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PICList Thread
'OT: Wire audio recordings'
1999\10\04@163009 by PJH

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face
Hey PICers,

This is rather off-topic but ... I
just was asked the question pasted
below. Does anybody know how those
old 'on wire' audio recordings were
done and what sort of pick-up head
you'd need to extract the signal?
(We're talking circa 1920's.)

I guess, you could use a 16c73 or
sim to digitise them. BUT magnetic
tapes have to be frequency
compensated before palyback, I seem
to recall from long ago. If wire
recordings need to be processed
similarly you'd probably do that
before the digitising stage to keep
the processing simple, I guess?

Need to think about a drive unit of
some sort for the wire spools as
well - wonder what sort of speed
you'd be looking at?

Any tips appreciated - PJH


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1999\10\04@174750 by Dave VanHorn

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> I guess, you could use a 16c73 or
> sim to digitise them. BUT magnetic
> tapes have to be frequency
> compensated before palyback, I seem
> to recall from long ago. If wire
> recordings need to be processed
> similarly you'd probably do that
> before the digitising stage to keep
> the processing simple, I guess?

The head will be the most critical piece. It's probably going to have an
unusually large gap. If the gap on the playback head is too small, then it
will be very noisy, among other problems.

The book "magnetic recording" by Finn Jorgenson may help, it's a tab book.

Good luck!

1999\10\04@224711 by William K. Borsum

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Used to have an old wire recorder when I was a kid--in the 50's or there
abouts.  Don't recall much about it except that the wire moved fairly fast
and broke easily.  TAPE usually was either amplitude or frequency modulated
carrier of about 100 KHz.  I don't know about the wire but would suspect
they would have to do the same thing.

The Smithsonia in Washington has a huge collection of old recordings--and
the equipment to play them back.  Might be a source of good
information--work a trade--they transcribe and get a copy for the arkives?

Kelly


At 06:26 AM 10/5/99 +1000, you wrote:
{Quote hidden}

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<borsumspamKILLspamdascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\04@230352 by Sean H. Breheny

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At 07:42 PM 10/4/99 -0700, you wrote:
>and broke easily.  TAPE usually was either amplitude or frequency modulated
>carrier of about 100 KHz.  I don't know about the wire but would suspect
>they would have to do the same thing.

What kind of tape are you talking about? You don't mean regular cassette
tapes,do you? AFAIK, those are just the raw audio applied to the recording
head (no carriers).

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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1999\10\04@232036 by Richard Prosser

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I think you're talking about bias.

In the early days DC bias was applied - I think this included wire
recording. These days AC bias is applied at something like 100kHz. This
reduces noise (or allows increased recorded level which results in the same
thing) and reduces distortion. In both cases the bias is simply added to the
signal - not modulated by it (although I guess in the DC case it amounts to
much the same thing).

I also have wondered what sort of heads were used for wire recorders .
(Weren't flight recorders a wire recording until reasonably recently?)

Richard P

> {Original Message removed}

1999\10\04@234820 by Brian Aase

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> I also have wondered what sort of heads were used for wire recorders .
> (Weren't flight recorders a wire recording until reasonably recently?)

As I recall, flight recorders used steel bands (and still may).

If one is looking to play back some pre-recorded wire, my
first suggestion is to pick up a wire recorder transport.  They
show up on the ebay auction from time to time.  I used to
own one, and it really didn't sound as bad as I expected.
The head had a very deep notch in the front to capture the
wire, but I never tried to take the thing apart to see how it
was built.

The transports (most were made by Webcor, a few by Brush)
did not use a capstan; instead, the takeup reel turned at a
more-or-less constant speed.  This means that the wire itself
passed over (through?) the head faster as more wire
accumulated on the takeup reel.
The head had a really cute mechanism to move it up and down
while running, to make sure the wire wound onto the takeup
in a nice even pattern.

It shouldn't be too hard to infer the playback EQ by inspecting
the rather simple electronics in the machine.  You might put out
a feeler on the rec.audio.tech newsgroup.  Some people there
do serious archiving work, and can probably elaborate at length.
For now, back to the pic world.

