Correcting varying speed tape

Archived from groups: rec.audio.tech (More info?)

Suppose you have an analog music tape that plays at varying speeds (or
perhaps was recorded at varying speeds) so that the frequencies are shifting
higher/lower than they should be, are there mathematical techniques to
detect and correct the shifts? Even though the frequencies in the music are
changing all the time, I thought it still might be possible to detect
synchronized shifts in all frequencies over time, which is what speed
changes would cause. There is no correct reference recording to compare
with.

David

Archived from groups: rec.audio.tech (More info?)

"David White" <no@email.provided> wrote in message
news:hFN3d.1204$pl.39386@nasal.pacific.net.au

> Suppose you have an analog music tape that plays at varying speeds (or
> perhaps was recorded at varying speeds) so that the frequencies are
> shifting higher/lower than they should be, are there mathematical
> techniques to detect and correct the shifts?

Their must either be a fixed reference in the recording, or presumptions
must be made about the content of the recording.

>Even though the
> frequencies in the music are changing all the time, I thought it
> still might be possible to detect synchronized shifts in all
> frequencies over time, which is what speed changes would cause.

This would seem to be an interesting problem in signal processing and
artificial intelligence.

> There is no correct reference recording to compare with.

But, you have now presumed that a reference of sorts exists. Your
presumption is that any frequency shift that affects all tones equally is
due to failings in the transmission medium, and can therefore be removed
without diminishing sound quality.

To do this right you'd have to process a lot of tones concurrently, detect
the FM in each, and determine which FMs correlated, and which did not. The
simple part would be applying corrective FM to the over-all signal. based on
the FM that correlated.

The relevant process that does work involves recovering the origional tape
recorder's bias signal. The signal is corrected as indicated by FM of this
bias signal. Tape heads with very small gaps were used.

Archived from groups: rec.audio.tech (More info?)

"Barry Mann" <zzzz@zzzz.zzz> wrote in message
news:41502d1a$1$avgroveq$mr2ice@wcnews.cyberonic.com...
> In <hFN3d.1204$pl.39386@nasal.pacific.net.au>, on 09/21/04
> at 01:55 PM, "David White" <no@email.provided> said:
>
> >Suppose you have an analog music tape that plays at varying speeds (or
> >perhaps was recorded at varying speeds) so that the frequencies are
> >shifting higher/lower than they should be, are there mathematical
> >techniques to detect and correct the shifts? Even though the
> >frequencies in the music are changing all the time, I thought it still
> >might be possible to detect synchronized shifts in all frequencies
> >over time, which is what speed changes would cause. There is no
> >correct reference recording to compare with.
>
> As others indicated, this is a difficult problem, but you could get
> lucky.
>
> If there is a recognizable signal on the tape, perhaps 50 or 60 cycle
> hum (or a harmonic), you could lock on an "correct" it.
>
> ----
>
> The pitch changes on the original tape are probably not random, but are
> likely to be obscure. Below are some possibilities. (note that any or
> all of these could be a property of the record and/or playback
> machine):
>
> Many cassette deck motors change speed after being off for a while. In
> my observation they usually slow down after a few seconds.
>
> There could be some speed variations related to the size of the tape
> pack on one or both reels.
>
> Temperature.
>
> Power line frequency (if one or both machines reference the power line
> and it is unstable).
>
> Power line voltage (only for low quality machines)
>
> Tape condition. (The adhesive on some older tapes may be leaching to
> the surface and change the coefficient of friction on an inch by inch
> basis. -- physically cleaning or conditioning the tape may help if it
> is only a playback problem) Keeping the playback heads clean is also a
> help.
>
> I don't know your level of experience, but it is common for beginners
> to confuse high frequency level changes (due to worn playback heads or
> wandering tape) with speed changes. Be sure you are barking at the
> right dog.
>
> ---
>
> Once you've figured out if you have a record and/or playback problem
> and can generate some sort of function to track the changes, applying
> the function to the playback deck is not so hard.
>
> "Pitch extraction" algorithms are being refined, but I'm not aware of
> any that would be foolproof for this application.
>
> If the tape is extremely important and you have the time, you could
> construct a computer file that incrementally controlled the playback
> pitch (either on a real tape deck or a computer file) and keep fussing
> with things till you are satisfied or run out of time.
>
> -----------------------------------------------------------
> spam: uce@ftc.gov
> wordgame:123(abc):<14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
> 13> (Barry Mann)
> [sorry about the puzzle, spammers are ruining my mailbox]
> -----------------------------------------------------------
>

