Re: [ARSCLIST] Baking tapes and high frequencies

["Peter@xxxxxxxxxxxxx" <lists@xxxxxxxxxxxxx>]

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limited applications was retracted by the person who made it when they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
>
>