🚀 go-pugleaf

In article <1992Sep2.041037.347@u.washington.edu> bunky@milton.u.washington.edu (Mike) writes:
>
>     
>     Does anyone know how macrovision allows a video tape to be watched on
>     a television but not on a vcr?  I recently looked at a copy protected
>     videotape and it appeared that there were 4 white bars on the last
>     8 or 9 lines in the vertical interval that periodicly (every 15 or 20
>     sec) faded in and out.  It looks like it's changing the average level
>     of the picture to mess up the vcr.  If that's the case could this be
>     fixed with a clamping circuit or some kind of dc restorer circuit?
>     If anyone knows the technical details of how the copy protection scheme
>     works I would be very interested. 

Macrovision inserts very bright pulses in the vertical blank interval.
Most VCR's use this when setting AGC video level, and get fooled into
thinking the video levle is much brighter than the picture area really
is. Most TV's don't. Dunno why.


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