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Abstract

An in-band television signal sync suppression scrambling system has two sync suppression modes which are aperiodically alternated. Horizontal and vertical synchronizing pulses are equally suppressed in one mode, and are suppressed to different levels in a dual modulus mode. In single modulus mode, horizontal and vertical sync pulses are both suppressed at 6 dB. In the dual modulus mode, sync suppression is dynamically shifted between 10 db suppression for horizontal sync pulses and 6 db for the vertical interval. Mode control data is conveyed on the audio carrier and is detected by decoding circuitry which enables the restoration of synchronization pulses in the scrambled television signal according to the mode indicated. Mode changes are aperiodically changed depending upon the scene content in the TV picture or a random generator. The system provides synchronized mode transitions by which data corresponding to an upcoming mode is transmitted a predetermined number of TV video fields prior to the transition by the head-end encoder to the new mode.

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First Claim