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RWG
Guest
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 Posted:
Mon Nov 29, 2004 6:19 pm Post subject:
Re: Cassette won't play, can't see why! |
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| Quote: | I've heard traces of a bias signal when playing back some commercial
tapes at low speeds. That could, in theory, be used to digitally
reconstruct a perfectly timed playback. The bandwidth of the sampling
rate would have to be very large, though. You'd probably need 24
bits/channel with a 192KHz sampling rate to find that faint tone in a
tape wobbling along with no capstan. 16 bits sampled at 96KHz might
work if the tape was slowed down and the signal got a good amount of
analog enhancement.
It's probably not worth the effort. Any tension on the reels crushes
the inner windings. It would be something you'd do to make a copy
before throwing the tape out. The original bias frequency might not
always be constant either.
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I don't think that would work all that well, would probably leave some
significant speed error, and IAC is probably not necessary. (It's an
intriguing idea, though, and might be doable--it would never have occurred
to me.) If you have a way of monitoring the size of the tape pack and the
speed of the reels, you can probably work it out. In fact, if you know the
maximum and minimum sizes of the tape pack, and the duration of the tape at
1.875 ips, you should be able to calculate how the takeup reel speed should
vary over the course of playback. You would just need to apply enough torque
to the feed reel to maintain a small, constant tension, and control
everything with the takeup reel.
There are two ways you could do that. Theoretically (as all this is really,
I'm sure you realize!), you could rig the tape machine to follow such a
speed profile, adjusting the takeup reel speed to keep the tape speed at the
head a constant 1.875 ips. OR you could just take up the tape at some
constant speed, and apply the corrections later in a digital editor.
However, you would have to also vary equalization over the course of the
playback. But again, I'd rather tackle working out some filtering curves for
a digital editor than sit and try to modify the mechanical and electronic
inner workings of a tape deck. I'm sure most others will feel the same.
But back to the real world, I think tonight I'm going to have a nice dinner,
get settled in and get my thoughts focused, and carefully work on winding
this tape from the existing shell to a new one. If that doesn't work, I
don't know what to do.
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Arny Krueger
Guest
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| Posted:
Mon Nov 29, 2004 6:19 pm Post subject:
Re: Cassette won't play, can't see why! |
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"Kevin McMurtrie" <mcmurtri@dslextreme.com> wrote in message
news:mcmurtri-1C76B8.23121923112004@corp-radius.supernews.com
| Quote: | In article <cnvpk5$fhq$1@gargoyle.oit.duke.edu>,
"RWG" <rgarrison1@nc.rr.com> wrote:
You really cannot use the reels on a cassette to control the speed
since the take-up reel gets bigger as the tape plays, so it will
speed up the tape. The capstan is there specifically to ensure a
constant playback speed.
You should try this tape in yet another tape deck, just to be sure.
I kind of ruled out tape decks since you had two decks that didn't
like the tape.
I know it's kind of a long shot, but I could picture a sophisticated
enough system could monitor the relative speeds of the two reels,
and compute the proper speed to deliver 4.75 mm/sec between the
reels.
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This has been done with tapes operating in the digital domain.
| Quote: | But I've not investigated whether anyone makes such a deck.
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As a product, I doubt it. The last new idea in cassette decks that I heard
of involved using DSP signal processing to compensate for amplitude-domain
issues. Prototypes were reported, but nothing seems to have made it to
market.
| Quote: | Another possibility BTW, IF you could get the capstans to actually
stay out of the way, would be to find a digital editor which would
apply just the right constantly varying speed correction. Again, I
don't know how hard this is to implement.
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Something like this has been done in the context of recovering good sound
from defective analog master tapes.
| Quote: | It might indeed be worth trying another tape deck first!
I've heard traces of a bias signal when playing back some commercial
tapes at low speeds. That could, in theory, be used to digitally
reconstruct a perfectly timed playback.
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That was done in the technology I just mentioned.
| Quote: | The bandwidth of the sampling
rate would have to be very large, though. You'd probably need 24
bits/channel with a 192KHz sampling rate to find that faint tone in a
tape wobbling along with no capstan.
