3. A method according to claim 1, wherein said calculating step comprises the step of applying an inverse Fast Fourier Transform process to the spectrum envelope to determine the cepstrum coefficients and applying regressive equations on the cepstrum coefficients.
3. A method according to claim 1, wherein said calculating step comprises the step of applying an inverse Fast Fourier Transform process to the spectrum envelope to determine the cepstrum coefficients and applying regressive equations on the cepstrum coefficients. 4. A method according to claim 3, wherein said regressive equations comprise following equations: ##EQU7##