Subband coder design incorporating recursive quadrature filters and optimum ADPCM coders

This paper presents the results of an analytical and experimental study of the use of IIR quadrature mirror filters and forward adapting ADPCM with optimum quantizers for octave band subband coders for speech. It is Shown that the underlying analysis-synthesis systems can be designed such that there is no interband aliasing distortion in the coded speech and such that the analysis-synthesis transfer function has no frequency distortion or no phase distortion, but not both. The experimental study showed that subband coders based on a mixture of infinite impulse response (IIR) and finite-duration impulse response (FIR) filters resulted in higher quality coding systems using fewer multiplies than for systems based on FIR filters alone. It was also found that a signal-to-noise ratio (SNR) gain of about 4 dB could be obtained using optimum quantizers for forward adapting ADPCM coders over linear quantizers in backward adapting APCM codes, but no perceptual quality gain was observed.