If the tape was made on a high enough quality machine, and at a high enough
speed, you may be able to recover the bias signal by slowing the tape down
enough - 100khz - 50kHz - 25kHz - 12.5k not likely to actually work very
well... perhaps finding a 60Hz buzz would be a better idea

Archived from groups: rec.audio.tech (More info?)

"Carl Valle" <cwvalle@swbell.net> wrote in message
news_%3d.23379$DR3.5674@newssvr24.news.prodigy.com
>
> If the tape was made on a high enough quality machine, and at a high
> enough speed, you may be able to recover the bias signal by slowing
> the tape down enough - 100khz - 50kHz - 25kHz - 12.5k not likely to
> actually work very well... perhaps finding a 60Hz buzz would be a
> better idea

Bias on legacy machines was often in the 35-70 KHz range. If the tape was
originally recorded at 7.5 or 15 ips, at 1 7/8 the bias would be at 1/4 or
1/8 the origional frequency. Other than modest losses in SNR there's no
unforgivable sin associated with using a head built for too-narrow tracks
like a cassette machine playback head, to get response @1 7/8 going up to
maybe even 15 KHz. 15 KHz @ 1 7/8 corresponds to 60 KHz at 7.5 ips and 120
KHz at 15 ips.

So find an open reel machine that played 1 7/8, shoehorn in some cassette
heads and transcribe away!

Archived from groups: rec.audio.tech (More info?)

In <pU24d.1271$pl.41218@nasal.pacific.net.au>, on 09/22/04
at 09:32 AM, "David White" <no@email.provided> said:

[ ... ]

>I haven't actually heard this recording yet. I've been told that it
>sounds "warbly", and that altering the speed of some sections improved
>it a little. But I also have a recording of my own containing what
>sounds very like a speed change in one spot, so I got to thinking
>about the problem, and I'll probably apply it to that spot at least if
>I can figure out a way to do it.

As you work with the tape, try to tie the speed changes with a tape
recorder characteristic. For example, if a reel bearing is tight or the
tape is rubbing a reel, then speed changes will be tied to reel
rotation (remember that the tape on the reel is essentially a spiral of
Archimedes and the spacing between the speed changes is tied to the
current pack radius.) If the supply reel bearing is tight, then the
tape may slow down as the pack shrinks.

"Warbly" suggests that the speed changes are fairly rapid. If this is
true, then your speed control system will need to be able to respond
accordingly.

-----------------------------------------------------------
spam: uce@ftc.gov
wordgame:123(abc):<14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
13> (Barry Mann)
[sorry about the puzzle, spammers are ruining my mailbox]
-----------------------------------------------------------

Archived from groups: rec.audio.tech (More info?)

"Arny Krueger" <arnyk@hotpop.com> wrote in message
news:QtOdnZK24qWqJc3cRVn-iQ@comcast.com...
> "Carl Valle" <cwvalle@swbell.net> wrote in message
> news_%3d.23379$DR3.5674@newssvr24.news.prodigy.com
> >
> > If the tape was made on a high enough quality machine, and at a high
> > enough speed, you may be able to recover the bias signal by slowing
> > the tape down enough - 100khz - 50kHz - 25kHz - 12.5k not likely to
> > actually work very well... perhaps finding a 60Hz buzz would be a
> > better idea
>
> Bias on legacy machines was often in the 35-70 KHz range. If the tape was
> originally recorded at 7.5 or 15 ips, at 1 7/8 the bias would be at 1/4
or
> 1/8 the origional frequency. Other than modest losses in SNR there's no
> unforgivable sin associated with using a head built for too-narrow tracks
> like a cassette machine playback head, to get response @1 7/8 going up to
> maybe even 15 KHz. 15 KHz @ 1 7/8 corresponds to 60 KHz at 7.5 ips and
120
> KHz at 15 ips.
>
> So find an open reel machine that played 1 7/8, shoehorn in some cassette
> heads and transcribe away!
>
>

