Hadamard-transformation technique of speech coding: some further results

The results of an extensive investigation of the properties of 64-point Hadamard transformed speech are presented. Detailed information is given about the probability density functions of the Hadamard coefficients, the average power-density spectrum in the Hadamard domain and the logical-autocorrelation function. The results indicate that good-quality speech can be reconstructed from 6 to 8 dominant Hadamard coefficients, but that the use of fewer coefficients is unlikely to lead to the reconstruction of speech of acceptable quality. The results of a preliminary series of listening tests are presented and these confirm conclusions drawn from the statistical properties of the transformed speech. It is shown that the number of bits needed for coefficient labelling constitutes a significant proportion of the total number of bits needed to represent Hadamard transformed speech. A technique is presented for reducing by more than 50% the number of labelling bits needed, and it is explained how, by using this technique, it should be possible to obtain good quality speech when using a transmission bit rate of 8 k bits/s.