Tape recorder bias oscillator questions...
Dan Akers
The recorder was not functioning in the record mode and after some
troubleshooting I found a bad cap in the bias oscillator circuit. I
replaced it and everything seems to be working fine. However, after
viewing the wave form to the record/playback head in the record mode
with a 1kHz audio tone at the input several questions have come to mind
about how the bias oscillator frequency is determined and how it is
"modulated". In this particular portable deck the oscillator runs at
about 50 kHz and applies this as a sinusoidal waveform to the recording
head at 80V (pk-pk). The audio signal is then superimposed on this
signal in such a way as to not truly modulate it in the AM radio sense,
but rather shift the signal "up" or "down" (for lack of a better term)
such that the peak to peak value remains constant and the entire wave
form "undulates" with the audio signal. To be honest, I had always
assumed that the record bias oscillator was simply amplitude modulated
by the audio, but as I discovered today this was not the case. Anyway,
my questions are these:
1. What determines the bias oscillator frequency?
2. How is this frequency related to tape bias; ie 120 uS for "normal"?
3. When calibrating a tape recorder, what should one look for at the
record head when setting up the record level meter for "0" db to ensure
the head is not "over-driven"? I am trying to calibrate to a particular
Maxell tape. (I drove it to "clip" the audio wave form then backed off
2 db at 1KHz. This gave me the best "response" from 50Hz to 18 KHz at
the head with comparable play-back to a standard "test" tape.)
I don't have a lot of information about this deck, just a
schematic. I realize that tape decks do differ, but I'm looking for
thumb-rules and tricks of the trade. If anyone could shed some light on
the mysteries of magnetic tape recorder set-up and bias oscillators I
would surely appreciate it.
-Dan Akers
Kevin Aylward
news:2410-39C...@storefull-241.iap.bryant.webtv.net...
You are quite correct in that tape bias is not AM. Tape bias works in the
same manner as a parametric amplifier. First, consider that there is an
effective non-linear expansion of the signal on the tape due to magnetizing
distortion:
H= a1.V1 + a2.V2^2 + a3.V^3 + a4.V^4 ....etc
Second, assume that the signal is symmetrical, i.e. no even harmonic
distortion, so that one considers the cubic term as dominant.
Now if you expand the V=(VsSin(Wst)+VmSin(Wbt))^3, were Wb is the bias
frequency, and collect terms, e.g. sin(x)^2 -> sin2x, you will find that not
only is the fundamental present due to the basic VsSin(Wst) term, but there
is also a cross product term that also generates something back at the
fundamental as well, proportional to both Vmand Vs. This effectively adds to
the fundamental, giving a net higher signal to distortion ratio. Hence
increasing the bias actually increases the total fundamental component, and
lowers the distortion.
Furthermore, if you examine the frequencies present you will see that Wb has
to be 3 (or 4, I can't remember which as it was 20 years ago when I first
worked it out) times, not twice as per sampling theorem, to eliminate other
distortion products.
The same technique is used with a nonlinear capacitor and a bias oscillator
to amplifier a signal with very little noise, called parametric
amplification.
Kevin Aylward , Warden of the Kings Ale
ke...@anasoft.co.uk
http://www.anasoft.co.uk - SuperSpice "Cheap, No Shit!", a currently free
GUI xspice, unlimited component, mixed-mode Windows simulator with Schematic
Capture, waveform display, FFT's and Filter Design.
Opinions of my employer are not necessarily indicative of my own
Oscillators don't, amplifiers do"
>
Mark Kinsler
>1. What determines the bias oscillator frequency?
I think it has a lot to do with the tape that's being used as well as the
design of the tape head. There's also an issue of tape machines that
contain an AM radio: the bias will beat with the incoming signal and cause
a whistle on the recording unless extra consideration is given.
>2. How is this frequency related to tape bias; ie 120 uS for "normal"?
I forget.
