Weird Reception of LAPD Frequencies
boneh...@netcom.com
I've noticed that whenever I'm listening to LAPD frequencies from my
area (a tad away from their location), REGARDLESS of what scanner I'm
using, its antenna, or my location (as long as I'm an acceptable distance
AWAY from their coverage area), LAPD frequencies seem to have a weird
"phase-in, phase-out, phase-in, phase-out" effect on their actual carrier
signals. Its as if the operator of any given frequency is turning a
variable repeater TX power dial between 100 watts and 10 watts, 100 watts
and 10 watts, etc. I guess the best way I can explain it is to give you
all an illustration, as dumb as this will be. :) Picture this as a heart
monitor-like graph of the carrier "strength" over the duration of one
second showing, which is respectively showing the signal strength:
Full scale: --------. .--------. .--------. .---------
Average: ` .` `. .` `. .`
Poor: . . .
As you can see, or maybe not, the signal drops to a hissy level about
three times a second at exact intervals, and then back up to full scale
again. What could be causing this? Depending on the frequency I'm
listening to, the interval is either faster or slower. And like I said,
it happens regardless of what scanner I'm using, whether I'm on the east
side of town or the west side of town, etc. I was thinking that it may
be a side-effect of out of phase co-phased antennas, but I'd be apt to
think that hetrodyne would be in order in such a case, not this. But
then again, maybe this is a result of being **SLIGHTLY** out of phase?
Someone help. I'm beggin' ya. I was put on this very earth to find
out the cause of this exact phenomenon, at this exact point in time.
Thank you all for your undivided attention. And I'll leave the light
on for 'ya. Bye now! Yeah, yeah, yeah .. .. .. ..
Mike
Could be an effect of a simulcast signal with one of the xmitters being
unstalble and drifting during the transmission. This could explain why
it is cyclic based on the period of the drift...
Just a thought...MikeN
CSupak
m. mattson wrote:
>
> boneh...@netcom.com wrote:
>
> >I've noticed that whenever I'm listening to LAPD frequencies from my
> >area (a tad away from their location), REGARDLESS of what scanner I'm
> >using, its antenna, or my location (as long as I'm an acceptable distance
> >AWAY from their coverage area), LAPD frequencies seem to have a weird
> >"phase-in, phase-out, phase-in, phase-out" effect on their actual carrier
> >signals. Its as if the operator of any given frequency is turning a
> >be a side-effect of out of phase co-phased antennas, but I'd be apt to
>
> our county-wide paging system, where the same signal is output from
> all antennas in the county. With "normal" radio coms, the signal is
> sent to the appropriate antenna (only) so that this doesn't occur.
>
> Mark
>
> The truth as I perceive it to be.
> Your perception will be different.
Or he could have a nearby paging transmiter that's almost swamping his
scanners front end, but not quite.
MD
--
First they came for the hackers. But I never did anything
illegal with my computer, so I didn't speak up.
Then they came for the pornographers. But I thought there was
too much smut on the Internet anyway, so I didn't speak up.
Then they came for the anonymous remailers. But a lot of nasty
stuff gets sent from anon.penet.fi, so I didn't speak up.
Then they came for the encryption users. But I could never
figure out how to work PGP anyway, so I didn't speak up.
Then they came for me. And by that time there was no one left
to speak up.
m. mattson
Larry Levy
Hi,
You thought your example was weird? Wait till you hear my theory on
this!!!! :)
I live in the NW San Fernando Valley and have a Pro-2006 connected to
a discone in the attic.
I have noticed this "phasing" you described on LAPD frequencies, but
only when the 5' mast that my discone is attached to in the attic has
moved off vertical. This used to happen quite a bit after the
Northridge earthquake when aftershocks would move the mast. I'd then
have to climb up into the attic and straighten the thing out.
I noticed that if the mast was at more than say a five or ten degree
angle from vertical, I'd hear what you described on the West Valley
and Devonshire Division frequencies. That was how I'd tell if I had to
make another trip up into the attic.
Needless to say, I now have the antenna mounted in a more stable way.
:)
Hope that helps,
-Larry
boneh...@netcom.com wrote:
>I've noticed that whenever I'm listening to LAPD frequencies from my
>area (a tad away from their location), REGARDLESS of what scanner I'm
>using, its antenna, or my location (as long as I'm an acceptable distance
>AWAY from their coverage area), LAPD frequencies seem to have a weird
>"phase-in, phase-out, phase-in, phase-out" effect on their actual carrier
>signals. Its as if the operator of any given frequency is turning a
>variable repeater TX power dial between 100 watts and 10 watts, 100 watts
>and 10 watts, etc. I guess the best way I can explain it is to give you
>all an illustration, as dumb as this will be. :) Picture this as a heart
>monitor-like graph of the carrier "strength" over the duration of one
>second showing, which is respectively showing the signal strength:
>Full scale: --------. .--------. .--------. .---------
> Average: ` .` `. .` `. .`
> Poor: . . .
