Browse Prior Art Database

Secure Audio Frequency Transmission System

IP.com Disclosure Number: IPCOM000080845D
Original Publication Date: 1974-Feb-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 47K

Publishing Venue

IBM

Related People

Featherston, JR: AUTHOR

Abstract

This system insures that an audio-frequency speech or other sound signal is unintelligible to any other than authorized listeners.

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Secure Audio Frequency Transmission System

This system insures that an audio-frequency speech or other sound signal is unintelligible to any other than authorized listeners.

An audio-frequency signal, as generated in a television broadcasting station, for example, is applied at input terminals 10 and thereafter recovered at output terminals 12 located in the receiver of an authorized listener. Between terminals 14 and 16, conventional radiofrequency broadcasting and receiving apparatus is arranged, with the normal electromagnetic signal radiating link therebetween. Unauthorized listeners may receive the electromagnetic radiation, but due to the circuit arrangement shown, the audio-frequency component of the radiated signal will remain scrambled unless the coded key to associated decoding circuitry is known.

A suitable random sequence pulse train, such as generated by a maximum sequence generating circuit of known construction, is applied at terminals 24 controlling the inversion of the audio-frequency signal. The conventional horizontal synchronizing signal is applied at terminals 22, along with the vertical synchronizing signals applied at terminals 24 for controlling the generation of the audio-signal inverting frequencies. These frequencies are generated by dividing circuit 26, a multiplying circuit 28, and another dividing circuit 30. The dividing circuits 26 and 30 are reset by the vertical synchronizing pulses applied at terminals 24.

An audio-signal inverting frequency of 7.875 KHz, obtained by dividing the horizontal synchronizing frequency by 2 in the divider 26, is applied to an AND gating circuit 32. Another inverting frequency of 8.400 KHz, obtained at the output of the divider circuit 30, is applied to another AND gating circuit 34. The AND gating circuits are alternately switched by the code signal applied at terminals 20, an inverting circuit 36 being inserted in one of the input leads to the AND ga...