Smoothed DPCM codes

Summary form only given, as follows. Rate distortion theory promises that autoregressive sources can be encoded optimally at small distortions (high rates) by a source coder with infinite encoding delay and zero delay at the decoder. However, for instrumentable systems with finite encoding delay and an unmatched code generator or for operation at low rates, decoding delay may provide a performance increment. The alphabet-constrained approach to data compression allows delay at both the encoder and the decoder, and Sethia and Anderson incorporate delay in a tree-coder code generator by combining a weighted linear interpolation scheme with DPCM. This system, called interpolative DPCM (IDPCM), was shown to outperform DPCM at rate 1 for several synthetic source models. In the present study, minimum-mean-squared-error (MMSE) fixed-lag smoothing is used in conjunction with DPCM to develop a code generator employing delayed decoding. This smoothed DPCM (SDPCM) code generator is compared with DPCM and IDPCM code generators at rates 1 and 2 for tree coding several synthetic sources and with a DPCM code generator at rate 2 for speech sources. The (M, L) algorithm is used for tree searching, and SDPCM outperforms IDPCM and DPCM at rate 2 for the synthetic sources with M = 1, 4, 8, and 12, and at rate 1 with M ≥ 4. For speech, SDPCM provides a slight improvement in MSE over DPCM codes that is also evident in sound spectrograms and subjective listening tests. The models on which the fixed-lag smoother is based must be chosen appropriately to achieve good SDPCM performance.