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Subband coding of speech using backward adaptive prediction and bit allocation

In this study it is our goal to improve the performance of ADPCM and subband speech coders at medium bit rates (9.6∼16 kb/s) without increasing the coder complexity substantially. Various major building blocks including the predictors (both fixed and adaptive), subband quadrature mirror filter (QMF) and bit assignment strategy (both static and dynamic) are investigated in detail. We have found that (1) the Least-Squares (LS) adaptive lattice predictor outperforms both the pole-zero adaptive predictor recommended by CCITT and a first-order fixed predictor. (2) more subbands can improve the coder performance (3) longer QMF can reduce the interband aliasing and improve the subjective performance of a subband coder (4) an optimal dynamic bit allocation scheme with an improvement of SNR as high as 5 dB is much more favorable than a fixed bit allocation. With all of the above finding we propose a 4-band hybrid subband coder with an LS adaptive lattice predictor and an optimal dynamic bit allocation strategy.