6. A storage medium readable by a computer, the medium having data stored thereon, the data, when executed by a processor of the computer, causes the processor to operate as an apparatus for determining, based on speech waveform data, a portion representing a feature of the speech waveform, said apparatus comprising: an acoustic/prosodic analysis unit which calculates, from said data, distribution of energy of a prescribed frequency range of said speech waveform along a time axis, and extracting, among various syllables, a first portion of said speech waveform that is generated stably by a source of said speech waveform, based on the distribution of energy and pitch of said speech waveform; a cepstral analysis unit which calculates, from said data, a frequency spectrum distribution of said speech waveform along the time axis, and estimating, based on the frequency spectrum distribution, a second portion of said speech waveform, for which change is well controlled by said source; and a pseudo-syllabic center extracting unit which determines the portion representing a feature of said speech waveform based on the first portion extracted by the acoustic/prosodic analysis unit and the second portion, wherein said cepstral analysis unit includes: a linear prediction analysis unit which performs linear prediction analysis on said speech waveform and outputting an estimated value of formant frequency; a cepstral distance calculating unit which calculates, using said data, a distribution of cepstral distance on the time axis based on the estimated value of formant frequency provided by said linear prediction analysis unit; an inter-frame variance calculating unit which calculates, based on an output from said linear prediction analysis unit, distribution of local variance of magnitude of delta cepstrum of said speech waveform on the time axis; and a reliability center candidate output unit which estimates, based both on said distribution of cepstral distance on the time axis based on the estimated value of formant frequency calculated by said cepstral distance calculating unit and on said distribution on the time axis of local variance of magnitude of delta cepstrum of said speech waveform calculated by said inter-frame variance calculating unit, a range in which change in the speech waveform is well controlled by the source.
9. The medium according to claim 6 , wherein said cepstral distance calculating unit includes: a cepstrum re-generating unit connected to receive said estimated value of formant frequency from said linear prediction analysis unit, for recalculating cepstrum coefficients based on said value of formant frequency; and a logarithmic transformation and inverse discrete cosine transformation unit connected to receive said speech waveform data for calculating FFT cepstrum coefficients based on said waveform data, wherein the cepstral distance calculating unit is configured to calculate cepstrum distance between the cepstrum coefficients recalculated by said cepstrum re-generating unit and the FFT cepstrum coefficients calculated by said a logarithmic transformation and inverse discrete cosine transformation unit, said cepstrum distance indicating a distribution of unreliability; and said cepstral analysis unit includes: a standardizing and integrating unit which combines the cepstrum distance and the distribution on the time axis of local variance of spectral change and outputting a combined data, wherein the reliability center candidate output unit estimates the range in which change in the speech waveform is well controlled by said source at a dip of the combined data output by said standardizing and integrating unit.