Jim:
Thanks, this is the best explanation I've heard so far. Testing has shown
that the "dimensional 'memory'" can sometimes be activated by the
application of heat. A controlled heat-application process is often
effective in using this "memory" to resize tapes that have been slightly
stretched or deformed.
An interesting consideration is that the coefficient of
expansion/contraction in magnetic tape (per the National Media lab
testing)
is greater for moisture than for heat (at lower levels). In addition, the
low-molecular-weight oligomers have greater volume than the corresponding
polymers from which they were created by hydrolysis.
This brings up the question of whether the tape being measured for
dimensional change was hydrolyzed and/or stretched before or after the
recording was made. If the damage/decay occurred after the recording,
"baking" should return the tracks to a closer approximation of what they
were originally. If the tape was hydrolyzed or deformed before the
recording was made, "baking" could alter the dimensions of the tape to a
closer approximation of an undamaged/un-hydrolyzed state and alter
tracking
that was recorded on a deformed tape. Makes it kind of tricky. How do
you
accurately restore a tape to a warped or deformed set of dimensions?
Lance:
As to "re-baking" a tape, the only report I've seen that stated baking has
limited applications was retracted by the person who made it when they
discovered that their oven wasn't turned on when they re-baked. As to why
someone would want to re-bake a tape, well, much as we all try- we can't
always control what our clients do. We have often seen commercial firms
pull a spot off of a restored tape without making a transfer of the entire
recording. When they want a different part of the recording two years
later, they need to have the tape re-baked.
Peter Brothers
-----Original Message-----
From: Association for Recorded Sound Discussion List
[mailto:ARSCLIST@xxxxxxx]On Behalf Of jim@xxxxxxxxxxxxxxxxx
Sent: Tuesday, April 12, 2005 11:31 AM
To: ARSCLIST@xxxxxxxxxxxx
Subject: Re: [ARSCLIST] Baking tapes and high frequencies
In video tapes where the tracks are helical I have experienced
some problems. Specifically - it appears that the base film can
undergo a non-linear dimensional change that can cause the tracks
to have additional skew. This additional skew can cause
mistracking and lower RF which is definitely a problem. I did at
one time actually use ferrofluidic analysis and a very accurate
microscope and measured both the track width and the angle before
and after baking and there was a change. I did this in the course
of a forensic case where I had to carefully document what I did.
I cannot say that this is the case for ALL tapes. It could have
been just the few I was working with. I think that there are so
many variables that it would be difficult to make any
generalizations because there were so many different base films
and thicknesses used over the years, and there well may be an
impact in terms of aging and storage conditions... but based on
allot of experience over the years I do believe that in many
cases there will be some slight dimensional shift which will mean
nothing for longitudinal recording but can cause some issues in
Helical Recordings - particularly with tapes that are wide and
have long tracks - 1" type C for example - Umatic as well.
My hypothesis - and i have NOTHING to support it in terms of data
- is that some base film has a dimensional "memory" that goes
back to when it was first made. This phenomenon is well
documented for many different types of plastics. Once the jumbos
are made and the coating and slitting occurs - the exact
dimensions may not correspond in all areas. So when you bake the
tapes - expansion or contraction will not necessarily be linear
in the length dimension and the width dimension - and distortion
can occur. In effect the tape can dimensionally "twist". You can
think of it as looking at a piece of tape in front of you - and
the top edge of the tape may not dimensionally change exactly as
the bottom edge does - and that would effect the entire width and
change in different places. But this is a hypothesis in terms of
how it happens. I
I have noticed that when I did some experimentation the tapes
that had been baked had in some cases apparently lower RF then
before baking. I did not have the precise equipment to measure it
at that time (I do now). I believe that this lower RF may be due
to slight mistracking due to the tracks distorting as described
above. Video tracks in almost all cases are a fraction of the
size of audio tracks and are not longitudinal - but helical - and
therefore in my opining far more vulnerable to issues relating to
dimensional changes. This is particularly true with VTR's that
are not able to "flex" their heads to maximize RF in a track.
I would think that Azimuth recordings would be particularly
vulnerable to this type of dimensional instability but I have not
had to time to investigate - maybe some day.
Regarding the loss of high frequencies due to baking. My feeling
is that this would not be the case for many reasons. I would
suspect that any loss in high frequencies would be related to
dirt in the head gap - which if the tape is baked and not cleaned
might accumulate - particularly if the head was not in great
shape to start off with.
I can say this. It is a very easy experiment to do if you really
want to know for sure.
Jim Lindner
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