A video signal scrambling system for use with a pay television system in which video signals are scrambled so that only specified subscribers can receive the program. Inversion of the video signal level for scrambling is performed only when the average gradation of the video signals varies relatively largely so that flickering is eliminated from the reproduced picture.
A video cipher processing system comprising a detecting means for detecting average tone variation of TV video signals, an extracting means for extracting at random at least some of outputs from the detecting means as a result of the detection and a ciphering means for ciphering video signals by inverting or non-inverting polarities of video signals in response to outputs extracted at random by the extracting means.
A video picture signal scrambling system for subscription braodcast or cablecast television systems which can scramble video signals without adversely effecting pictures received at a subscriber's terminal. The system includes a detector for detecting the brightness level of the picture signal over a predetermined interval, a comparator for comparing the brightness level with a first predetermined reference value and a scrambling circuit responsive to the comparator for inhibiting reversion of the picture signal when the brightness level exceeds the first reference value, and for selectively reversing the picture signal in response to changes in the brightness level in successive intervals of the picture signal.
A means for scrambling video signals by the application of a periodic scrambling waveform which has the effect of altering the normal amplitude relationship between horizontal sync and the video picture portion of a horizontal line uses the comparison between the brightness level of the video signal during a predetermined time period with a reference as a means for varying the amplitude of the periodic scrambling waveform.
A means for coding and decoding video signals by the application of one of a plurality of available scrambling signals includes means for comparing the brightness level during a predetermined time period of the video signal with the brightness level of the video signal over the succeeding predetermined time period. There are means for changing from one scrambling signal to another if the difference between brightness levels in successive predetermined time periods exceeds a given value.
A technique for toggling the coding mode in a television signal encoder comprises establishing a rectangular window defining a relatively small and substantially centrally located portion of the field image intervals of the television signal. The coding mode of the encoder is toggled whenever the APL of the television signal occurring during the window exhibits a change for successive video fields exceeding a predetermined threshold level.
