A Comparison of Four Methods for Analog Speech Privacy

Four well-known procedures for analog speech privacy have been compared in terms of residual intelligibility, bandwidth expansion, and encoding delay. Intelligibility scores have been determined from a perceptual experiment where about 70 untrained listeners were given the task of recognizing each of 200 spoken digits that occurred in a balanced set of 50 encrypted four-digit utterances, and by averaging resulting probabilities of correct digit recognition. Bandwidth expansion has been expressed in terms of a new segmental measure that is more sensitive to short-time bandwidth manipulations than a conventional, long-time-averaged power spectrum measurement. Encoding delay is a straightforward function of analog scrambler parameters. The scrambling procedures that have been compared are sample permutation (
S
), block permutation (
B
), frequency inversion (
F
), and a combination of methods
B
and
F
, denoted by [
BF
]. Sample permutations involved a contiguous set of L
S
(2 to 128) 8 kHz samples, while block permutations operated on a contiguous set of N
B
(4 to 128) speech segments each of which was L
B
(8 to 256) samples long. Frequency inversion is obtained by simply inverting the sign of every other Nyquist (8 kHz) sample. The parameters,
L_{s},N_{B}
, and L
B
, determine residual intelligibility as well as transmission properties such as encoding delay and bandwidth. The comparisons in our study provide a quantitative justification for the popular approach [
BF
]. For example, with
N_{B} = 8
and
L_{B} =128
, although the encoding delay is as much as 128 ms, the bandwidth expansion is only about 100 Hz (using the new segmental measure), and the digit intelligibility
I
is 20 percent. Note that in the specific problem of recognizing ten digits, purely random (input-independent) listener responses correspond to
I = 10
percent.