Adaptive differential pulse-code modulation with adaptive bit allocation

Abstract

A new method of speech coding is described. The method, which is referred to as adaptive differential pulse-code modulation with adaptive bit allocation (ADPCM-AB), is a combination of adaptive differential pulse-code modulation with adaptive first-order prediction and switched pre-emphasis and adaptive bit allocation. The adaptive bit allocation technique is based on the short-term variation of the prediction error. An extensive subjective test has been carried out in which the system operating at data rates of 16 kbit/s and 32 kbit/s has been compared with conventional adaptive differential pulse-code modulation, with fourth-order prediction, operating at the same data rates, and with 7 bit logarithmically companded (A-law) PCM oprating at 56 kbit/s. The analysis of the results of the subjective test indicates that at 32 kbit/s the performance of the system is indistinguishable from that of conventional ADPCM and better than 7 bit logarithmically companded PCM. At 16 kbit/s the performance of the new ADPCM-AB system is preferred to the conventional ADPCM system. In addition, the technique of switched pre-emphasis has been applied to conventional adaptive differential pulse-code modulation, and the resulting modified ADPCM system has been compared with the new ADPCM-AB system. The results of subjective tests indicate that at 16 kbit/s the two schemes are considered to be subjectively equivalent. But, by using long data blocks in connection with the adaptive bit allocation technique, the ADPCM-AB system can be further improved with respect to its perceptual quality and made significantly superior to ADPCM with switched pre-emphasis.