believe its also true.
Dersu u
>>plane into the ground.
I have never seen a 1/2 second lockout with any of the Futaba 1024 stuff. I
have checked any number of times by turning on two transmitters on the same
channel to lockup the receiver. The receivers restore the instant that one of
the transmitters is turned off, not a 1/2 second later. Some people confuse the
re-boot time of the transmitter as receiver lockout time. But, people do not
ordinarily switch their transmitter on and off in flight. I have 15 Futaba
R-129DP receivers flying in all types of models. The last 11 yrs with them has
yet to see any sort of problem. Can't say the same for some of the PPM stuff
that I have.
As for the transmitters, I find that the Futaba 5,7,8 and 9 series have all
done well for me on PCM. Currently my choice is the WCII 9 chan. with the
synthesized module and a 8 UAPS with a handfull of modules as a backup.
The Natural Philosopher
Dunno. I'll see if I can get the mag and quote verbatim...
This is their wish list
-(i) Warn users to program failsafe for zero throttle
(ii) Warn users that switching to PPM and back to PCM may zero teh
failsafe.
(iii) warn users that switching of RX *may* remove failsafe settings
stored in Rx.
(iv) Upgarde all software for low throttle in teh event of low RX
battery
(v) Put failsafe (zero) throttle on all new PPM Rx's
(vi) Better manuals :-)
(vii) put reciever on TX so that same cannot be switched on if channel
in use.
How do you program the failsafe anyway? I assume that the TX sends it to
the RX at some point and teh RX stores it. What then happens if the RX
is switched off?
Yerrs. I remember years ago I went to the Nats and a rather nice
wakefield rubber model wanderd into the C/L combat circle, and emerged
as shrredded wheat - the competitors simply went for it as it was 'in
their way'.
> Yet, in fairness, I often see the exact opposite as well. Take the TOC.
> Prestige, a winners purse in the tens of thousands of dollars, and fierce
> competition at a world class level. Yet, when a competitor looses his
> plane, he has offers up and down the flight line from his direct
> competitors offering to let him fly their planes. He could then continue to
> compete against them, with their own planes.
Yeah, his competitors. Bet he doesn't even lend a dead battery to the
poor teenager trying to get his wot4 up in the air....
>
> I believe I fit in the middle somewhere. I like the social aspects of the
> hobby. I like camping at two and three day events, since there is a lot of
> social activities going on in the evening. Some days, I only fly once or
> twice. Others, I'm in the air all day long.
>
> But, the flying aspect is what captures me the most. I love watching one of
> the big planes in high alpha flight, or with smoke on in a elevator or
> harrier, or a nice low high alpha knife edge pass. Then coming in for a
> nice slow gentle landing. Its memorizing, and i get lost it in.
Mesmerizing, and lost in it ?
> The
> addiction is the way these larger planes can handle the high alpha and slow
> flight with grace. Something the smaller ones just can't do because of
> physics.
>
> I know you know the feeling. The smell of leather. Pleasing sounds through
> your ears. The feel of a nice S curve through the countryside. The soft
> feel of the interior....
>
Sure.
:-)
I just like the buzz of whacking the sticks into a corner and going for
the pigeons meself.
I *May* have a plane fast enough to outrin a pigeon now :-)
The Natural Philosopher
Steve wrote:
>
> The Natural Philosopher <Now...@there.org> wrote:
>
> >Technically. Not in overall cost benefit analysis. Depending on what
> >benefit YOU derive from your flying. I just want the model to come back
> >to me. Preferably in one piece, without spending a fortune. If I wreck a
> >PPM RX it is only the cost of a tank of gas. A PCM RX is at least two
> >tanks of gas...
> >
>
> If it took me 100 hours to build a $50 plane, it might be worth the $150 to
> protect my investment. But, if I bought the plane ready to fly for $50,
> then its not.
>
> It takes me around 250 to 300 hours to build a giant scaler, and at a cost
> of $6,000 or so. The $150, plus another $60 for a glitch detector is
> peanuts in the scheme of things.
