Experimental comparison of forward and backward adaptive prediction in DPCM

Results of experimental comparisons of forward- and backward-adaptive prediction in differential pulse code modulation (DPCM) of speech are presented. Two different types of comparisons were conducted. In one comparison, both predictors were used with the same three/five-level pitch compensating quantizer (PCQ). For this comparison, forward prediction clearly outperforms backward prediction, but with the penalty of a 10% increase in data rate due to the need to transmit coefficients. In the second comparison, the forward-prediction DPCM system and the backward-prediction DPCM system are constrained to have the same data rate of 16 kbits/sec. The backward-adaptive predictor outperforms forward prediction for this latter comparison. The speech data base for the simulations is one sentence spoken by a male speaker in four different languages, English, French, German, and Arabic. The performance comparisons are based on signal-to-quantization noise ratio, signal-to-prediction error ratio, sound spectrograms, and formal subjective listening tests.