Comparison of segment quantizers: VQ, MQ, VLSQ and unit-selection algorithms for ultra low bit-rate speech coding

We consider the class of segment quantizers used for low to ultralow rate speech coding, namely, vector quantization (VQ), matrix quantization (MQ), variable-length segment quantization (VLSQ) and two more recent unit-selection based segment quantization algorithms which represent an important shift in using large unclustered continuous codebooks in contrast to the conventional clustered codebooks of VQ, MQ and VLSQ. We examine the advantage, if any, in this shift from small clustered codebooks of VQ, MQ and VLSQ ( 10-14 bits/segment), to the larger continuous unit databases ( 16-18 bits/segment) in the unit-selection framework, by comparing their rate-distortion curves. We show that while the conventional VQ, MQ have higher distortions and VLSQ satuarates in its distortion reduction, unit-selection algorithms provide lower distortions and steeper reductions at marginally low increase in bit-rates and justifies exploring their potential further.