The Analog Voice Privacy System

The Analog Voice Privacy System (AVPS) is a voice scrambler that permutes individual output samples from a subband coder analysis filterbank. The system has 125! possible permutation keys, giving it the cryptanalytical strength of a digital encryption system. However, it retains the good voice-quality characteristics of analog scramblers. The AVPS has been implemented in a real-time hardware prototype designed for evaluation in telephone environments and works with any modular telephone and standard 120V ac electrical power. The unit contains two circuit boards — one for analog and one for digital processing — that each use four digital signal processors. To date, we have successfully tested it over long-distance telephone connections, several analog and digital PBXs and telephone switches, and a channel simulator. The quality of the decrypted speech is considered very natural, and speaker recognition is retained — a significant advantage over digital vocoders. This paper describes the underlying principles of the AVPS algorithm, its implementation, and laboratory test results.