Toshiyuki Morii

Dispersed-pulse codebook and its application to a 4 kb/s speech coder

This paper presents a dispersed-pulse codebook for CELP coder. This codebook generates an excitation vector by convoluting dispersion vectors with signed pulses in an algebraic codevector. The dispersion vectors are obtained through training so the coding distortion to be reduced. An objective evaluation result shows that the coding distortion with this codebook is smaller than that with an algebraic codebook. The dispersed-pulse codebook is applied to a 4 kb/s CELP coder. Subjective evaluation results show that: (1) the fundamental performance of the 4 kb/s coder is equivalent to that of G.726 32 kb/s coder, and (2) the performances of the 4k b/s coder under some error and background noise conditions are equivalent to those of G.729 8 kb/s coder.

Predictive vector quantization of wideband LSF using narrowband LSF for bandwidth scalable coders

For implementing a bandwidth-scalable coder, a wideband line spectral frequency (LSF) quantizer was developed. It works in combination with a narrowband LSF quantizer. A new predictive vector quantization was introduced to the wideband LSF quantizer. The predictive vector quantizer is based on the use of several predictive contributions, which include first-order auto regressive (AR) prediction and vector quantization (VQ) codebook mapping. One feature of the new predictive vector quantizer is exploitation of the correlation between wideband and narrowband LSFs quantized in the previous frame for estimating wideband LSF in the current frame. A 16-bit switched predictive three-stage vector quantizer was used to encode estimation residues. Results showed that introduction of the predictor brought about a performance improvement of 0.3dB in spectral distortion. This paper describes procedures of designing the predictor and the three-stage codebook, as well as simulation results.

A high quality 4-kbit/s speech coding algorithm based on MDP-CELP

This paper presents a high quality 4-kbit/s speech coding algorithm based on the CELP algorithm. This coder operates on speech frames of 20 ms. Its main features are as follows: modification of a target vector for LSP-VQ which improves the subjective quality of the coded speech signal, a dispersed-pulse structure which improves pulse-based fixed-codebook performance at a low bit rate, a high performance fixed-codebook consists of a partial algebraic codebook and a random codebook, backward mode switching which improves fixed codebook performance for unvoiced and stationary noise regions, a stationary noise generator which significantly improves the subjective quality under background noise conditions. Subjective testing indicates that the quality of this coding algorithm is better than that of ITU-T G.726 (32-kbit/s ADPCM) under error-free and noise-free conditions and equivalent to that of ITU-T G.729 (8-kbit/s CS-ACELP) under some background noise conditions.