Microprocessor-Based Analog Voice Scrambling Techniques

Abstract

Analog voice privacy techniques provide the advantage of being compatible with the 3 kHz audio bandwidth of the existing radio and telephone channels. The degree of privacy provided by an analog voice scrambling technique, however, is proportional to the number of time and frequency elements into which the voice signal can be divided as well as to the number of permutation patterns according to which the elements are scrambled. This implies the requirement for a high degree of signal-processing capability. Microprocessorbased implementations of an analog voice scrambling device provide a large potential for signal processing and scrambling. Furthermore, they provide this potential at a reasonable cost, small volume and moderate power consumption. In addition, a single microprocessorbased analog voice privacy device can be configured in software to yield various degrees of privacy, depending on a particular use and circumstances. Also, a variety of auxiliary functions such as timing, code generation, synchronization and analog-to-digital conversion can be time-shared within the same microprocessor chip, thus minimizing the requirement for support hardware. The purpose of this paper is twofold: (1) to provide an overview of the existing analog voice privacy techniques, and (2) to specifically outline the capabilities of the microprocessor-based analog voice privacy system design, with a particular emphasis on achieving an analog scrambled signal compatible with the 3 kHz nominal audio bandwidth of the existing radio and telephone channels. Also, workable algorithms used for microprocessor-based analog voice scrambling in frequency as well as in time domain are described. Tape recordings of the voice scrambled and recovered with these algorithms are presented for comparison.