Eric W. M. Yu

Harmonic+noise coding using improved V/UV mixing and efficient spectral quantization

This paper presents a harmonic+noise speech coder which uses an efficient spectral quantization technique and a novel voiced/unvoiced (V/UV) mixing model. The harmonic magnitudes are coded at 23 bits/frame using the magnitude response of a linear predictive coding (LPC) system. The difference between the harmonic magnitudes and the sampled magnitude response is minimized by the closed-loop approach. The V/UV mixing is modeled by a smooth function which is derived from the speech spectrum envelope based on the flatness measure. The V/UV mixing model allows noise to be added in the harmonic portion of speech spectrum so that buzzyness is reduced. The V/UV mixing information is determined from the spectral parameters available in the decoder, no bits are needed for transmitting the V/UV information. A 1.4 kbps harmonic coder is developed. The speech quality of the coder is comparable to other harmonic coders operating at higher rates.

Phase and transient modeling for harmonic+noise speech coding

A high quality harmonic+noise speech coder is presented. This coder operates in either the steady mode or the transient mode. An efficient phase coding technique based on polynomial fitting is proposed. In this technique, the frequency deviations that are used to minimize the phase prediction errors are modeled by a 6/sup th/-order polynomial. The polynomial coefficients are quantized in closed-loop by a split vector quantizer. For the transient speech, the unvoiced part is modeled by an 8/sup th/-order all-pole model while the voiced part is modeled by the low-frequency harmonic components. The transitional behavior is characterized by the onset time and growth rate of each harmonic component. An analysis-by-synthesis procedure is proposed to determine the onset times and growth rates in the time domain. The proposed coder retains the temporal events of the speech signal and produces high quality speech at 2.4 kbps.

Variable bit rate MBELP speech coding via V/UV distribution dependent spectral quantization

A variable bit rate multiband excited linear predictive speech coder is proposed in this paper. Speech signals are compressed at different bit rates ranging from 0.88 kbps to 2.6 kbps according to the mode of operation and the optimum V/UV transition frequency. An average bit rate of 1.24 kbps is achieved. The proposed speech coder improves the speech quality by splitting the non-stationary speech segments for analysis. The V/UV distribution of a short-time speech spectrum is represented efficiently by using a closed-loop minimised V/UV transition frequency. Depending on the V/UV transition frequency, the spectrum envelope is quantized at variable bit rates through embedded differential predictive scalar and vector quantizations of the LSP parameters. The proposed spectral quantization scheme results in a spectral distortion comparable to a fixed 24-bit 2-dimensional differential scalar quantization scheme.

Harmonic+noise coding based on the characteristics of human phase perception

This paper presents a variable-rate harmonic+noise coder for high-quality coding of speech at an average bit rate of 1.98 Kbps. The proposed coder employs an efficient phase coding technique that reduces the number of bits for phase quantization by taking account of the characteristics of human phase perception. The proposed perceptual phase coding technique takes advantage of the poor phase sensitivity of the human auditory system in the frequency region below the critical phase frequency and in the unvoiced band. The number of bits for quantization of the frequency deviations is assigned according to a perceptual weighting function which is derived from the measurements of just-noticeable difference of phase. Speech quality of the harmonic+noise coder is improved significantly by employing the proposed phase coding technique. Experimental results show that the proposed coder outperforms the 2.4 Kbps sinusoidal transform coder.

Frequency-domain postfilter for multiband excitation coding of speech at low bit rates

Postfilters for suppressing coding noises in low bit rate speech coders usually operate in the time domain with high complexity and also introduce nonlinear phase distortion to the speech signals because they are recursive filters. In this paper, a frequency-domain postfiltering technique for multiband excited linear predictive (MBELP) coders is proposed. The idea is to firstly sample the postfilter magnitude spectrum at the pitch harmonics and then scale the signal band magnitudes by the sampled postfilter magnitudes. This approach does not need to specifically perform the filtering process, and the signal energies before and after postfiltering can be easily equalized by simply re-scaling the signal band magnitudes. Another obvious advantage of using this frequency-domain postfiltering technique is that the phase information utilized in multiband synthesis will be retained because only the magnitude spectra are modified. In this paper, a fast method is given to sample the postfilter magnitude spectrum which is characterized by line spectral pair (LSP) parameters. The proposed frequency-domain postfilter has a much lower implementation complexity than its time-domain counterpart.

A variable-rate harmonic speech coder with efficient spectral quantization

This paper presents a high quality harmonic coder for variable-rate speech coding. An efficient variable-rate quantization technique is adopted for the quantization of the speech spectrum envelope. In this technique, the line spectrum pair (LSP) parameters of each speech frame are split and the prediction residuals of the LSP parameters are subsequently quantized by using the variable-dimension vector quantization (VQ) method. The splitting of LSP parameters is based on a voiced/unvoiced (V/UV) transition frequency. This V/UV transition frequency is determined from the closed-loop minimization of a V/UV matching error in the frequency domain. The proposed coder achieves good quality for speech communications at an average bit rate of about 1.9 kbps.