>
> 300 hours is 300 hours that I didn't share with my family, flying, or being
> paid for work. You can't recoup that time. Once spent it gone. If you need
> to put a dollar value to it, even at minimum wage its almost $2,000
Well there ya go. I hate spending more than a coupla days on a plane. I
want to *FLY*. A big scaler would be matchwood before I got it over the
trees, given my current skills :-)
Not in a glider. Not up at 200ft. I often get dead zones. As long as
they don't throw the model into a flat spin, if theres altitude, there's
time. Even a 60mph dive is only 88ft/s. That gives a good three seconds
to pull out of a 60mph vertical dive from 300ft. Even if the plane
doesn't pull itself out. Program a little 'up' in your failsafe :-).
> Quick now, you have a half second. Recover.
> Rolling left, pushing to the wheels, throttle decreased, yawing right.
> Times up...
>
Thats why I fly cheapo planes. Its gotta be instinctive, like turning
into the skid in a car. That moment when the wheels lock, and theres a
corner coming....and somehow you can see the sunny side of the road
isn't iced, and its head for it BRAKE and EASE the car round...and then
stop and wait till the knees stop knocking. Yep. 1/2 second is enough,
if the reflexes and teh brain are there. My last crash some little kid
said 'make it come back' just as I was doing a low level turn downwind.
Grr.. PCM won't stop THAT brat from squeaking just when I don't need to
hear it....
However, I have no religion on this. I will fly cheapo PPM until I have
something whose structure costs more then the RX, then I will think
again...
Yes, but the point the other guy was making, is that maybe its better to
know that you lost it for that one second. If you were gliding, hands
off, you wouldn't know....with a PPM you get a 'twitch' and its maybe
time to bring her in fast. Something is wrong...With PPM on electric,
the motot falters under signal loss. And thats mainly what I get - not
wild servos fluctuationg - just signal loss.
The Natural Philosopher
Dersu u wrote:
>
> >>These guys no longer use PCM, because a half second lockout will put the
> >>plane into the ground.
>
> I have never seen a 1/2 second lockout with any of the Futaba 1024 stuff. I
> have checked any number of times by turning on two transmitters on the same
> channel to lockup the receiver. The receivers restore the instant that one of
> the transmitters is turned off, not a 1/2 second later. Some people confuse the
> re-boot time of the transmitter as receiver lockout time. But, people do not
> ordinarily switch their transmitter on and off in flight.
No, but a strong masking signal, or a loose aerail or a faulty Xtal, or
a burst of static from a wire-to-wire type joint have exactly that
effect - momentary loss of *all* information.
Or are yiu saying that the TX doesn't come on from off imediately?
Steve
>
>Dunno. I'll see if I can get the mag and quote verbatim...
>
>This is their wish list
>
>-(i) Warn users to program failsafe for zero throttle
>(ii) Warn users that switching to PPM and back to PCM may zero teh
>failsafe.
>(iii) warn users that switching of RX *may* remove failsafe settings
>stored in Rx.
>(iv) Upgarde all software for low throttle in teh event of low RX
>battery
>(v) Put failsafe (zero) throttle on all new PPM Rx's
>(vi) Better manuals :-)
>(vii) put reciever on TX so that same cannot be switched on if channel
>in use.
>
Nice wish. I agree with it, especially warning users of throttle failsafe,
and making it the default failsafe setup.
>How do you program the failsafe anyway? I assume that the TX sends it to
>the RX at some point and teh RX stores it. What then happens if the RX
>is switched off?
>
From what I understand, the failsafe information is downloaded into the RX
when the link is established with the Tx/Rx. I don't know if it stays until
its reset by the TX, or if powering off will clear it. Something I'll ask
Futaba about if I get the chance.
Steve
>
>> Yet, in fairness, I often see the exact opposite as well. Take the TOC.