>3. When calibrating a tape recorder, what should one look for at the
>record head when setting up the record level meter for "0" db to ensure
>the head is not "over-driven"? I am trying to calibrate to a particular
>Maxell tape. (I drove it to "clip" the audio wave form then backed off
>2 db at 1KHz. This gave me the best "response" from 50Hz to 18 KHz at
>the head with comparable play-back to a standard "test" tape.)
Well, theoretically it should peak into the red but hang around zero db
most of the time. Again, it depends upon the tape formulation: the head
won't be overdriven, but the tape might be, so you theoretically have to
set your record level to suit your particular brand of tape. I used to
show my more fervent audiophile customers how to do this and send them
home, happy in the knowledge that they'd be fooling with levels for the
next three months and I wouldn't have to hear from them in the meantime.
If you have other things to do with your life, just go roughly by the
meter and don't expect great things out of a cassette machine.
> I don't have a lot of information about this deck, just a
>schematic. I realize that tape decks do differ, but I'm looking for
>thumb-rules and tricks of the trade. If anyone could shed some light on
>the mysteries of magnetic tape recorder set-up and bias oscillators I
>would surely appreciate it.
Well, the cassette recorder was originally designed as a dictating machine
and wasn't supposed to be used for music. Philips did market it as a
music medium fairly early on, though, and a lot of research went into
getting good response from a very narrow, very thin, slow-moving tape.
The head gaps were narrowed to some ridiculously small space, chromium
dioxide tape was developed, and Dolby designed some new noise-reduction
circuitry. Then metal tapes improved things somewhat, though these need
bias levels similar to what you'd find in the erase head.
You can find a zillion articles on cassette recorder setup in the various
hi-fi magazines from the 1970's and '80's. Check your local library for
bound copies or microfilms.
M Kinsler
--
512 E. Mulberry St. Lancaster, Ohio USA 43130
Voice: 740.687.6368 Fax: 740.687.6108
Home of the "How Things Work" engineering education program.
See http://www.frognet.net/~kinsler
Paul Grohe
in the newsgroup sci.electronics.basics,
golden...@webtv.net (Dan Akers)
from WebTV Subscriber thoughtfully posted:
> I am in the process of repairing a friends cassette tape recorder.
> The recorder was not functioning in the record mode and after some
> troubleshooting I found a bad cap in the bias oscillator circuit. I
> replaced it and everything seems to be working fine. However, after
> viewing the wave form to the record/playback head in the record mode
> with a 1kHz audio tone at the input several questions have come to mind
> about how the bias oscillator frequency is determined and how it is
> "modulated". In this particular portable deck the oscillator runs at
> about 50 kHz and applies this as a sinusoidal waveform to the recording
> head at 80V (pk-pk).
Sounds about right.
> 1. What determines the bias oscillator frequency?
The designer. Record head properties. You will see frequencies
between 40KHz and 200KHz. High enough not to cause artifacts in
the audio, but not so high as to be difficult to generate.
The AC current through the record heads is more important than
the *exact* frequency. Don't be too concerned about the *exact*
frequency.
> 2. How is this frequency related to tape bias; ie 120 uS for "normal"?
That's related to the pre-emphasis/de-emphasis processing added
to the audio signal. Basically, boost the high frequency audio on
record, and them cut them on playback to create an overall "flat"
response. "Normal" tapes use a 120uSec boost, and "metal" tapes
use a 70uSec boost.
BTW: FM radio utilizes a similar boost/cut system (75uSec for US,
50uS for Europe).
> 3. When calibrating a tape recorder, what should one look for at the
> record head when setting up the record level meter for "0" db to ensure
> the head is not "over-driven"? I am trying to calibrate to a particular
> Maxell tape. (I drove it to "clip" the audio wave form then backed off
> 2 db at 1KHz. This gave me the best "response" from 50Hz to 18 KHz at
> the head with comparable play-back to a standard "test" tape.)
