Television signal scrambling method for CATV system

摘要

A television signal scrambling and descrambling method and apparatus for use with a community antenna television system in which the scrambling mode employed is changed from time to time so that one cannot illegally receive the transmitted signals over a long period of time if only one of the scrambling techniques employed is decoded. In the central facility, a plurality of scrambling circuits are provided, one for each scrambling mode desired. A selected one of the scrambling circuits is selected by a switch device which is controlled so as to switch among the scrambling circuits as desired. An identifying signal is transmitted which indicates which of the scrambling circuits is active. In the terminal units at each subscriber's home there is provided a corresponding set of descrambling circuits. The received signal is coupled to the inputs of the descrambling circuits and the appropriate one of the descrambling circuits selected in accordance with the demodulated identifying signal.