Pay TV system

Claim 1

Apparatus for coding video information arranged in a plurality of lines comprising means for selectively inverting video information in groups of lines, each group of lines being less than a full field and consisting of one or more lines, a random timing device to control the number of lines in successive groups, said random timing device including a plurality of AND gates and a random data generator connected to each gate, a line counter connected to each AND gate, with any AND gate providing a video inversion signal when it has simultaneous inputs from said line counter and random data generator, the video signals in one group being inverted and the video signals in the succeeding group being non-inverted, said random timing device being reset to an initial condition at the start of each field, means for converting audio information into a digital signal, and means to add the digital signals of said random data generator and the converted audio signals to provide a coded audio signal.