High security subscription television transmission system

摘要

A system for scrambling and descrambling a composite video signal having video program information, vertical sync pulses, and horizontal sync and blanking information includes an encoder and a decoder. The encoder scrambles the video signal by selectively biasing the horizontal information, selectively inverting the video program information and the non-biased horizontal information. The scrambled and nonscrambled portions of the video signal are modulated with data identifying the manner in which has been scrambled. The decoder detects the data identifying the manner of scrambling of the video program information. The decoder reinverts the sync by a fixed sequence which is triggered by normal sync. The decoder also selectively debiases the biased horizontal information, by a fixed sequence which is triggered by the normal sync, thereby producing the original composite video signal.