NotTaR of Television Sets : AGC and copy protection                 
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AGC and copy protection

(From: Jeroen H. Stessen (Jeroen.Stessen@philips.com).)

  1. RF-AGC which compensates for different signal strength at the aerial, it measures RF amplitude and is NOT sensitive to video contents because with negative modulation the sync is the peak and is constant, this AGC will not work on CVBS (baseband video) inputs.

  2. Video-AGC which normalizes baseband signals which enter AFTER the tuner-IF. A.o. this compensates for different signal strengths when you connect two VCRs together. It measures peak-white, so it IS sensitive to video content and thus to the Macrovision pulses.

And: A television does NOT have a video-AGC, unless you want to call the beam current limiter circuits an AGC. (Exception: the Secam-L system with positive modulation requires an RF-AGC which measures peak-white instead of peak-sync.)

The RF-AGC does not see the peak-white of the anti-copy pulses. If you connect the VCR to the TV via the CVBS (baseband) input, then the RF-AGC is not even in the path. Still, it may be disturbed. But the sync separator may see the extra inserted Hsync pulses, and due to the phase disturbance the video clamping may be disturbed too.