Wednesday, October 24, 2012

How VHS and other magnetic tapes degrade

VHS, VHS-C, SVHS, Hi 8, Digital 8, and even DV all share one thing in common: they are magnetic media. And the trouble with magnetic charge ("remanence") is that it is fundamentally impermanent. But that's just the beginning of the trouble for video tape, for there is a long list of ways it can deteriorate:
  • Magnetic particles gradually lose their charge, in a process called remanence decay. The rate depends somewhat on the exact chemistry of the particles used in the tape, but in general if this happens, you can expect some color shift toward weaker hues and loss of detail overall.
  • Magnetic particles may be accidentally demagnetized. This can be from storing too near a magnetic source (like an audio loudspeaker) or even from the playback machine itself, whose heads can be become partically demagnetized if not maintained perfectly. With a poorly maintained VCR, every playback actually erases information from the tape!
  • The lubricant in the binder layer is used up, with each playback. As it erodes, the binder layer itself takes on more wear, which can directly affect the magnetic particles and cause information loss.
  • The binder layer can become a sticky, unplayable mess. The binder's polymers will absorb water (in even a moderately humid environment), in a process known as hydrolysis, and eventually delaminate. Engineers often refer to this as sticky-shed syndrome. Trying to play an affected VHS, VHS-C, or other magnetic tape is an invitation to damage of both the tape and the playback machine.
  • The backing and substrate can become stretched, from multiple rewindings and playback. This causes tracking errors that can dramatically reduce playback quality.
  • Successive recordings can lose information and synch signals. As an analog medium, each generation of recording loses substantial information. If the tape you are trying to preserve was in fact a 2nd or 3rd generation copy, it has already irretrievably lost a great deal of information (see this light-hearted VHS example of what is of course a serious problem).

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