ZERO-CROSSING PRESERVATION IN LOW-BIT-RATE PSEUDORANDOMLY DITHERED QUANTIZATION OF SPEECH SIGNALS

In earlier investigations, it has been found that, at low bit rates, the dithered quantization of speech samples in PCM is perceptually pleasing but is less intelligible than normal fixed-level quantized PCM. Also, it has been suggested that a possible cause of the reduced intelligibility of the dithered PCM is the effect that the dithering has on the zero crossings of the speech signal. This paper reports on an investigation into this suggestion. Several methods of dithered quantization are considered in which the zero crossings of the speech signal are preserved, and the results of intelligibility tests carried out to compare the various methods of dithering are presented. The statistical analysis of the results shows that, for 3, 4 and 5 bits per sample quantization, two of the methods of dithered quantization are significantly more intelligible than normal, correspondingly low-bit-rate PCM and the advantage is better than 1 bit per sample.