Re: [ARSCLIST] Fw: [78-l] ^ Distortion on a cassette tape

[John Ross <johnross@xxxxxxxxxxx>]

To Norman Field (78-l):

I've had cassettes with this problem. I believe the original 
recording occurred with the cassette improperly inserted into a 
(probably portable) recorder, such that the tape did not pass between 
the capstan and the pinch roller, but on the "outside" of the capstan.

Because the capstan and pinch roller normally keep the tape moving at 
constant speed, this tape was at the mercy of the take-up spool's 
tension, which was trying to pull the tape at a higher speed, but it 
was held back by the take-up tension. The differential between the 
supply and take-up spools caused the change in speed.

As for the distortion, I'm guessing that the tape was slightly 
removed from the record head (or the record/play head) as it moved 
along. As the tape moved farther from the record head, the magnetic 
field from the head decreased, and so volume level dropped and the 
amount of distortion increased -- the exact rate would depend on the 
ever-changing geometry of supply spool, tape guides, capstan and take-up spool.

Alas, I don't think there is any way to eliminate this distortion, 
since it was created at the time of the (defective) recording process.

John Ross

At  6/21/2006 07:47 PM, you wrote:
> Since ARSCLIST has all the serious magnetic tape expertise, perhaps you
> can answer Norman's question (and I, or someone, can forward the solution
> back over to 78-L...)
>
> Steven C. Barr
>
> ----- Original Message -----
> From: "Norman Field" <jazz@xxxxxxxxxxxxxxx>
> To: "78-l" <78-L@xxxxxxxxxxxx>
> Sent: Wednesday, June 21, 2006 1:10 PM
> Subject: [78-l] ^ Distortion on a cassette tape
>
>
> > OK, it's well off topic, but there's a lot of expert folks up here. And
> I'm
> > curious as to how this problem occurred.... somebody here may well know?
> >
> > This C90 cassette (of speech) was recorded on a faulty machine, in that it
> > began very slow, & then ran slower & ever slower.
> >
> > In fact, it ended up so slow, that 32 minutes of one 45-minute side, when
> > roughly re-pitched, ran for about 75 minutes!
> >
> > Of course, it started off like the Chipmunks... and ended up like
> > hyper-active Chipmunks breathing practically pure helium & rushing around
> in
> > hopeless confusion... 8^}
> >
> > The starting speed, when roughly halved, gave perfectly acceptable
> results,
> > as was expected.
> >
> > But: as the speed fell & fell, finally requiring ~100% pitch reduction, a
> > distortion gradually crept in.
> >
> > So that by the end, the voices were fragmentary & broken, sounding like a
> > conversation half-heard in a dream, as it were. And totally
> incomprehensible
> > of course.
> >
> > Why should the material have become distorted?
> >
> > If the cells in the cassette recorder were losing voltage, that would
> > account for it... but surely they'd never drive the motor for half an hour
> > in that state?
> >
> > The only thing I can come up with is that the tape was somehow 'saturated'
> > with signal, because it was passing far too slowly over the recording
> head?
> > And yet the modulation level on the tape was very low...
> >
> > Has anyone here come across such a phenomenon before?
> >
> > Previously, I've recovered stuff recorded on reel tape at 15/16 ips (~2.4
> > cm/sec) when the lowest speed I have here is 3 3/4 ips (9.5 cm/sec), which
> > requires a similar re-pitching, and it's been pretty well OK.
> >
> > Norman.
> >
> > P.S. At least I'm not charging my client for the 25% of the stuff I did
> > manage to recover. We here on this list, many of us the 'progeny' of John
> > R.T. Davies, don't like to be beaten! Come to that, I wonder what he would
> > have done..?
> >
> > -------------------------------------------
> > To unsubscribe, see http://www.78online.com
> >