A Comparison of Two Methods for Very-Low-Rate Speech Coding

Vector quantization (VQ) has been used recently for developing vocoders operating below 800 b/s. We describe in this paper two systems that use VQ and transmit intelligible speech in the range of 300 to 600 b/s. The frame vocoder which uses VQ for quantizing the spectral parameters of a single frame of speech was found to be most effective at the higher rate of 600 b/s. The segment vocoder which uses VQ for quantizing the spectral parameters of a sequence of frames yielded better intelligibility at the lower 300 b/s rate. We present the quantization algorithms and bit allocation for the two vocoders and compare their performance for varying bit rates and different noisy speech conditions.