Shinya Takahashi

Apparatus for linear predictive coding and decoding of speech using residual wave form time-access compression

A speech coding and decoding apparatus for use in linear predictive coding of a speech signal includes coding apparatus having a pitch analyzing element for separating an analysis frame into one or more blocks and calculating the strength of correlativity between pitch periods of a residual waveform in each block. A residual partially compresing element compresses the time axis of the residual waveform in a block having a high correlativity strength. A residual quantizing element quantizes the compressed residual waveform while preferentially allotting quantization allotment bits to the compressed portion of the residual waveform. Decoding apparatus includes a residual inverse quantizing element for inversely quantizing the residual waveform by the same bit allotment. A residual partially expanding element expands the compressed portion of the residual waveform to its original length and supplies it to a linear predictive synthesis filter.

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.