A segment vocoder at 150 b/s

In this paper we investigate several methods for reducing the bit rate of a segment vocoder [1] by 35% to 150 b/s. In the original vocoder we used a random sample of vectors as a set of templates for vector quantization. We demonstrate in this paper that this random quantizer is near-optimal by comparing it with quantizers that use clustering algorithms for quantizing speech segments. The reduction of the bit rate of the segment vocoder was achieved primarily by using a segment network, i.e., not all segment templates are allowed to follow a given segment template. The spectral continuity of speech is used to determine the subset of templates, that can be used to quantize an input segment. To achieve the low rate of 150 b/s, we also reduced the bit rate for coding pitch, gain, and segment duration. Finally, we present the bit allocation used for transmitting speech at 150 b/s as a single speaker segment vocoder.