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The Macrovision protection introduces high amplitude pulses at the end of the vertical blanking interval. If the set has slow vertical retrace the Macrovision pulses will be seen. You will see the pulses on an oscilloscope or waveform monitor.
Either the signal can be modified to block the pulses from the last few VBI lines or else the vertical retrace blanking circuitry of the set modified to conceal the pulses. I did the former once with a circuit I built to block the active line during vertical retrace while gating in the horizontal sync.
It is funny how the composite analog video interface was used to convey all sorts of extra junk over the years.
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