1999\10\05@131906 by Harold M Hallikainen

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       You should be able to find wire recorders at garage sales,
antique stores, etc.  My high school girlfriend's grandmother had one
that we played with.  They SHOULD still exist.  You shouldn't have to
build your own.  I think her machine was a Webcor.

Harold


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1999\10\05@194918 by William K. Borsum

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At 11:00 PM 10/4/99 -0400, you wrote:
>At 07:42 PM 10/4/99 -0700, you wrote:
>>and broke easily.  TAPE usually was either amplitude or frequency modulated
>>carrier of about 100 KHz.  I don't know about the wire but would suspect
>>they would have to do the same thing.
>
>What kind of tape are you talking about? You don't mean regular cassette
>tapes,do you? AFAIK, those are just the raw audio applied to the recording
>head (no carriers).
>

Yup--Or at least thats the way I remember it from my engineering days at
Ampex.  Scientific and hi frequency analog recorders general y used a HF
carrier, that was typically FM modulated. Frequency response depended
heavily on the speed of the tape across the head gap versus the width of
the gap. Very narrow gaps could run the tape slower.  In the olde days,
good audio was typically run at 15-30 ips, and I don't recall seeing any 3
ips tapes until about the time I left the industry.  Video got around the
problem by spinning the head on a diagonal across the tape.   Audio--and
I'm trying to remember data from 35 years ago--needed to be written with an
AM modulated carrier to get the ferrite particles in the tape to magnetize
correctly.  Playback appeared to be normal audio frequencies straight off
the heads.  100Khz sticks in my mind for the carrier.

Ampex put out a really good technical book on magnetic recording--might
still be available.

Kelly

William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems
<borsumspamspam_OUTdascor.com> & <http://www.dascor.com>San Diego, California, USA

1999\10\06@015838 by Nigel Goodwin

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In message <@spam@61E63DF92FF1D1119A2800805FA64017014C56ECKILLspamspamexchange1.swichtec.> co.nz>, Richard Prosser <KILLspamRPROSSERKILLspamspamSWICHTEC.CO.NZ> writes
>In the early days DC bias was applied - I think this included wire
>recording. These days AC bias is applied at something like 100kHz. This
>reduces noise (or allows increased recorded level which results in the same
>thing) and reduces distortion. In both cases the bias is simply added to the
>signal - not modulated by it (although I guess in the DC case it amounts to
>much the same thing).

The bias is to overcome the hysteresis (I'm not sure about that
spelling!) in the recording medium. As you say, modern recorders use
about 100KHz for the bias, the recorded audio is added to this, and
amplitude modulates it. The bias is usually a fairly high signal level,
up to 100 volts peak to peak, and the audio is very low, only a few
100mV. This makes it very hard to see the modulation on an oscilloscope,
sometimes you can just see it with the gain turned up and the vertical
shift fully one way :-).
--

Nigel.

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1999\10\06@021339 by Sean H. Breheny

face picon face
This discussion is confusing me more and more as it continues ;-)

At 06:44 PM 10/5/99 +0100, you wrote:
>The bias is to overcome the hysteresis (I'm not sure about that
>spelling!) in the recording medium. As you say, modern recorders use
>about 100KHz for the bias, the recorded audio is added to this, and

If this is so, then why when you look at the specs do they give the
frequency response of the recording head as something like -3dB at 19kHz?

Also, if I understand magnetic recording, you NEED hysteresis to make a
recording (in order for the tape to effectively retain the information, the
tape must be magnetized, which means that you are on one of the flat parts
of the hysteresis loop). The audio itself must overcome the hysteresis and
reverse it (if needed) in order to be strong enough to produce a good
recording. I don't see how adding an additional 100kHz signal helps this
process. I would assume that the hysteresis loop would be a lot more linear
at 100kHz (and have a LOT less slope than in the vertical region of the
low-freq. loop).

I just want to make sure that we're talking about the same thing: I'm
talking about the walkman that I buy at the corner store for $25, not some
high-end specialty audio recorders or digital decks.

>amplitude modulates it. The bias is usually a fairly high signal level,

How does adding two signals together (in a roughly linear system) cause one
to modulate the other?

