Digital coding of speech in sub-bands

TLDR

Digital coding of speech is proposed to be performed in sub‑bands of the total spectrum. The goal is to control and reduce quantization noise in speech coding. The method quantizes each sub‑band with perceptually‑based bit allocation, employing techniques such as low‑pass translation, integer‑band sampling, complex demodulation, envelope/phase‑derivative representation, and adaptive quantization with parsimonious bit allocation across bands. Simulations show that sub‑band coding yields higher quality at 16 and 9.6 Kbits/s than single full‑band coding.

Abstract

A rationale is advanced for digitally coding speech signals in terms of sub-bands of the total spectrum. The approach provides a means for controlling and reducing quantizing noise in the coding. Each sub-band is quantized with an accuracy (bit allocation) based upon perceptual criteria. As a result, the quality of the coded signal is improved over that obtained from a single full-band coding of the total spectrum. In one implementation, the individual sub-bands are low-pass translated before coding. In another, "integer-band" sampling is employed to alias the signal in an advantageous way before coding. Other possibilities extend to complex demodulation of the sub-bands, and to representing the subband signals in terms of envelopes and phase-derivatives. In all techniques, adaptive quantization is used for the coding, and a parsimonious allocation of bits is made across the bands. Computer simulations are made to demonstrate the signal qualities obtained for codings at 16 and 9.6 Kbits/sec.