Analog scramblers for speech privacy

Abstract Despite significant strides in digital speech technology, scrambling of the analog speech signal continues to be an important method for achieving casual privacy in many classes of voice communications systems. This paper explains the principles of important classes of analog speech scramblers, with emphasis on algorithms that possess demonstrated robustness in the context of real-channel operation. At the forefront of the art are so-called two-dimensional analog scramblers that manipulate the speech signal in both time and frequency domains, to obtain impressive combinations of residual intelligibility, communication delay and transmission robustness. Our review includes several examples of such two-dimensional algorithms.

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