>up to 100 volts peak to peak, and the audio is very low, only a few
>100mV. This makes it very hard to see the modulation on an oscilloscope,
>sometimes you can just see it with the gain turned up and the vertical
>shift fully one way :-).

I don't know where in a normal tape deck there would be a 100Vp-p 100kHz
supply!! I am thinking more and more that we are talking about different
things.

>--
>
>Nigel.

Sean



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1999\10\06@030519 by Michael Rigby-Jones

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Sean, all analogue magnetic tape recorders use some kind of high frequency
bias, even your cheapo walkman (as long as it records).  With out the bias
the recording would be very poor.  Have a look at
http://www.vex.net/~pcook/rec.audio.pro/analog.html (section Q4.2) and it
explains.

Mike Rigby-Jones

> {Original Message removed}

1999\10\06@054715 by paulb

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William K. Borsum wrote:

> Scientific and hi frequency analog recorders generally used a HF
> carrier, that was typically FM modulated.

 As do "Hi-Fi" video tape recorders now.

> 100Khz sticks in my mind for the carrier.

 For an FM carrier, it would be.  For bias, 30 to 40 KHz I would have
thought would be generous.  The article Michael Rigby-Jones references
does however go along with the 100 KHz as does my final point below.

Sean H. Breheny wrote:

> This discussion is confusing me more and more as it continues ;-)

 So it seems.

> If this is so, then why when you look at the specs do they give the
> frequency response of the recording head as something like -3dB at
> 19kHz?

 Why do you suppose you need a *large* bias?

> Also, if I understand magnetic recording, you NEED hysteresis to make
> a recording (in order for the tape to effectively retain the
> information, the tape must be magnetized, which means that you are on
> one of the flat parts of the hysteresis loop).

 You confuse retention with hysteresis.  Putty has fairly good
retention, but little hysteresis; you bend it and it stays bent.  Steel
has hysteresis; you bend it but it springs back.  Bend it further, and
after springing back, it stays "set" to a lesser position than you bent
it.  Of course the steel resists alteration better overall.

 Magnetic recording materials are generally "soft" (interestingly, this
often parallels their mechanical properties) with low hysteresis.  A
very crude recorder without bias will record on a good material, but you
will get significant distortion.  More bias is needed for "harder"
materials such as Chrome, thus the switches on the recorder.

> The audio itself must overcome the hysteresis and reverse it (if
> needed) in order to be strong enough to produce a good recording. I
> don't see how adding an additional 100kHz signal helps this process.

 But that's exactly the point.  The audio *doesn't* overcome the
hysteresis, the bias signal (added, not multiplied) does.  In effect the
audio "biasses" the bias signal to control the point to which the
magnetism falls back after the head passes.

> I would assume that the hysteresis loop would be a lot more linear
> at 100kHz (and have a LOT less slope than in the vertical region of
> the low-freq. loop).

 You got me there!

> I just want to make sure that we're talking about the same thing: I'm
> talking about the walkman that I buy at the corner store for $25, not
> some high-end specialty audio recorders or digital decks.

 Even Walkmans have to deliver *some* fidelity, for which you need
bias.  *Good* machines use the same bias oscillator (at higher level) to
drive the erase head (which has a much larger gap) while "junk" machines
use DC.

> How does adding two signals together (in a roughly linear system)
> cause one to modulate the other?

 It certainly doesn't.  What it does is cycle the material through a
loop which is well beyond the hysteresis level.  The signal component
affects the *average* (not the *amplitude*) of this cycle, and that
defines the level of magnetization that results.

 There are two other ways of looking at this.  The tape contains
"domains" each of which have basically two states.  The bulk
magnetization is the proportion of each state.  Demagnetization is when
equal numbers of domains are in each state, while "saturation" is when
virtually all domains are in the same state.  That should make it a lot
clearer.  It explains also why tape is a noisy medium as the domains are
quantised.

 The first way of looking at it is to say that as the tape leaves the
gap, the domains on the very edge of the gap are aligned in proportion
to the magnetization at that point.  The alternative is to view the
addition of bias and signal and application of this to a hysteretic gate
as generation of PWM, just as you could do with a 74HC14.

