Subjective improvements in DPCM-AQ performance based on adaptive noise shaping

This paper reports subjective improvements in DPCM coders with adaptive quantizers (DPCM-AQ coders). The improvements were observed in simulations with real speech. An adaptive feedback filter shaped the output noise spectrum, and subjective quality increased despite a decrease in signal-to-noise ratio. The noise spectrum in the best case was shaped like the speech spectrum but was below it at almost all frequencies by 20 dB. This result is consistent with work on adaptive noise shaping in ADPCM-AQ systems. The improvements were enough to make 32 kbits/s DPCM-AQ systems sound equivalent to 40 kbits/s systems. Subjective improvements were observable, with diminished effectiveness, down to 24 kbits/s. Since the feedback filter does not have a counterpart at the receiver, transmission of the adaptive filter coefficients is unnecessary.