We previously proposed a new segment quantization method called Lempel-Ziv segment quantization (LZSQ), which is a modified version of Lempel-Ziv (LZ) coding that can be applied to a continuous information source. In the present paper, LZSQ is applied to the mixed excitation linear prediction (MELP) coder, which is a standardized vocoder-type speech coder that operates at 2.4 kbit/s, in order to reduce the bit rate to below 2.4 kbit/s, while preserving the quality of the coded speech. LZSQ is applied to six coding parameters of the MELP coder to reduce the total bit rate as much as possible. As a result, the total bit rate of the modified MELP coder was reduced to 1.57 kbit/s, while the subjective quality of the modified MELP coder is equivalent to that of the previous MELP coder.

A new segment quantization method using the Lempel-Ziv algorithm is proposed, and it is applied to quantize line spectral frequency parameters in speech codec. The proposed segment quantizer can save four bits per frame, compared with the ITU-T G.729 speech codec (18 bits/frame), without degradation of subjective or objective speech quality

This paper presents a new segment quantization method, which is based upon the Lempel-Ziv algorithm, and it is applied to quantize LSF segments of speech signals. The proposed quantizer is compared with other practical coders such as the G.729 and the MELP, by substituting those original LSF quantizers with the proposed segment quantizer. As a result, at the same cepstral distortion, nine bits per frame were saved compared with the scalar quantizer of the MELP (25 bits/frame), and three bits per frame were saved compared with the MA prediction VQ of the G.729 (18 bits/frame), respectively.

In this paper, a very low bit speech coder at 1.2 kbps is newly proposed. Like the LPC vocoder, it only requires gain, pitch, and spectral information, but its quality is far superior. The synthesis method is one of harmonic coding, using sinusoids whose frequencies are multiples of the fundamental frequency, where the amplitudes of the sinusoids are adaptively modulated using gammatone filters as a perceptual weighting filter. The sinusoids' phases are also adjusted so as to maximize the perceptual quality. In order to reduce the total bit rate to 1.2 kbit/s, a new segment coder for spectral information (LSP coefficients) using DP matching is also proposed. The quality of the synthesized speech was improved by 0.45 in the mean opinion score (MOS) compared with that of the simple LPC vocoder operating at the same rate, and it was comparable to that of 2.4 kbit/s MELP coder.

A new vector quantization method is proposed which can reduce search complexity and code book memory size by reducing the number of code vectors without increasing quantization distortion. This method uses hyper-columnar clusters, and an input vector is quantized to a cluster, center axis of which is nearest to the input vector. Thus, one vector and one scalar must be coded and transmitted. The proposed method was applied to four geometrically different distributions and LPC cepstrums of speech signals. As a result, the number of code vectors was decreased compared with that in an ordinary vector quantizer, in all of the above distributions. Then the reduction of memory size and search complexity were evaluated.>