>> Prestige, a winners purse in the tens of thousands of dollars, and fierce
>> competition at a world class level. Yet, when a competitor looses his
>> plane, he has offers up and down the flight line from his direct
>> competitors offering to let him fly their planes. He could then continue to
>> compete against them, with their own planes.
>
>Yeah, his competitors. Bet he doesn't even lend a dead battery to the
>poor teenager trying to get his wot4 up in the air....
>
That would be atypical of the people I am referring to. In fact, they would
be the first to help out, and have been. I can name many people that I have
flown with of TOC, Top Gun, or IMAC caliber that have given up practice
time over and over, to help trim out a newbies plane, teach someone a new
maneuver, and or just help someone with a problem.
Granted, there will be some that won't give you the time of day, but thats
just people being people, and different than asking the time of day to
someone on the street. The majority that I have met, and flown with, are
very helpful, and generous with their time, regardless of their level.
Steve
>Well there ya go. I hate spending more than a coupla days on a plane. I
>want to *FLY*. A big scaler would be matchwood before I got it over the
>trees, given my current skills :-)
>
Don't be so sure. Its a well known secret that bigger flys better and
easier. However, don't tell anyone....
Seriously, if you can fly a high wing .40 or .60 without making matchwood
out of it, you can fly a high wing 40% size with the same, if not better,
results.
If you can fly the Great Planes Cap 231/ex, or the Yellow or Hangar 9 Cap,
and bring it home in one piece, you will be amazed that the 40% Cap feels
more like a big pussy cat in comparison.
But again, don't tell anyone....
>>
>> Heading straight down, and the difference between making it and loosing it
>> at only 1/2 second, and I don't care what scheme you are running, an
>> interference event of 1/2 second will cause the demise of either aircraft.
>>
>> Say One thousand one. Now say it twice as fast. That's all you got to
>> figure out what happened, what the attitude of the aircraft is, what you
>> need to do to recover, then to implement it. You're out of altitude, out of
>> ideas, and out of time.
>>
>
>Not in a glider. Not up at 200ft. I often get dead zones. As long as
>they don't throw the model into a flat spin, if theres altitude, there's
>time. Even a 60mph dive is only 88ft/s. That gives a good three seconds
>to pull out of a 60mph vertical dive from 300ft. Even if the plane
>doesn't pull itself out. Program a little 'up' in your failsafe :-).
But here is the example that Matthew gave, and the one that I was referring
to. 600 to 700 feet, 200mph, wait for 50 meters, then pull up. 1/2 second
is nothing at those speeds and heights. Its a terminator in 3D aerobatics,
and a sheer play on fear. 1/2 second too soon, you miss your altitude, 1/2
second too late, and its dust.
Can you say One One Thousand fast!
-->In F5B, the pilots used to climb to about 600-700 feet, power down the
-->motor, and dive straight down to gain speed to about 200 mph. (this task
-->was eliminated by the FAI this year) At about 50 meters (the minimum
-->distance allowed) from a sighting device, they level off at about 2M off
-->the ground, and fly at very high speed (power off in a glide at 180-200
-->mph) to pass under the sighting device. (the device is at 3M). There is
-->enough retained energy after this "limbo" task to climb to about 600
-->feet without power, and do a 5 minute thermal duration glide, with out
-->starting the motor (which subtracts points if run)
>
>Yes, but the point the other guy was making, is that maybe its better to
>know that you lost it for that one second. If you were gliding, hands
>off, you wouldn't know....with a PPM you get a 'twitch' and its maybe
>time to bring her in fast. Something is wrong...With PPM on electric,
>the motot falters under signal loss. And thats mainly what I get - not
>wild servos fluctuationg - just signal loss.
>
I understand his point, and to an extent, I agree with it.
But for me, I hardly ever fly hands off. While flying aerobatics, you are
generally on the sticks the entire time making corrections. A 1 second loss
would be felt most of the time. If its persistent, then it will definitely,
and you can take actions. Granted, if you fly lots of hands off, you might
not notice it like you might on a PPM system.
