Re: [ARSCLIST] Baking tapes and high frequencies

[jim@xxxxxxxxxxxxxxxxx]

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