🚀 go-pugleaf

userDANC@mts.ucs.UAlberta.CA (unknown) writes:

>Is anyone aware if a scheme exists to 'copy protect' commercial
>VHS tapes?  In running a recent tape through a second video recorder,
>it looked as if the signal had become scrambled, though viewing it on
>a normal television worked fine.  As a second year Electrical Engineering
>student, I would be very curious in learning the details of the protection
>scheme.  (looks like I have a bit of catching up to do).
>Dan

Yes Dan there is a copy protection scheme on VHS tapes. The name is called
Macrovision, and the principle is somewhat simple. On I believe Line 14 of
Field 1, you will notice some flashing white lines. This is the copy
protection scheme. The method works on the demodulation/remodulation mode. The
TV set is happy because it does not remodulate the baseband video. However,
when you try to copy the tape, the 2nd VCR has to remodulate the baseband video
into FM.  I am guessing after this. The copy scheme senses this and increments
or decrements a count, which then enables the protection portion of the code.
Then, when the copied tape is demodulated into baseband, the setup portion of
the video constantly moves up and down, giving the VCR fits and the TV.

On a purely thereoretical basis, I would suppose that someone could defeat the
scheme by (a) recording in FM only, or (b) running the original tape through a
processing amplifier, which will strip out the vertical interval and replace
it with it's own. I would think this would work, I don't know for a fact.


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