Zenith Electronics Corporation

Oak Industries, Inc.

Ronald L Forbes

John A Lund

Pieter Den Toonder

Dennis E Burke

James F Stoffer

Harold E Mann, Iii

A two channel audio scrambling circuit for a television signal encoding system includes synchronized digital carrier switching means for deriving two suppressed carrier, switched audio signals that are 90 degrees apart, and combining them into a single output signal. A digital phase shifter, supplied with a carrier related to the horizontal line frequency develops properly phased outputs for controlling electronic switches to produce two phase displaced digital carrier switching signals. A plurality of scrambling modes are determined by logic means under control of a vertical interval signal, a video inversion signal and a tone signal input for controlling the digital phase shifter to change the carrier frequency. Complementary decoding circuitry for recovering the so-encoded audio signals is also described.

Two channel audio scrambling system

A multi-mode audio scrambling system includes an all-pass filter and a 90 degree phase shift circuit for supplying balanced modulators with 90 degree phase displaced audio information and 90 degree phase displaced carriers derived from the horizontal line frequency of a television receiver for producing a single sideband, suppressed carrier audio spectrum. A plurality of offset frequencies are derived from the horizontal line frequency and are used to further modulate the resultant signal to produce a single sideband displaced audio spectrum. A logic circuit selects the offset frequency (mode) in response to vertical interval signals, video inversion signals and audio tone or data signals. The final output is thus scrambled in different modes with different offset frequencies. A complementary unscrambling system is also described.

Dynamic audio scrambling system

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.

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McHenry, IL, United States of America

Ronald L Forbes