>As you can see, or maybe not, the signal drops to a hissy level about
>three times a second at exact intervals, and then back up to full scale
>again. What could be causing this? Depending on the frequency I'm
>listening to, the interval is either faster or slower. And like I said,
>it happens regardless of what scanner I'm using, whether I'm on the east
>side of town or the west side of town, etc. I was thinking that it may
>be a side-effect of out of phase co-phased antennas, but I'd be apt to
Paul Grohe
On Wed, 23 Oct 1996 18:43:22 -0700,
in the newsgroup rec.radio.scanner,
Mike <mi...@dxnet.com>
from Microserve Information Systems (800)-380-INET thoughtfully
posted:
> Could be an effect of a simulcast signal with one of the xmitters being
> unstalble and drifting during the transmission. This could explain why
> it is cyclic based on the period of the drift...
>
> Just a thought...MikeN
Yep.
Not so much "unstable", but that one transmitter is simply 3Hz
off frequency from the other.
If it is an 800MHz system, 3Hz is probably within spec. And of
course, 800MHz systems use multiple transmitters on the same
channel.
Two signals coming in at relatively the same level can add or
subtract from each other as they "beat" against each other.
You would really hear this effect if it was AM, but the limiters
and capture ratio of the FM receiver tend to "even" it out ("full
scale"), except for when the signals severely cancel each other
out (the "valleys").
You must be between the two transmitters. Try to find a "null"
spot where one of the transmitters signals is weak, or use a
directional antenna, and the received signal should clear up.
The two signals are cancelling each other out *at* the antenna,
so there is little the scanner can do.
Also, the transmitters may not even be carrying the same traffic!
But their carriers will interfere just the same, and you will
hear the audio from the stronger of the two.
It's a normal (and annoying) "phenomenon".
Cheers,
Paul Grohe
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Here I. Am
We sometimes, in radio broadcasting, use a "booster" which is an
on-frequency gap fill transmitter. There will always be an interference
zone where a receiver will receive both transmitter sites, the main and
the booster(s).
Some of the more sophiticated boosters are synchronized (phase locked)
to minimize the interference. LAPD no doubt is using a similar
technique to ensure total coverage of a particular area, especially for
portables.
Donald Gray
boneh...@netcom.com wrote:
>I've noticed that whenever I'm listening to LAPD frequencies from my
>area (a tad away from their location), REGARDLESS of what scanner I'm
>using, its antenna, or my location (as long as I'm an acceptable distance
>AWAY from their coverage area), LAPD frequencies seem to have a weird
>"phase-in, phase-out, phase-in, phase-out" effect on their actual carrier
>signals.
What you are hearing is probably the results of picking up 2 or more
transmissions from the LAPD.
It could be a quasi-synchronous network that they have. If it is a
quasi-synchronous system, the following may help to understand what is
going on and how you get this fading in & out or chuffing sounds.
Quasi-sync can be used with both AM and FM transmissions. The sound
effect in the overlap area is different on each.
For certain services, a single transmitter may not have the coverage
needed to give assured service in a given geographical location. To
get round this, multiple transmitters are put in prime locations, ALL
of them on the same frequency and transmitting simultaniously. These
locations are chosen to give an overlapping coverage.
This give rise to two major problems:-
1) The mobile users in the overlapping areas can receive 2 or more
signals. These signals can and do interfere with each other.
2)Frequency stability. The standard transmitter oscillator is stable
enough when it is used on its own, but totally inadequate when put
into service with multiple transmitters
If the stability of the frequency is not good enough, the interference
can be so bad that the transmissions could be unreadable, SO... ...
They use very expensive, High Stability Master oscillators and attempt
to "lock" the frequencies to within 12 - 15 cycles. This actually does
work - IF the frequency is separated very, very slightly!
What you are hearing is a Q-S system that needs the radio engineer to
do his rounds and reset the master oscillators to within 12 - 15 hertz
of each other! (Even the best of kit needs a regular tweek from time
to time.)
With an AM Q-S system this "closeness" of frequency will give rise to
the receiving equipment detecting the signal being increasing in
strength when the two frequencies are in phase with each other. When
they are out of phase, the signal decreases in strength. Because the
signals are coming in and out of phase at about 12 to 15 times a
second, this is rapid enough for the audio (whilst degraded slightly)
to be quite readable and the 12/15 cycle chuffing not being
objectionable. (The principle is that the mobiles are travelling
through the overlap area and the signal cleans up in non-overlap
areas.)
If the frequencies are locked together at a closer relationship, say 3
- 5 cycles, the in & out of phase becomes so detectable as to be
objectionable and gives rise to a regular "swishing" or chuffing
sound.
With an FM Q-S system, the signal strength remains fixed but the audio
becomes "ragged at the edges" almost as if the speaker wants to cough.
The base (despatcher) stations do not hear this multiple path
transmissions, even though 2 or more of the repeater receivers can be
picking up signals from the mobile. What happens here, is that all the
receiver signals from the various repeater locations are fed into a
"black box" (a diversity unit). This box is also given information as
to the signal strength of each of the signals detected and it only
allows the strongest to be fed to the dispatchers speaker (or back to
the transmitters if it is on talkthrough.)