 How's that sound (did you ever think of generating PWM that way for a
"class D" amplifier)?  You can see why the bias needs to be much larger
than the signal to make the process linear, but too much bias reduces
the signal.

> I don't know where in a normal tape deck there would be a 100Vp-p
> 100kHz supply!

 You look in a tape deck (recorder) and you'll see at least one "IF"
transformer!
--
 Cheers,
       Paul B.

1999\10\06@105031 by Wagner Lipnharski

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To be able to extract the same recorded signal, high frequency bias is
required during recording.

First of all, you can not record very well low frequencies in magnetic
media, and even if you can, the retrieving of that signal would be very
difficult.  High frequencies are easily recorded, so the tricky thing is
to record your audio together with some kind of high frequency carrier.

Think as if you inject pure audio to a radio transmitter antenna, you
will transmit something, but very weak, no use for it right? ok, it is a
gross comparison, but makes sense, you need a high frequency carrier
that works better in *that* media.  This is why you "modulate" the radio
frequency (carrier) with the audio signal, in AM (amplitude modulated),
FM (frequency), DPSK (phase), QAM (quadrature), and some other
modulation techniques.

The magnetic tape media has its own characteristics, and we need to
"adapt" our audio signal to be recorded. As the frequency response of
the tape is better at high frequencies, we need to use that frequency
band as our working zone, so this is why we use a carrier around 100kHz
to be modulated by the audio signal.

In real you are not recording audio, but a modulated radio frequency of
100kHz on tape, that's it.

In a radio receiver, you need a circuit to remove the radio frequency
from the incoming signal and leave only the audio, this is called
"detector" or something else. This job in the tape player is done by the
magnetic head itself and little electronics around it to better
compensate the frequency response.

This is used in *all* magnetic tape recorders, from $5 units from KMart,
answering machines, up to professional expensive players.

VCR units should use this only for audio (since it is recorded in the
same way as a audio cassette unit, longitudinal recording), while video
recording frequency is quite high, and I guess it doesn't need it.

Wagner

1999\10\06@122011 by Dave VanHorn

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Ok, this needs a severe debunking.

> >The bias is to overcome the hysteresis (I'm not sure about that
> >spelling!) in the recording medium. As you say, modern recorders use
> >about 100KHz for the bias, the recorded audio is added to this, and

True. But it has <<NOTHING>> to do with playback.

The bias signal is simply there to "shake up" the tape, and make it possible
to record at lower signal levels than before.   Imagine a tray full of
rocks, You could write in them with your finger, but not very well. If you
shake the tray so the rocks are in motion, then you can move them around
with ease.   If you had a way to shake the rocks that are near your finger,
and then leave them alone, you'd have a bias generator.

What's recorded on audio tape (not video tape!) is simply audio. Hook a
radio shack amplifier to a head, and rub the head on the tape.  The head
isn't capable of reproducing anything at the bias frequency, the wavelength
(as defined by how much tape moves past a point during one cycle) is too
short by far.

The audio in an audio tape recorder is not recorded as some sort of
modulated carrier, or encoded, or anything else fancy.

1999\10\06@122019 by Mike Keitz

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On Wed, 6 Oct 1999 10:31:11 -0400 Wagner Lipnharski
<RemoveMEwagnerlspamTakeThisOuTEARTHLINK.NET> writes:
> This is used in *all* magnetic tape recorders, from $5 units from
> KMart,
> answering machines, up to professional expensive players.

The very cheapest units at least used to use "DC bias" for recording.  I
think this involves just flowing some DC through the record / play head
(obviously causing a problem later during playback, when it is still
magnetized).  DC can also be used for erasing.  The erase head may just
be a permanent magnet.  It is possible to make a recognizeable recording
this way, but quality is poor.


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1999\10\07@151930 by Nigel Goodwin

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In message <37FB5D2F.2142C5BEEraseMEspam.....earthlink.net>, Wagner Lipnharski
<EraseMEwagnerlspamEARTHLINK.NET> writes
>VCR units should use this only for audio (since it is recorded in the
>same way as a audio cassette unit, longitudinal recording), while video
>recording frequency is quite high, and I guess it doesn't need it.

