A comparison of methods for 300-400 b/s vocoders

In this paper we discuss several algorithms that can be used to reduce the transmission rate for LPC vocoded speech to around 300 to 400 b/s, with only a modest degradation in speech quality relative to that of fixed-rate 2400 b/s LPC vocoders. We limit the discussion to vocoders that transmit information for single frames (as opposed to whole segments of speech). We start with vector quantization, which reduces the bit rate to around 800 b/s accompanied by a significant but tolerable loss in quality relative to a typical fixed-rate 2400 b/s vocoder. Then we reduce the frame rate using one of two techniques: Fixed-Rate Transmission with Variable Interpolation, or Optimal Variable-Frame-Rate Transmission. We also reduce the data rate necessary for the source parameters (pitch, voicing, gain) from 400 b/s to about 100 b/s by taking advantage of their statistical dependence on the spectrum and some perceptual factors. The final result at 300 b/s has a quality comparable to that of the fixed-rate 800 b/s vector quantization vocoder. At 400 b/s, the quality is, in many respects, better than that of the 800 b/s vocoder and comparable to the 2400 b/s LPC vocoder.