"0dB" is far below clipping. There is a standard as to what "0dB"
is in magnetic flux intensity on the tape. The only way to get
that is to use a calibrated manufacturers test tape, or one made
on a known good machine. Find a good deck and record a series of
400Hz, 1KHz, 8KHz and 15KHz tones at -20dB and 0dB levels (about
5 minutes each). Record them in sequence without stoping the tape
(for constant track alignment). If you do not have a tone
generator, there are some PC sound editing programs that will
generate tones through your sound card.
Always set the playback levels first - then set the Record levels
to match. The deck should play back "standard" tapes correctly
before attempting any recording adjustments.
Use the 400Hz tone to set the 0dB level. Make sure you have the
right tape formulation selected and that the Noise Reduction is
off.
**BE CAREFUL** There are probably separate pots to adjust the
preamp playback level and the meter levels. Getting these
out-of-sync will make it difficult to set the correct levels. You
might have the playback too low and the meters set too high - or
visa-versa. Only the manual can tell you how to correctly set the
meter levels (meters are usually the first things calibrated).
You will get the best frequency response with lower recording
levels. Notice that most decks specify the frequency response
using -20dB level signals. Remember: The high frequencies are
boosted - so they are "louder" going onto the tape - so they
saturate sooner.
With "normal" tape, you should have the music peaks hit +3dB,
with the average being below 0dB. With CrO2/Metal tapes, peak at
+6dB and keep peaks below +3dB. Ideally, stay below 0dB - but
this is hard to do with sources like CD's.
18KHz with "Normal" tape is pretty good. That's what I would have
expected with CrO2 tape. Metal should get you to at least 20KHz.
Note: Make sure DolbyNR is off. Most Dolby circuits contain a
"pilot" low-pass filter to eliminate saturation from the 19KHz FM
pilot tone.
> I don't have a lot of information about this deck, just a
> schematic. I realize that tape decks do differ, but I'm looking for
> thumb-rules and tricks of the trade. If anyone could shed some light on
> the mysteries of magnetic tape recorder set-up and bias oscillators I
> would surely appreciate it.
Do you have a three-head deck?
The simple-and-stupid way is to record a white noise source (FM
radio interstation-hiss) and adjust the bias for a slight peak in
the audio. Too much bias will make it sound tinny - and too
little bias will make recordings sound muffled. Somewhere in
between there is a "happy medium".
The service manual would have you set the bias current by
measuring the head current through a specified test point. Truly
determining the bias point requires a distortion meter, tone
generator and a "standard reference" tape.
You never mentioned the model of your deck. Perhaps someone here
may be familiar with it.
Years ago, I spent days calibrating my Nakamichi deck - and that
was with the service manual, factory alignment tapes and a
(borrowed) Nakamichi test set. Geez, what a nightmare! (We won't
even talk about head alignments!!!)
Almost every deck I have worked on had the speed set wrong. Use
the 8KHz test tone tape to set the speed (newer DMM's have the
frequency function - or audibly "beat" the original signal source
against the tape playback). The speed adjustment is located on
the board (follow motor leads) - or through a little flap on the
back of the motor (the motor driving the capstan [the post of the
pinch roller] on a two-motor deck). Even a 1% difference in speed
is noticeable. If the error is conciderable, ask the owner if
they want it adjusted - since adjusting it will make their tape
library too fast/slow when they play it back with the new
settings.
Fine tuning a deck is an art. Most consumer decks are designed to
be fairly centered "out-of-the-box", with minimal tweaking. If it
is a upper-end deck, it probably about as good as it is going to
get.
Don't work yourself up trying to squeeze the last bit of
performance out of your deck - you'll never be satisfied! Get it
"good enuf" and leave it alone. Believe me!!!! :^)
Cheers,
Paul Grohe
Mark Kinsler
I learned a lot. About the only thing I might add is that you can make a
pretty good tape to check the machine's speed if you have access to a
cassette recorder whose speed is accurate. Just record a telephone dial
tone onto a tape with the good machine. To check the speed of any other
machine, listen to the tape on the machine being tested and pick up the
nearest telephone at the same time. Dial tones are accurate, and you'll
be able to adjust the speed to within a very close tolerance.