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Note - you can roll the tape as slowly as you want, if you use a modified
deck. This would move the bias frequency down to something that you could
easily recover with conventional heads and playback electronics. |
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Richard Crowley
Guest
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| Posted:
Tue Nov 30, 2004 6:06 pm Post subject:
Re: Cassette won't play, can't see why! |
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"RWG" wrote ...
| Quote: | It's relatively easy to transfer a tape pack to a new shell.
Since the tape in question is so valuable, practice on a few
junk tapes first.
Well, I've done it, by the spooling it reel-to-reel by hand, without
opening the original case; and the results worked well,.....
Thanks to all for your discussion and recommendations. Even if your
solution was not the one that worked for me, it stimulated creative
thinking on my part, and helped me to get to success.
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Just curious why you rejected the method of opening the original
shell and just transfering the reels to a screw-together shell?
IMHO, subjecting such fragile tape (particularly a valuable copy)
to manual spooling is more dangerous than transfering the whole
reels. |
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RWG
Guest
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| Posted:
Tue Nov 30, 2004 6:07 pm Post subject:
Re: Cassette won't play, can't see why! |
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| Quote: | Just curious why you rejected the method of opening the original
shell and just transfering the reels to a screw-together shell?
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Because it would have required destoying the original case, which was not
screwed together but apparently put together with some kind of
plastic-welkding process. I wasn't even sure where the weld points were.
| Quote: | IMHO, subjecting such fragile tape (particularly a valuable copy)
to manual spooling is more dangerous than transfering the whole
reels.
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Oh tell me about it! As I said, it was quite tricky to do, with the tape at
times wanting to flop all over the place. I got the tape started, then put
the shell together and spooled from one closed shell to another. (It took
about as long as playing the tape would.) Then I opened the case again,
carefully so as not to disturb the tape any more than necessary, and hooked
up the other reel, and put it back together.
Worse, the tape was recorded right up to the end of the tape at a couple of
points, with no leader. Fortunately, I recorded those same stretches
previously, and I don't think they have any wow problems in those regions. I
also have a second-hand copy that someone loaned me years ago and I
digitized, so I can fall back on that if there are any dropouts. In fact,
there is a small one in the middle of the tape (not due to handling) that I
may wind up fixing using the old copy. |
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RWG
Guest
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| Posted:
Tue Nov 30, 2004 6:07 pm Post subject:
Re: Cassette won't play, can't see why! |
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| Quote: | It's relatively easy to transfer a tape pack to a new shell. Since the
tape in question is so valuable, practice on a few junk tapes first.
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Well, I've done it, by the spooling it reel-to-reel by hand, without opening
the original case; and the results worked well, except that the tape pulled
loose from the hub at each side change. (But that didn't stop me from
ripping the audio, which is what I really cared about.) Looking back on it,
I do think this was the best way to go, of all the alternatives (at least
all the ones that didn't involve a whole research project). But I wouldn't
call it exactly "easy." You need to choose a time when your nerves are
steady, and to give this your undivided attention.
Thanks to all for your discussion and recommendations. Even if your solution
was not the one that worked for me, it stimulated creative thinking on my
part, and helped me to get to success.
--Ronald W. Garrison |
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Richard Crowley
Guest
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| Posted:
Wed Dec 01, 2004 6:08 am Post subject:
Re: Cassette won't play, can't see why! |
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| Quote: | Just curious why you rejected the method of opening the original
shell and just transfering the reels to a screw-together shell?
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"RWG" wrote ...
| Quote: | Because it would have required destoying the original case, which was not
screwed together but apparently put together with some kind of
plastic-welkding process. I wasn't even sure where the weld points were.
|
Usually just spot welded in a few places. You can frequently pop
them apart with a good sharp knife. Even if they are welded all the
way around a Dremel tool with a tiny saw blade will do a very
smooth job of splitting it open. Even if you have to saw it apart,
still preferable to manually spooling from one shell to another,
at least IMHO. |
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