Low delay tree coding of speech at 8 kbit/s

A low delay tree coder for coding speech at 8 kbit/s that incorporates a new backward coefficient adaptation structure is presented. The coder has an algorithmic delay of 2.5 ms, produces good quality speech over ideal channels, and maintains robustness to bit errors for independent bit error rates up to and including 10/sup -2/. The tree coder consists of a randomly populated fractional rate tree, a weighted squared error distortion measure, the (M,l) tree search algorithm, an incremental path map symbol release rule, a long-term predictor, an all-pole short-term predictor, and the newly-obtained parameter adaptation algorithms; The sensitivity to channel errors is reduced by using the receiver excitation sequence for adaptation of the short-term predictor coefficients, but the ability to track rapid changes in the speech is retained by shaping the excitation sequence with an all-zero filter. The output speech of the tree coder is evaluated using segmental signal-to-noise ratios (SNRSEG), narrow band spectrograms, a paired comparison with pulse code modulation (PCM), and an equivalent noise paired comparison using modulated noise reference unit (MNRU).<
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