Paul Grohe
in the newsgroup sci.electronics.basics,
kin...@frognet.net (Mark Kinsler)
from thoughtfully posted:
> Paul Grohe's response about tape deck setup was far better than mine, and
> I learned a lot. About the only thing I might add is that you can make a
> pretty good tape to check the machine's speed if you have access to a
> cassette recorder whose speed is accurate.
Thank you - Thank gawd for CD's....Even the worst CD alignment is
better than a misbehaving cassette deck. I still have the Nak
deck...wonder if it is still in cal after 10 years....Hmmmm....or
maybe I should just take my car apart instead - less trouble.
> Just record a telephone dial
> tone onto a tape with the good machine. To check the speed of any other
> machine, listen to the tape on the machine being tested and pick up the
> nearest telephone at the same time. Dial tones are accurate, and you'll
> be able to adjust the speed to within a very close tolerance.
I did that a few times - then I discovered that on some phones
(Ma Bells old desk phones) - if you hold down two keys at once at
just the right point - you can get a single tone.
If you have a musical keyboard - you are all set!
The sound card generator is the best method if you do mot have a
stable tone generator and frequency counter. Some of those
programs also have "spectrum analyzer" display, which you can use
to measure the playback frequency. Check out Cooledit.
Cheers,
Paul Grohe
Dan Akers
information. Anyway, I got hold of a test tape (ordered it through
radioshack.com). This is not a very expensive tape deck nor is it very
sophisticated, but the principles of magnetic tape recording have for
the moment at least, captured my attention. One last question(s); what
is the significance of the 120 uS and 70 uS when referring to the
recommended bias? Does this refer to the bias network inflection point
(buck/boost); ie 8.3 kHz and 14.3 kHz; ie wavelength of the inflection
point on the frequency response curve of the network? I have been able
to identify the equalization RC network on the schematic. Am I to
understand that when dealing with magnetic tape the "equalization" is
performed during the recording process as apposed to equalization being
performed in the playback process for say an LP record (RIAA
equalization network)? In other words, it appears that the playback
signal does not pass through any sort of equalization network.
There doesn't seem to be a lot information available concerning the
detailed "physics" of magnetic tape recording; ie the interaction and
relationship between the bias oscillator/tape head/magnetic
tape/equalization. Everything I've found seems to be quite basic or
glosses over the very details I seek. Forgive my ignorance but my
experience has been mostly in the area of industrial control technology;
not much experience with consumer audio. I'm trying to get a handle on
the esoteric technical concepts associated with it now that I've entered
my "tinkering", retirement years. Thanks...
-Dan Akers
Paul Grohe
in the newsgroup sci.electronics.basics,
golden...@webtv.net (Dan Akers)
from WebTV Subscriber thoughtfully posted:
> Thanks for all the advice. This group is a treasure trove of
> information. Anyway, I got hold of a test tape (ordered it through
> radioshack.com). This is not a very expensive tape deck nor is it very
> sophisticated, but the principles of magnetic tape recording have for
> the moment at least, captured my attention. One last question(s); what
> is the significance of the 120 uS and 70 uS when referring to the
> recommended bias?
There are two properties that need to be set for each tape
formulation - bias current in the record head and
record/playback audio equalization.
The "tape" switch controls two separate circuits. The audio time
constant has *nothing* to do with the tape bias oscillator. Some
decks even have separate switches for "tape type" (bias) and
Equalization (120/70uSec).
The time constant was chosen for the particular tape formulation
to correct for the natural frequency response of the tape type
(and also masks playback noise as a side benefit). Metal tapes
are lower noise and can handle higher frequencies better - so
they do not require as much high-end "boost" as the "normal" tape
requires. But Metal tapes do require higher bias drive levels.
> Does this refer to the bias network inflection point
> (buck/boost); ie 8.3 kHz and 14.3 kHz; ie wavelength of the inflection
> point on the frequency response curve of the network?
It's a "time constant", not time period. Gotta stick a
"1/2*Pi*F" in there..
t= 1/(2*pi*F)
70uSec = 2274 Hz
120uSec = 1326 Hz
Low frequency pole is 3180uSec, or 50Hz, for all formulas.
