Re: [AV Media Matters] Digitizing Audio and Video

[rohre@ARLUT.UTEXAS.EDU]

Good discussion going on about when tapes need rewinding.  Since
converting
to back coated tapes in the early 70's; those of us in the scientific
instrumentation recording field have rarely had to cope with cinched
tapes
in storage, rewinding,etc.

As Jim Wheeler said, in those cases where a tape packing problem from
improper recorder tension happens, you simply rewind the tape on a good
machine a few times until it relaxes and winds properly.  The worst two
cases we ever encountered from tapes coming from outside our facility
required unwinding the affected tape into a lint free box, and allowing
its
mylar to relax, then carefully rewinding it onto a precision reel to
well
center it onto the hub.  Proper storage environments, temperature and
humidity, keep the tape from having changes in length while wound on the
reel.

However, backcoating does present a long term storage issue.  We have
recently been contracted to revive some 28 year old analog data on 14
track
one inch tapes.  We find that sticky shed syndrome has invaded these
tapes
even with laboratory storage.

All the tapes that old have had to be baked before use.  Even brand new,
used once working test tapes 10 years old, have had some sticky shed and
required baking.  The main culprit is shed of the back coating because
of
hydrolysis of the urethane binder.

If you play mylar tapes that are old and have not been used in years,
they
are likely to gum up guides and transfer backcoating gunk to oxide
surfaces
and gum up heads or cause head clogs during playback.

A good convection oven, or a laboratory oven, can easily be used to
recondition the tapes.  We have found the older ones need longer baking
periods and do seem to improve if they are baked on open sided metal
reels
both heads out and tails out. (Bake once, let rest a couple of days,
mount
and play forward (not over heads), and then bake them on the take up
reel.)
We are baking at 115 degrees F, but the main thing is to bake in a dry
laboratory, and immediately seal up the tape to prevent humidity
affecting
it.  Work out of metal cans or sealed tape boxes to limit the dry tape's
further humidity exposure even in office and lab environments which are
air
conditioned, unless you rigidly control relative humidity to low levels,
more favorable to tape.

We will broad band digitize the analog tracks once the tapes are stable,
and
then data can migrate from one digital format to another.  For our
purposes,
a minimum of 12 bits and more desirable, 16 bits are the digitizers
used.
Other analog signals may need the higher bit accuracy of audio digital
formats.

Stuart Rohre
Applied Research Labs
Univ. of Tx

Moderators Comment:
I am not trying to make a habit of making these comments - but...
I have long thought that backcoating can be a "bad actor" in tape
deterioration, particularly in cases of extreme sticky shed. I have
mentioned this to many people over the years, and I have gotten many
reasons why people think that it isn't. Nevertheless my personal
experience suggests that it is at least an element in some cases of
severe deterioration. Without some organic chemistry it would be hard to
"prove", but it certainly looks that way on some tapes.

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