Kind of all goes back to my saying that PPM and PCM are both very good
receivers, and its just a choice of matching the receiver type with the
users preference and application. In his application, he may feel
comfortable with PPM because he feels he gets better noitce of glitches. In
the same application, operating at those distance extremes, I would want
the PCM to gain the advantages of the better signal quality, and try to
avoid the glitches in the first place.
Both receiver types will work fine. In some cases, the PPM might save an
airplane. And in others, the PCM might save the airplane. Problem is, you
just never know when you will confront the situation that is going to take
your plane down, or you could swap in whatever receiver will save it.
If he got hits, and landed, it may have saved his plane, while the PCM
plane might not know he has a problem until its too late. However, if he
gets hit with sustained interference, his plane may hit the dirt, while the
PCM pilots is bringing his plane in for a landing because it felt sluggish
or notchy. Different situations, different results. I want PPM on the
former, and PCM on the latter.
Which will next week?
Steve
>
>
>Dersu u wrote:
>>
>> >>These guys no longer use PCM, because a half second lockout will put the
>> >>plane into the ground.
>>
>> I have never seen a 1/2 second lockout with any of the Futaba 1024 stuff. I
>> have checked any number of times by turning on two transmitters on the same
>> channel to lockup the receiver. The receivers restore the instant that one of
>> the transmitters is turned off, not a 1/2 second later. Some people confuse the
>> re-boot time of the transmitter as receiver lockout time. But, people do not
>> ordinarily switch their transmitter on and off in flight.
>
This is part of where the old wives tale came from, and what the Futaba
engineer was relating to when they stated that as soon as a valid frame is
received, it is acted on. No 1/2 second delay. Yes, there is a self test
run when you first power on your transmitter. If you try to use that as a
test to mimic interference, you will indeed see a delay. This test isn't
even close to reality, and this delay will never been seen in flight unless
you turn off your transmitter.
The other part of the old wives tale, is that there is a 1/2 second delay
before the receiver goes into failsafe, from when it first saw an invalid
frame. And while that is true, its taken out of context, and assumed that
if an invalid frame is received, you will automatically experience that 1/2
second, even if you receive a good signal back in less than a 1/2 second.
And this is false.
Think of this 1/2 second delay as a timer. When a bad frame is received,
its starts the timer, and rather than going directly into failsafe, it just
holds the position of the controls. If it hasn't received a valid frame in
that 1/2 second time frame, then it goes into failsafe. If it receives a
valid frame immediately after the proceeding bad one, it will come out of
the 1/2 second delay, and return control to the pilot. I don't remember how
long each frame of data is, but coming out of hold mode could be as small
at 20milliseconds. You can't even blink that fast if you tired real hard.
Certainly not a 1/2 second mandatory hold period as some believe.
>No, but a strong masking signal, or a loose aerail or a faulty Xtal, or
>a burst of static from a wire-to-wire type joint have exactly that
>effect - momentary loss of *all* information.
>
>Or are yiu saying that the TX doesn't come on from off imediately?
>
See above. There is a self test performed at power up.
Much better than doing it at power down, randomly while flying, or not at
all :-)
Matthew Orme
> Think of this 1/2 second delay as a timer. When a bad frame is received,
> its starts the timer, and rather than going directly into failsafe, it
just
> holds the position of the controls. If it hasn't received a valid frame in
> that 1/2 second time frame, then it goes into failsafe.
We design speed controls. We have this problem. How do you identify a "valid
frame"? We use a running average an 8 element stack and reject anything that
is not within the normal range. This means that at 50Hz, it will take the
speed control 160 milliseconds to go from off to full throttle, and the same
back. (it is easier on the controller also)
Picture this. You are flying with a setting of 1.3 millisecond pulse width.
(roughly 10 percent for everyone but Futaba, which would be 90%, with the
normal output of any channel from 1.2 millisecond pulse width to 1.8
milliseconds), and you loose your signal for 200 milliseconds. Now, you get
a "valid" frame at 1.8 millisecond pulse width. Is it really valid, i.e.,
did you move the stick to 100%, or was it something else that appears as a
valid frame? Don't tell me it is not possible. This is not a modem that can
talk back to the source to check.
