Abstract: Even though television signals are transmitted as analog signals over cable from the transmitter to the receivers of customers of the cable system, programs for which charges to the customers are payable are scrambled at the transmitter and unscrambled at the receiver on a digital basis, requiring digital-to-analog and analog-to-digital conversion both at the transmitter and at the receiver. In some cases there is such conversion at the receivers already for normal reception processing. By the provision of digital scrambling and unscrambling, it is possible to use a digital key which is subject to variation by a random signal generator, this key being transmitted during a one-line period that forms part of the vertical blanking interval. The unscrambling circuit has a built-in microprocessor so that its mode of operation corresponding to that of the control circuit at the transmitter is not readily detectible.

Abstract: In a television receiver for a transmission system in which the video signal is scrambled to allow only for its authorized reception, a video signal processor (8) is provided in which the scrambled video signal is de-scrambled. A low rate encrypted signal present at an output (11) of a sound/data signal processor (10) is decrypted and employed to periodically set (say once every one or two seconds) a master pseudo ramdom pulse generator (12B) producing an output of pseudo random configuration at the frame rate which is used to periodically set a video slave pseudo random pulse generator (15) to produce an output of pseudo random configuration at line frequency. This is applied to an input S of the processor (8) and forms the scrambling key for the line-by-line de-scrambling of the video signal.

Abstract: In an apparatus for descrambling a scrambled television signal, an elastic surface wave filter is used. This elastic surface wave filter has an interdigital electrode having an electrode pattern similar to at least portion of the waveforms of a luminance signal section of the scrambled television signal to be input. The elastic surface wave filter generates a response pulse only when the waveforms which are analogous to the electrode pattern of the interdigital electrode of the scrambled television signal were input, thereby descrambling the television signal using this response pulse.

Abstract: A television signal is scrambled by suppressing horizontal and vertical synchronizing signals while leaving unsuppressed a reference level during a selected reference interval. Descrambling of the received scrambled signal is achieved by comparing the unsuppressed reference level with the suppressed synchronizing signals and by adding a level representative of that comparison to the suppressed synchronizing signals, thereby to restore the latter to their proper levels.