These are the -3dB "corner frequencies".
> I have been able
> to identify the equalization RC network on the schematic. Am I to
> understand that when dealing with magnetic tape the "equalization" is
> performed during the recording process as apposed to equalization being
> performed in the playback process for say an LP record (RIAA
> equalization network)? In other words, it appears that the playback
> signal does not pass through any sort of equalization network.
It does - somewhere. Check around the "Normal/Metal" switches.
Simply switching out an RC network (or just a "C") will provide
the correct equalization.
Frequencies above 1326/2274Hz are boosted on record, and
frequencies above 1326/2274Hz are rolled off upon playback.
You can mix-and-match the EQ for desired sonic results if
"accuracy" is not your main concern. If you want your CrO2 tape
to sound "brighter", then play it back on 120uSec ("Normal") EQ.
Note that some decks "auto-sense" what type of tape is being used
by detecting slots on the top of the tape shell. "Normal" tapes
will not have any slots. CrO2 tapes will have a "wider" record
tab hole - and Metal tapes will have an additional slot. Stack
the tapes and compare their tops to see.
There are "fingers" that hang down and detect the
presence/absence of these slots. From these slots - the deck can
determine the bias and equalization settings it should use.
Look around the playback amp and the "tape type" switches. You
should see some sort of RC network. The equalization may also be
part of a multi-function IC.
> There doesn't seem to be a lot information available concerning the
> detailed "physics" of magnetic tape recording; ie the interaction and
> relationship between the bias oscillator/tape head/magnetic
> tape/equalization.
1. The tape and tape heads have a natural frequency response -
which needs to be corrected for (equalization).
2. Each tape formulation needs to be excited with different flux
levels (bias).
> Everything I've found seems to be quite basic or
> glosses over the very details I seek. Forgive my ignorance but my
> experience has been mostly in the area of industrial control technology;
> not much experience with consumer audio. I'm trying to get a handle on
> the esoteric technical concepts associated with it now that I've entered
> my "tinkering", retirement years. Thanks...
The actual physics is beyond me - a magnetics expert can explain
the gory details.
Roughly, the tape is a non-linear device - a slight "S" curve
with severe crossover distortion. The large HF AC bias is used to
"pave the road" (saturate) the tape for the audio signal and
"center" it on the curve. Audio is applied to the record head
with a low power audio amplifier and the bias signal is cap
coupled in with the audio at the head (a choke isolates the AC
bias signal from the audio amplifier). There are no AM or FM
"modulation" circuits.
Not bad for a 60 year old technology involving mylar tape and
rust...
Cheers,
Paul Grohe
Lizard Blizzard
> "Dan Akers" <golden...@webtv.net> wrote in message
> news:2410-39C...@storefull-241.iap.bryant.webtv.net...
> > I am in the process of repairing a friends cassette tape recorder.
> > The recorder was not functioning in the record mode and after some
> > troubleshooting I found a bad cap in the bias oscillator circuit. I
> > replaced it and everything seems to be working fine. However, after
> > viewing the wave form to the record/playback head in the record mode
> > with a 1kHz audio tone at the input several questions have come to mind
> > about how the bias oscillator frequency is determined and how it is
> > "modulated". In this particular portable deck the oscillator runs at
> > about 50 kHz and applies this as a sinusoidal waveform to the recording
> > head at 80V (pk-pk). The audio signal is then superimposed on this
> > signal in such a way as to not truly modulate it in the AM radio sense,
> > but rather shift the signal "up" or "down" (for lack of a better term)
> > such that the peak to peak value remains constant and the entire wave
> > form "undulates" with the audio signal. To be honest, I had always
> > assumed that the record bias oscillator was simply amplitude modulated
> > by the audio, but as I discovered today this was not the case. Anyway,
> > my questions are these:
> >
> > 1. What determines the bias oscillator frequency?
> > 2. How is this frequency related to tape bias; ie 120 uS for "normal"?