Yeah that might actually work - I dont know how old the machine was, my
akais have narrow gap glass heads but the bias is 120kHz and it won't run at
1.825. Actually I can make my 630 run at that speed, but it isn't equalized
for it and it's unstable. The problem with most reel machines is you double
the wow and flutter when you halve the speed, unless they are direct drive.
You could copy up to higher speed tape to slow it down. Say take a 3 to 15
then drop back to 3. Then I guess you would have to have some way of
continuously varying the sample rate based on the pitch change. What happens
when the flutter and the wow are different frequencies, which they should
be. I guess you could use multiple passes correcting every periodic you can
find. I think i would just use a pitch control and try to fix gross pitch
problems by ear and call it a day.
Carl

Archived from groups: rec.audio.tech (More info?)

On Tue, 21 Sep 2004 22:37:20 -0400, zzzz@zzzz.zzz (Barry Mann) wrote:

>As you work with the tape, try to tie the speed changes with a tape
>recorder characteristic. For example, if a reel bearing is tight or the
>tape is rubbing a reel, then speed changes will be tied to reel
>rotation (remember that the tape on the reel is essentially a spiral of
>Archimedes and the spacing between the speed changes is tied to the
>current pack radius.) If the supply reel bearing is tight, then the
>tape may slow down as the pack shrinks.
>
>"Warbly" suggests that the speed changes are fairly rapid. If this is
>true, then your speed control system will need to be able to respond
>accordingly.

I doubt anyone was contemplating a MECHANICAL speed-correction system
:-) But it's an intrieging idea!

Archived from groups: rec.audio.tech (More info?)

>
>On Wed, 22 Sep 2004 09:07:43 +1000, "David White" <no@email.provided>
>wrote:
>
>>Thanks for the responses. There are some promising ideas there. Locking
>onto
>>a buzz or the bias would do the trick very well.
>
>I was going to suggest using the bias signal, but had assumed that it
>was too high in frequency to get recorded. Is it in fact recorded?
>Would it need a special playback head to retrieve it? If possible,
>this would seem the ideal system.
>

The bias signal is on the tape and i have been able to hear it on machines
played back at very slow speesd such as while scrubbing on a reel-reel.

Recovering the bias is another thing altogether.

Richard H. Kuschel
"I canna change the law of physics."-----Scotty

Archived from groups: rec.audio.tech (More info?)

"Laurence Payne" <l@laurenceDELETEpayne.freeserve.co.uk> wrote in
message news:m5g2l0te4g2ok1sp5dquun151rin1nvadk@4ax.com
> On Wed, 22 Sep 2004 09:07:43 +1000, "David White" <no@email.provided>
> wrote:
>
>> Thanks for the responses. There are some promising ideas there.
>> Locking onto a buzz or the bias would do the trick very well.
>
> I was going to suggest using the bias signal, but had assumed that it
> was too high in frequency to get recorded. Is it in fact recorded?

Yes, and there are accounts on the web of commercial audio applications of
bias recovery for the purpose of speed correction.

> Would it need a special playback head to retrieve it?

I believe that the specific example I am thinking of used a special playback
head. However, playing the tape at a low speed could work as well.

> If possible, this would seem the ideal system.

In the cases I'm thinking of, the demos showed dramatic improvements.

After some searching around, I found the web page I was thinking of:

http://www.plangentprocesses.com/


Here's the RAP thread that brought this to my attention:

http://groups.google.com/groups?se [...] .panix.com

Credit to the encyclopedic audio knowledge of Scott Dorsey (again!). ;-)