Hirohisa Tasaki

Spectral postfilter design based on LSP transformation

A new spectral postfilter that has high flexibility of design is proposed. In this method, two LSP (line spectrum pair) transformation blocks compute ARMA filter coefficients for the postfilter. The characteristics of the proposed postfilter can be flexibly designed in the frequency domain, because an effect of LSP transformation can be estimated in the frequency domain. As examples of design, an approximation to a conventional LPC-based postfilter, enhancement in selected frequency bands, and a suitable design for a tandem coding are demonstrated.

New excitation codebook search methods to reduce perceptual degradation of CELP

In this paper, we propose three new excitation codebook search methods for a low-bit-rate CELP coder using an algebraic excitation codebook to reduce code selections that may result in pulse-like degradation. The proposed methods are: (1) adaptive distortion weighting based on the states of the input speech; (2) distortion weighting based on the amplitude distribution of the direction of time; and (3) distortion-ratio limitation to algebraic excitation codebook search. Subjective evaluation results show that the performance of 4 kbit/s CELP is improved by the proposed methods both in clean conditions and with background noise.

CELP Coding Below 3kbps Using LPC Residual Phase Coding

In Code-Excited Linear Prediction (CELP) [l] coding at rates of 3kbps OT lower, efficient representation of the LPC-excitation signal is most important. However, at present, pitch periodicity and short-time phase characteristics of the LPC-excitation signal represented only by excitation Wdevectors are insuficient t o reproduce good quality speech. In this paper, we present a new CELP Coder, Phase Coding CELP (PC-CELP). In this coder, short-time phase characteristics of the LPCez&ation signal are extracted and encoded directly. Informal listening tests indicate that good quality synthetic speech is obtained at 2.5kbps.