> > 3. When calibrating a tape recorder, what should one look for at the
> > record head when setting up the record level meter for "0" db to ensure
> > the head is not "over-driven"? I am trying to calibrate to a particular
> > Maxell tape. (I drove it to "clip" the audio wave form then backed off
> > 2 db at 1KHz. This gave me the best "response" from 50Hz to 18 KHz at
> > the head with comparable play-back to a standard "test" tape.)
> >
> > I don't have a lot of information about this deck, just a
> > schematic. I realize that tape decks do differ, but I'm looking for
> > thumb-rules and tricks of the trade. If anyone could shed some light on
> > the mysteries of magnetic tape recorder set-up and bias oscillators I
> > would surely appreciate it.
> >
> > -Dan Akers
>
> You are quite correct in that tape bias is not AM. Tape bias works in the
> same manner as a parametric amplifier. First, consider that there is an
> effective non-linear expansion of the signal on the tape due to magnetizing
> distortion:
>
> H= a1.V1 + a2.V2^2 + a3.V^3 + a4.V^4 ....etc
>
> Second, assume that the signal is symmetrical, i.e. no even harmonic
> distortion, so that one considers the cubic term as dominant.
>
> Now if you expand the V=(VsSin(Wst)+VmSin(Wbt))^3, were Wb is the bias
> frequency, and collect terms, e.g. sin(x)^2 -> sin2x, you will find that not
> only is the fundamental present due to the basic VsSin(Wst) term, but there
> is also a cross product term that also generates something back at the
> fundamental as well, proportional to both Vmand Vs. This effectively adds to
> the fundamental, giving a net higher signal to distortion ratio. Hence
> increasing the bias actually increases the total fundamental component, and
> lowers the distortion.
>
> Furthermore, if you examine the frequencies present you will see that Wb has
> to be 3 (or 4, I can't remember which as it was 20 years ago when I first
> worked it out) times, not twice as per sampling theorem, to eliminate other
> distortion products.
>
> The same technique is used with a nonlinear capacitor and a bias oscillator
> to amplifier a signal with very little noise, called parametric
> amplification.
And you posted this to sci.electronics."basics"??? Hrrmm... Not really
needed for repair. He should have poste it to the s.e.repair ng too.
Let's just put it this way. You can't put the audio directly on the
tape because the magnetic medium is highly nonlinear. You have to
continually switch the tape's magnetic domains between north and south
poles while you're recording, and the average net magnetization is zero
with no audio. As the audio level changes, the net magnetization goes
from north to south at the level of the modulation.
Isn't it so much easier to record digitally? Just ones and zeroes, and
no worry about distortion, etc. And you get much better S/N ratio, too.
Lizard Blizzard
[snip]
> tape/equalization. Everything I've found seems to be quite basic or
> glosses over the very details I seek. Forgive my ignorance but my
> experience has been mostly in the area of industrial control technology;
> not much experience with consumer audio. I'm trying to get a handle on
> the esoteric technical concepts associated with it now that I've entered
> my "tinkering", retirement years. Thanks...
Most of what you want to know should be available at a local library,
since many of the books they usually have are from the time when tape
recorders were mostly analog. The repair manuals should have a lot of
good info.
As for the other followup that said that compact cassettes were
originally for dictating machines, I thought that Philips originally
aimed these at the mass consumer market as portables in the early 60s,
and with improvements in sound quality and tape composition, they took
over the high end audio market later.
> -Dan Akers
Paul Grohe
in the newsgroup sci.electronics.basics,
Lizard Blizzard <nos...@postoffice.pacbell.net>
from Stewed Spasm & Handsome Ladder thoughtfully posted:
Yes, they were originally for the Norelco (Phillips) dictating
machines (and answering machines?).
It was probably just one of those "just the right thing at just
the right time" things (sprinkled with reasonable licensing
requirements from Philips).
Open reel was too bulky and clumsy (ever have a 3600' reel fly
off during rewind? Ouch!) and 8-track was just too unreliable
(jams between tracks created some interesting music "mixes").