BTW, we have measured PCM receivers working between 30Hz, and 70Hz,
depending on the brand, and frequency. Don't ask me which, because I can't
recall. It makes a major difference with analog devices.
>
.If it receives a
> valid frame immediately after the proceeding bad one, it will come out of
> the 1/2 second delay, and return control to the pilot. I don't remember
how
> long each frame of data is, but coming out of hold mode could be as small
> at 20 milliseconds.
Pulses come every 16 milliseconds at 60Hz. with 1 dropped frame, there is a
32 millisecond gap between valid frames. Still pretty darn fast. What I want
to know is how many frames are needed before the receiver says, ok, we are
working again. I bet it is more than 1, because I bet (without knowing for
sure) that it would be possible to get "valid" frames all within the normal
range, that are random in value. (Postulate a fault before the front end of
the transmitter, so the transmitter is transmitting junk a lot of the time.
Broken trace on a circuit board, etc. I would doubt that if you got only 1
valid frame every 200 milliseconds that the receiver would think all is ok.
(but it might)
Matthew Orme
The FAI F5B team selection was done a bit ago in San Diego. Most of these
pix probably will bore most of you, but if you are interested in High
Performance (fast) electrics, this is basically the state of the art
http://sefsd.org/2001Events/F5B%20Trials/f5btrialsfor2002.htm
Here are a couple of the pix I was talking about. All planes are hand built
hollow molded kevlar, CF, fiberglass. The molds for the San Diego guys were
machined out of a big block of aluminum. Dieter is using an imported German
plane. He is the guy launching a plane in the pix below
http://sefsd.org/F5B/1st/pa210042.jpg
Thomas Pils
http://sefsd.org/F5B/2nd/pa210009.jpg
Steve Neu
http://sefsd.org/F5B/2nd/pa220018.jpg
The Contestants
http://sefsd.org/F5B/2nd/pa220030.jpg
The Team
http://sefsd.org/F5B/2nd/pa220038.jpg Dieter is the Alternate
Steve
>
>
>> Think of this 1/2 second delay as a timer. When a bad frame is received,
>> its starts the timer, and rather than going directly into failsafe, it
>just
>> holds the position of the controls. If it hasn't received a valid frame in
>> that 1/2 second time frame, then it goes into failsafe.
>
>
>
>We design speed controls. We have this problem. How do you identify a "valid
>frame"? We use a running average an 8 element stack and reject anything that
>is not within the normal range. This means that at 50Hz, it will take the
>speed control 160 milliseconds to go from off to full throttle, and the same
>back. (it is easier on the controller also)
>
>Picture this. You are flying with a setting of 1.3 millisecond pulse width.
>(roughly 10 percent for everyone but Futaba, which would be 90%, with the
>normal output of any channel from 1.2 millisecond pulse width to 1.8
>milliseconds), and you loose your signal for 200 milliseconds. Now, you get
>a "valid" frame at 1.8 millisecond pulse width. Is it really valid, i.e.,
>did you move the stick to 100%, or was it something else that appears as a
>valid frame? Don't tell me it is not possible. This is not a modem that can
>talk back to the source to check.
>
>BTW, we have measured PCM receivers working between 30Hz, and 70Hz,
>depending on the brand, and frequency. Don't ask me which, because I can't
>recall. It makes a major difference with analog devices.
>
>
You should probably check directly with Futaba on how they do this. I
personally don't have the background to help you, and only relayed what was
posted to the mailing list from a Futaba engineer.
My understanding is correct in what I read, but I could be interpreting the
next valid frame as meaning the next frame received, when it could really
be the second, third, or fourth frame, before its actually determined to be
valid. However, once its determined to be valid, I know for a fact that it
is acted upon, and does not wait for any 1/2 second delay.
