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Abstract

A method and system for scrambled transmission of an information signal having a regular, repeating information rate from which a receiver of the information signal derives synchronizing information as in a television video signal. The information signal is summed with a scrambling signal having a frequency which is a function of the information rate plus an error quantity within a predetermined bandwidth, namely that of the synchronizing circuitry in the normal information receiver. The sum of the information signal and the scrambling signal is transmitted as the scrambled information signal. In a television scrambling system, the information signal is a television video signal and the information rate is a video synchronizing rate, preferably the horizontal line rate. The television information signal also includes an audio signal which is scrambled by mixing the audio signal with a carrier signal realted to the video synchronizing rate, to produce a lower sideband which is transmitted as the television audio signal. The frequency of the scrambling signal F.sub.j is defined by the equation F.sub.j =A.multidot.F.sub.h +B, where A is an integer, B is the value error quantity, and F.sub.h is the information rate. To increase scrambling security, the frequency of the scrambling signal may be modified periodically. Also disclosed is a method and system for decoding or unscrambling the scrambled information signal.

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First Claim