Other formats never took off because they were too proprietary
(L-Cassette?).
The introduction of Dolby NR, Cr02 tape and car players in the
late 70's sped up acceptance. This is about the same time FM was
surpassing AM in popularity - so the public's "sonic taste" had
refined. Cassette had sounded fine in the days of AM, but now it
sounded "just as good" as FM.
The mid 80's saw some of the best decks and highest popularity -
three heads, frequency response to 22KHz, 60dB+ S/N ratio, search
functions, indexing, real-time counters - manufacturers were
falling all over themselves adding features. Cheap blank tapes,
"Boom Boxes", high-speed duplicating decks and the flood of
"Walkman" tape players added fuel to the fire.
Then CD's hit. The performance bar was lifted for the high-end
decks to compete against - but consumers started abandoning tape
for CD's. They no longer had to transfer their albums to tape
since they could tote CD's along with them due to the portable CD
player.
Nobody wanted to pay $400-$800 for a good 3-head, two-motor,
Dolby B/C/dBX deck anymore - so the major manufactures just
produced single head "vanilla" decks. The CD player was now the
"jewel" of the system. The majority of consumers were copying
CD's onto tape just to play in their car.
Cassette is still clinging to life - mostly because of the huge
"installed userbase" and low "media" cost. But CD-R/RW's, as well
as flash-based MP3 players - will give the cassette it's well
deserved rest.
We are now seeing the same thing happen to VCR's. VHS won over
Beta in the early 80's. VCR's peaked in the early 90's with
functions and features, and now VCR's are starting to be
displaced by DVD's and have been stripped of their features to
where they are now cheap "commodities". Now the HDD based "VCR's"
are threatening the tape VCR - but you still can't "share" your
recordings. Wait until DVD-RW's based decks appear - kiss tape
goodbye!
IMHO: The reason the "Philips" cassette was accepted was because
it was adopted as a "standard" from the start. Any attempts to
bastardize it were rejected - the standard has stayed fairly
intact for 30+ years. A tape made today would still play back on
one of those original dictating machines.
The manufactures of MP3-type playback equipment should take their
cue from the humble Cassette - and develop a "standard" interface
and format. We have seen what the market can do to "proprietary'
formats. Beta was killed because Sony was tight with the
licensing - and tapes and equipment were expensive. JVC was
liberal with the VHS licensing - and competition drove the prices
down and the selection up. Beta may have been a superior system -
but the consumers sided on "cheap and available".
We are seeing the early days of the tape recorder in the MP3
players. There are multiple vendors with multiple formats right
now. Some are standardizing on the flash card format - but what
is put *on* the card is not standardized yet. Each uses it's own
"recorder" software.
We won't see the MP3 player market take off until consumers are
sure that they can record, play back, and share the "card" with
their friends - no matter what player they have - just like they
used to do with the "old" cassettes.
Enuf driveling...
Cheers,
Paul Grohe
Dan Akers
"And you posted this to sci.electronics."basics"??? Hrrmm... Not really
needed for repair. He should have poste it to the s.e.repair ng too.
Let's just put it this way. You can't put the audio directly on the tape
because the magnetic medium is highly nonlinear. You have to continually
switch the tape's magnetic domains between north and south poles while
you're recording, and the average net magnetization is zero with no
audio. As the audio level changes, the net magnetization goes from north
to south at the level of the modulation."
Actually the repair was no problem. I was looking for "theory".
And by the way, thanks for the reply. I think it is starting to gel in
my mind how these things operate; not to mention a bit of familiarity
with the liberally applied esoteric terminology. I'm working on
acquiring a new guinea pig from a local repair shop (abandoned by
customer); something with all the bells and whistles on it that I can
experiment with and tear into without care. My neighbor is jumping up
and down for me to remove his "precious" tape recorder from my
dissection table. "You're going to destroy it!", or some such concern.
So much for the free repair. How quickly they forget... Oh well, I
think I've gleaned all I can from it anyway...
-Dan Akers