Analog Scramblers for Speech Based on Sequential Permutations in Time and Frequency
Nikil S. Jayant, Richard Vandervoort Cox, Barbara J. McDermott, A. M. Quinn · Bell System Technical Journal · 1983
Permutation of speech segments is frequently utilized in scramblers for analog speech privacy. This paper discusses a “sequential” permutation procedure that has better segment-separation properties than the well-known procedure of “block” permutation, where contiguous segments are arranged in blocks of appropriate size, and permuted within such blocks. It further proposes the application of the sequential procedure to a novel technique for simultaneous permutations in time and frequency. The paper also presents results of a subjective experiment where we measured residual speech intelligibility at the output of scramblers using permutations in time [time segment permutation (TSP)], or permutations in time and frequency [time-frequency segment permutation (TFSP)]. The experiment included examples of block TSP, sequential TSP, and sequential TFSP. We measured spoken-digit-intelligibility as a function of the communication delay introduced by the scrambling operation. We found that even with a delay of 512 ms, the residual intelligibility in a TSP scrambler is no lower than about 50 percent; however, a sequential TFSP scrambler can realize an average digit intelligibility in the order of 20 percent with a delay of 256 ms. A companion paper discusses the implementation of the sequential TFSP scrambler, and the quality of descrambled speech in the context of real-channel operation.