Analog Scramblers for Speech Based on Sequential Permutations in Time and Frequency

Analog Scramblers for Speech Based on Sequential Permutations in Time and Frequency By N. S. JAYANT, R. V. COX, B. J. McDERMOTT, and A. M. QUINN (Manuscript received July 20, 1982) 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)J. 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.