An Optimal Unit-Selection Algorithm for Ultra Low Bit-Rate Speech Coding

In this paper, we first analyze an algorithm proposed recently by Lee and Cox, 2002, which attempts to perform a segment unit-selection for segmental quantization and show how it intrinsically suffers from several sub-optimalities. We then propose a generalized unit-selection algorithm for ultra low bit-rate segment quantization based on a modified one-pass dynamic programming algorithm. We show that this proposed algorithm is exactly optimal for both fixed and variable-length segments and also how it solves the sub-optimalities of the Lee-Cox-2002 algorithm. From rate-distortion curves from a very large continuous speech multi-speaker database, we show that our algorithm has a significantly superior performance than the algorithm of Lee and Cox by achieving considerably lower spectral distortions (up to 3 dB lower distortions) as well as much lower bit-rates for a given distortion over a range of unit database sizes.