Correct, the video is recorded as an FM signal at a high power and high
frequency, and doesn't require any bias. The same applies to HI-FI audio
as well, but the conventional linear audio uses bias just like an audio
tape recorder. In fact it's a fairly common fault that the bias
oscillator packs up, this also stops the erase as well, but the video
signal has enough power to erase the tape anyway, so the picture's OK,
but the old sound is left on the recording - you can get quite amusing
results sometimes :-).
--

Nigel.

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1999\10\07@151942 by Nigel Goodwin

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In message <006301bf1016$6d79ffa0$0200a8c0@xemu>, Dave VanHorn
<RemoveMEdvanhornspam_OUTspamKILLspamCEDAR.NET> writes
>Ok, this needs a severe debunking.

I wouldn't say so, as it's what happens!.

>> >The bias is to overcome the hysteresis (I'm not sure about that
>> >spelling!) in the recording medium. As you say, modern recorders use
>> >about 100KHz for the bias, the recorded audio is added to this, and
>
>True. But it has <<NOTHING>> to do with playback.

Very true, I never suggested it had!

>The bias signal is simply there to "shake up" the tape, and make it possible
>to record at lower signal levels than before.   Imagine a tray full of
>rocks, You could write in them with your finger, but not very well. If you
>shake the tray so the rocks are in motion, then you can move them around
>with ease.   If you had a way to shake the rocks that are near your finger,
>and then leave them alone, you'd have a bias generator.

Perhaps one of the easiest ways to understand what bias does is to
imagine towing a car using a bungee rope. When you start pulling, the
rope stretches and the car doesn't move at first, the same applies to
magnetic tape, the signal has to overcome the 'reluctance' to be
magnetised and this causes massive distortion. To overcome this a high
level, high frequency signal is used to 'shift' the audio signal above
this distortion area. A very high level is required because neither the
head or the tape are sensitive at these frequencies.

>What's recorded on audio tape (not video tape!) is simply audio. Hook a
>radio shack amplifier to a head, and rub the head on the tape.  The head
>isn't capable of reproducing anything at the bias frequency, the wavelength
>(as defined by how much tape moves past a point during one cycle) is too
>short by far.

Exactly, that's why the bias frequency is chosen to be where it is!

>The audio in an audio tape recorder is not recorded as some sort of
>modulated carrier, or encoded, or anything else fancy.

No, just plain audio, the bias is too high to be recorded.
--

Nigel.

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1999\10\07@151947 by Nigel Goodwin

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In message <EraseME3.0.3.32.19991006021034.0096fc10spamspamspamBeGonepostoffice2.mail.cornell.ed> u>, Sean H. Breheny <RemoveMEshb7KILLspamspamCORNELL.EDU> writes
>I just want to make sure that we're talking about the same thing: I'm
>talking about the walkman that I buy at the corner store for $25, not some
>high-end specialty audio recorders or digital decks.

It applies to almost all recordable tape decks, including the vast
majority of recordable Walkman's.

>>amplitude modulates it. The bias is usually a fairly high signal level,
>
>How does adding two signals together (in a roughly linear system) cause one
>to modulate the other?

Stick a scope on a record head and have a look

>>up to 100 volts peak to peak, and the audio is very low, only a few
>>100mV. This makes it very hard to see the modulation on an oscilloscope,
>>sometimes you can just see it with the gain turned up and the vertical
>>shift fully one way :-).
>
>I don't know where in a normal tape deck there would be a 100Vp-p 100kHz
>supply!! I am thinking more and more that we are talking about different
>things.

It comes from a small (IF can sized) transformer, usually driven by a
single small transistor, although high end units sometimes use two or
more transistors. The actual level of bias required depends on the tape
in use, that's what the tape selection switches do, my Kenwood 3 head
cassette deck has variable bias, which you can adjust while you monitor
the recording off the third head. As a very rough guide, the more bias
you use the less treble you get, the adjustment technique is to compare
the source and recording, and adjust the bias so they sound as identical
as possible.

--

Nigel.

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