SUBSCRIPTION TELEVISION APPARATUS AND METHODS

摘要

Subscription apparatus and methods wherein an encoder begins inversion of a video signal during the last horizontal trace line of a randomly selected vertical interval and continues to invert trace lines in the following field of the video signal. The encoder also suppresses the amplitude of randomly selected horizontal blanking pulses only during fields of the video signal while maintaining unaltered the absolute amplitude differential between the horizontal blanking pulses and associated horizontal synchronization pulses. A decoder reinverts trace lines of the field following detection of an inverted last horizontal trace line of a vertical interval. The decoder also selectively rebiases the video signal to restore the horizontal blanking pulses upon detection of a suppressed amplitude during the front porch of a horizontal blanking pulse. Reinversion of the trace liens and rebiasing of the blanking pulses is achieved by selective reference biasing of a video modulator circuit in the decoder.