A new approach for low bit rate coding of speech in a pitch-excited vocoder context is presented in this paper. The new technique, which operates at about 800 bps, has quality equivalent to a 2400 bps fixed-rate LPC vocoder while requiring only slightly more storage and computational resources. The improved performance of this system is based on two specific developments. The first is a novel use of line spectrum pair (LSP) coefficients in a structure which allows a low average bit rate while constraining the resulting distortion to have a low perceptual impact. The second is a frame-to-frame parameter interpolation algorithm which both reduces the bit rate and simultaneously insures more speech-like formant trajectories than those derived from vector quantizers at comparable bit rates.
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