I hope that this explanation helps to understand what you are hearing
and how it works.(I haven’t gone into other problems of the audio
coming out of each of the transmitters at a slightly different times
due to propagation delays, nor the need for identical audio phase
envelopes to be transmitted - I thought that it might confuse the
subject!
BTW: I operate a four site FM Q-S system and also "look after" an FM
Q-S system for a local County Council. They can be a nightmare to set
up and keep happy! Our local police use an AM VHF Q-Ssystem and their
transmittions have been terrible with a 1 to 2 hz seperation - almost
made me switch off!
Happy days to you all
Donald Gray
Free Agent V1.0 -
Eudora Lite V1.5.4
Please do not short the output or severe damage will be caused to the fuse.
(On a Sony power supply leaflet!)
MEM&GWM
boneh...@netcom.com wrote:
>I've noticed that whenever I'm listening to LAPD frequencies from my
>area (a tad away from their location), REGARDLESS of what scanner I'm
>using, its antenna, or my location (as long as I'm an acceptable distance
>AWAY from their coverage area), LAPD frequencies seem to have a weird
>"phase-in, phase-out, phase-in, phase-out" effect on their actual carrier
>signals. Its as if the operator of any given frequency is turning a
>variable repeater TX power dial between 100 watts and 10 watts, 100 watts
>and 10 watts, etc. I guess the best way I can explain it is to give you
>all an illustration, as dumb as this will be. :) Picture this as a heart
>monitor-like graph of the carrier "strength" over the duration of one
>second showing, which is respectively showing the signal strength:
>Full scale: --------. .--------. .--------. .---------
> Average: ` .` `. .` `. .`
> Poor: . . .
>As you can see, or maybe not, the signal drops to a hissy level about
>three times a second at exact intervals, and then back up to full scale
>again. What could be causing this? Depending on the frequency I'm
>listening to, the interval is either faster or slower. And like I said,
>it happens regardless of what scanner I'm using, whether I'm on the east
>side of town or the west side of town, etc. I was thinking that it may
>be a side-effect of out of phase co-phased antennas, but I'd be apt to
>think that hetrodyne would be in order in such a case, not this. But
>then again, maybe this is a result of being **SLIGHTLY** out of phase?
>Someone help. I'm beggin' ya. I was put on this very earth to find
>out the cause of this exact phenomenon, at this exact point in time.
>Thank you all for your undivided attention. And I'll leave the light
>on for 'ya. Bye now! Yeah, yeah, yeah .. .. .. ..
The LAPD is probably putting into effect the NEW 484.**** Frequencies
that were supposed to go into service 1 Jan 1996. Some are and some
aren't. They are running frequencies in tandem.
Joe Leikhim
boneh...@netcom.com wrote:
The LAPD system is simulcast from several geographically seperated
transmitter sites at once. The signals from each transmiter have the
potential to reinforce each other when the signals are received in
phase and on exactly the same carrier frequency. This is the reason
simulcast systems are used to provide improved building penetration in
many systems.
However, if the signals differ slightly in frequency you will hear the
signals beating against each other like you described. If you are
closer to one site than another the signal from the closer site will
capture (8 to 12 dB ratio), If you are far outside of the coverage
area, the signals from one site to another may differ in the time
taken to travel from the transmitter to your receiver. RF signals
travel the speed of light 5.4 microseconds per mile. If the signal
from one site arrives 125 microseconds later than another site the
distortion can be quite severe. If the simulcast ystem is not properly
designed the signals from the multiple sites can be destructive.
There are many other "problems" that simulcast systems present
including the following:
All carrier frequencies must be closely locked together using high
stability oscillators, Rubidium, WWVB, GPS.
Transmitter modulation (FM deviation etc) must be nearly identical
from each transmitter.
Audio frequency response of the audio chain from the dispatch console
and reciever voters must be nearly identical before transmission at
each site.
Transmitter site separation must be within the maximum allowable for
the modulation used. FM audio distorts at 125 microseconds, while high
speed data distorts at lower transmitter site separations as a
function of the baud rate.
FM capture ratio can overcome some of the phase delay problem but not
all.
Richard Evans
Mike (mi...@dxnet.com) wrote:
: Could be an effect of a simulcast signal with one of the xmitters being
: unstalble and drifting during the transmission. This could explain why
: it is cyclic based on the period of the drift...
: Just a thought...MikeN
Could be another transmitter on a very very similar frequency, but not
quite the same frequency.
When two frequencies are similar but not exactly the same, they drift
in and out of phase with each other. When they are in phase they
re inforce each other creating a slightly stronger signal. When
they are out of phase they work against each other creating a
weaker signal. If they are of similar strength then they will
virtually anialate each other.
The speed at which the signal strength changes is exactly equal to
the difference in frequencies between the two signals. So if as you
say your signal is dropping out 3 times a second, then the difference
in frequency between the two signals is 3 Hz.
Richard.