Audio signal cryptographic system

Claim 1

An audio decrypting system comprising: receiving means for receiving frames of pulse code modulated audio information, the frames including encrypted audio data portions and audio control portions; first extraction means, connected to said receiving means, for extracting the encrypted audio data portions of the frames; second extraction means, connected to said receiving means, for extracting the audio control portions of the frames; decryption key storage means for storing a plurality of unencrypted decryption keys; decryption key selection means, connected to said decryption key storage means, for selecting one of said plurality of decryption keys; decryption means, connected to said first extraction means and said decryption key selection means, for decryption extracted audio data portions by inverting bits thereof according to a selected decryption key, said decryption means comprising a programmable logic device; and authorization means, connected to said second extraction means, for determining from said audio control portions whether to enable said decryption key selection means to select a decryption key for decrypting the audio data portions, wherein the receiving means comprises a pulse code modulation circuit, the first and second extraction means comprise a headend unit, the decryption key storage means comprises an SRAM, the decryption key selection means and authorization means comprise a microprocessor.