Fractional rate multi-tree speech coding

The authors present both forward and backward adaptive speech coders that operate at 9.6, 12, and 16 kb/s using integer and fractional rate trees, unweighted and weighted squared error distortion measures, the (M, L) trees search algorithm, and incremental path map symbol release. They introduce the concept of multitree source codes and illustrate their advantage over classical, multiple-symbol-per branch, fractional-rate trees for speech coding with deterministic code generators. Performance results are presented in terms of unweighted and weighted signal-to-noise ratio and segmental signal-to-noise ratio, sound spectrograms, and subjective listening tests. Using the (M, L) tree search algorithm and a frequency weighted distortion measure, the multitree coders yield speech ranging from near toll quality at 16 kb/s to speech with good quality and intelligibility at 9.6 kb/s. The 9.6-kb/s backward adaptive multitree coder substantially outperforms APC (adaptive predictive coding) and has an encoding delay of less than 2 ms.