An Efficient Objective Intelligibility Measure for Frequency Domain Scramblers
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Research Article An Efficient Objective Intelligibility Measure for Frequency Domain Scramblers ´ A. M. C. R. Borzino,1 J. A. Apolinario Jr.,1 and D. G. da Silva2 1 Department
of Electrical Engineering, Military Institute of Engineering (IME), Prac¸a General Tib´urcio 80, 22290-270 Rio de Janeiro, RJ, Brazil 2 Center for Telecommunications Studies (CETUC), Pontifical Catholic University of Rio de Janeiro (PUC-Rio), Rua Marquˆes de S˜ao Vicente 225, 22453-900 Rio de Janeiro, RJ, Brazil Correspondence should be addressed to J. A. Apolin´ario Jr., [email protected] Received 17 August 2007; Accepted 3 December 2007 Recommended by E. Magli An objective performance measure is proposed to evaluate the intelligibility of a speech signal having its frequency subbands permuted. The proposed tool can be used to generate efficient keys for frequency domain scramblers as well as to assess the results of cryptanalysis. Copyright © 2007 A. M. C. R. Borzino et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1.
INTRODUCTION
The main goal of a scrambler is to make a speech signal unintelligible via the permutation of its frequency subbands. In this work, we assume a sampling frequency of 8 kHz and a filter bank with 8 channels, each using a filter with 128 coefficients. With N being the number of (permutable) subbands, there are N! possible permutations. However, not all of them are efficient [1]; this is due to the fact that some of them result in a signal with high residual intelligibility. Objective measures in the technical literature, such as spectral distortion and segmental signal-to-noise ratio [2], are not appropriated for the task of evaluating intelligibility in scrambled speech; this is so because the objective is not the difference in spectra but how intelligible the signal is. In [3], a scheme for obtaining scores was proposed. This scheme (Beker score) took into account subbands shift, number of subbands kept in their original positions, continuity of the subbands, and occurrence (or not) of subband inversion (not taken into consideration here due to the structure of our frequency scrambler). From our experiments, we noted that even with a close to perfect continuity as in permutation [5 6 7 8 1 2 3 4]T , the listener could not understand the signal. Therefore, the objective measure proposed herein does not take into account the continuity of subbands; instead, it introduces a subband
weighing as each subband has its own degree of importance, mainly determined by the presence of formants [4]. 2.
POSITION DISTANCE
By means of subjective tests, it was observed (for our 8subband scrambler) that if all subbands are shifted in two or more positions, the scrambled signal becomes completely unintelligible for a nonexpert listener. Nevertheless, if the shift is of only one position, the signal is partially intelligible. In order to set a weight to the shift o
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