Scrambler and decoder for secure television system

摘要

A scrambler for a television system alternately inverts the phase of the carrier which is modulated by the television signal. A binary coding signal controls the manner of the carrier-phase inversion. Phase inversion is obtained by combining the modulated normal carrier with an out of phase carrier having a maximum peak amplitude greater than the peak amplitude of the modulated carrier. The coding signal also frequency modulates the carrier for the scrambled television signal thereby providing a key for decoding at the television receiver. The decoder at the receiver reproduces the coding signal from the scrambled signal, removes the frequency modulation and combines the scrambled signal with a reproduced normal carrier at the time intervals corresponding to the presence of the phase-inverted carrier. An automatic gain control circuit insures that the decoded television signal has constant sync pulse amplitude for normal carrier intervals and inverted phase carrier intervals by controlling the reproduced normal carrier amplitude as a function of sync pulses in the normal carrier portion of the scrambled television signal.