Method and apparatus for scrambling and descrambling video signals in a multichannel system

Claim 1

In a video communication system which provides an unscrambled composite video signal having a video information signal interrupted by (1) amplitude-detectable horizontal sync pulses occurring within a horizontal blanking interval which follows the scan of a horizontal line and (2) periodic sets of pulses, each such set including vertical sync pulses and occurring within a vertical blanking interval which follows the scan of a predetermined fixed number of horizontal lines, apparatus for limiting access by a receiver set to the video information contained within the video information signal of the unscrambled composite video signal, the apparatus comprising: a scrambler having the unscrambled composite video signal as an input, the scrambler comprising: amplitude modifying means for (a) reducing the amplitude of predetermined horizontal sync pulses to levels indistinguishable from the amplitude of the video information signal of the unscrambled composite video signal and (b) maintaining the amplitude of (1) at least some of the pulses occurring during each vertical blanking interval and (2) substantially all of the video information signal, the scrambler thereby providing a selectively suppressed scrambled composite video signal as an output; and a descrambler connected to receive the scrambled composite video signal as input, the descrambler comprising: means for deriving the time intervals of the amplitude-reduced horizontal sync pulses from the amplitude-maintained pulses occurring within the vertical blanking intervals, said video communication constituting a system wherein each periodic set of pulses includes a fixed number of adjacent broad pulsewidth vertical sync pulses and a plurality of narrow pulsewidth equalizing pulses, the scrambler further comprising: (a) means for determining a time period which contains a plurality of equalizing pulses, at least some of which are adjacent in time; (b) means for dividing the intervals between at least two adjacent equalizing pulses in the determined time period into time slots; and (c) flagging means for selecting one of a plurality of broadcast identifier flags and for generating a flag pulse during one of the time slots, the flag pulse in the time slot corresponding to the selected identifier flag, said flagging means further comprising: means for inserting the generated flag pulses into the selectively suppressed composite video signal when (a) the flag pulse occurs during a preselected time slot between two equalizing pulses, (b) the preselected time slot occurs during the determined time period, and (c) the determined time period occurs within a fixed relative time interval during each vertical blanking interval, said flag inserting means further comprising: modulator means for effecting a relative increase in the selectively suppressed scrambled composite video signal during the presence of an inserted flag pulse, said time interval deriving means comprising: means for producing a reference pulse at substantially the same relative time during each vertical blanking interval, said reference pulse producing means comprising: (a) means for integrating the vertical sync pulses; (b) means for comparing the output of the integrating means with a threshold level which is less than the maximum output integrated by the integrating means; and (c) means for signalling when the output of the integrating means exceeds the threshold level, (d) means, receiving the scrambled composite video signal as input, for providing a pip signal in response to either only the rising edge or falling edge of each pulse in the srambled video signal; and (e) means, having as inputs the output of the pip signal providing means and the output of the integrating means, for generating a pulse output when a pip from the pip providing means is first provided after the signalling means indicates that the threshold level has been exceeded.