Tree encoding of speech

Recently developed methods of tree source coding with a fidelity criterion are applied to speech coding. We first demonstrate that tree codes are inherent in A-D speech convertors of the waveform following type and point to ordinary and adaptive delta modulation and differential pulse code modulation (DPCM) as examples. Insights of coding theory improve these trees at low rates; we offer two new code classes, one obtained by smoothing the DPCM tree and one using the rate-distortion theory of autoregressive sources. Using these codes, we study the performance of a simple synchronous tree searching algorithm called the
M
-algorithm that maintains a small fixed number of paths in contention. 1 and 2 bit/sample code trees, used to encode actual speech at 8, 10, and 16 kbits/s, yield improved dynamic range and channel error resistance, and 4-8 dB improvement in mean-square error (mse) over ordinary single-path searched DPCM. These improvements in excess of analytical estimates suggest that tree coding methods perform better with real-life sources than previously thought.