How many frames are in a 1/2 second? Is three for four significant, or are
we talking noise level here?
>>
>.If it receives a
>> valid frame immediately after the proceeding bad one, it will come out of
>> the 1/2 second delay, and return control to the pilot. I don't remember
>how
>> long each frame of data is, but coming out of hold mode could be as small
>> at 20 milliseconds.
>
>Pulses come every 16 milliseconds at 60Hz. with 1 dropped frame, there is a
>32 millisecond gap between valid frames. Still pretty darn fast. What I want
>to know is how many frames are needed before the receiver says, ok, we are
>working again. I bet it is more than 1, because I bet (without knowing for
>sure) that it would be possible to get "valid" frames all within the normal
>range, that are random in value. (Postulate a fault before the front end of
>the transmitter, so the transmitter is transmitting junk a lot of the time.
>Broken trace on a circuit board, etc. I would doubt that if you got only 1
>valid frame every 200 milliseconds that the receiver would think all is ok.
>(but it might)
>
Again, I don't know the answer to this, and its worth asking Futaba or JR
on how they do this. I'd like to know. If there is any error correction, I
would think they could validate the frames with as little as a couple
frames worth of data. However, I don't think they are using error
correction, at least that is my understanding, but I haven't asked anyone
either.
James D Jones
>
> How do you program the failsafe anyway? I assume that the TX sends it to
> the RX at some point and teh RX stores it. What then happens if the RX
> is switched off?
>
That depends on the manufacturer of the system. Hitec sends the
failsafe information in a special coded frame only once when the
transmitter is turned on. so, if you are using a Hitec PCM system,
standard procedure is to power on the receiver first. Powering on
after the transmitter is on or powering the receiver off will cause
the loss of the failsafe data.
Futaba transmitters periodically send the failsafe information, so
there are no special precautions. I don't know abut JR or Airtronics.
Jim - AMA 501383
Paul Nesbitt
gyros, you must turn on the TX first.
Paul
James D Jones wrote:
>
> The Natural Philosopher wrote:
>
> That depends on the manufacturer of the system. Hitec sends the
> failsafe information in a special coded frame only once when the
> transmitter is turned on. so, if you are using a Hitec PCM system,
> standard procedure is to power on the receiver first. Powering on
> after the transmitter is on or powering the receiver off will cause
> the loss of the failsafe data.
>
> Jim - AMA 501383
Steve
Very good point Paul!
My Futaba GY-501 gyro wanted to be powered on after the transmitter for
calibration as you mentioned. It did have a way to re calibrate it manually
after the fact. However, you need to be aware that there is an issue, and
then always calibrate manually with the PCM Hitec receivers. I bet few know
of this...
Steve
medi...@mindspring.com
RC web Site http://webpages.charter.net/mediashop/rc-giant-scale/
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Jimmy Huang
I think you are oversimplifying how noise/interference works.
Let's go with the example of interference lasting 1.5 seconds. In 1.5
seconds, there would be what, about 2000 frames going by? Each frame
carries all 4 or 8 channels or whatever, if the parity or whatever
check on the frame is wrong, *everything* for that time slice gets
dumped. So we have some *strong* interference that lasts 1.5 seconds.
Since a whole frame is yea _____ long. The PCM will probably dump all
the frames (it will always find a bit error somewhere). In the end,
all channels get locked out for the full duration of the interference.
But, the PPM on the other hand, may get some seepage. All you need is
a valid channel info on one channel (hopefully the elevator) once in a
while, cuz it takes everything it can decode as valid. Thusly, in
cases of strong sudden interference, PPM some control (some servos may
travel to their endpoints tho), PCM lockout until inteference is gone.
What they need to do is UP the bandwidth and do parity or CRC checks
on a per channel basis instead of a whole frame. This should fix the
problem up real good. So even in a case like this, PCM will at least
match PPM.
Jimmy
On Mon, 03 Dec 2001 15:13:57 -0500, Steve <medi...@mindspring.com>
wrote: