Speech decoder, speech decoding method, program and storage media to improve voice clarity by emphasizing voice tract characteristics using estimated formants

A speech decoding method, comprising the steps of:

• restoring a vocal tract characteristic and a vocal source signal by separating a received voice code;
• estimating a plurality of formants in said vocal tract characteristic;
• calculating a plurality of amplification ratios, each corresponding to each of the plurality of estimated formants, for the vocal tract characteristic based on the plurality of estimated formants;
• emphasizing the vocal tract characteristic based on the calculated plurality of amplification ratios;
• and
• outputting a voice signal by synthesizing the modified vocal tract characteristic modified by the emphasizing step and the vocal source signal obtained from the voice code, wherein
• said estimating step includes estimating a plurality of pairs, each having a formant frequency and a formant amplitude at said formant frequency,
• each of the plurality of pairs corresponds to each of the plurality of estimated formants,
• said calculating step includes calculating a constant amplification reference power from said vocal tract characteristic and determining the plurality of amplification ratios of the respective plurality of formants so as to match the formant amplitude of each pair of the plurality of pairs with the same constant amplification reference power, and
• said emphasizing step includes emphasizing the vocal tract characteristic by using each of the plurality of amplification ratios of each of the respective plurality of formants.

A program embodied in a computer-readable medium, comprising instructions for performing the steps of:

• restoring a vocal tract characteristic and a vocal source signal by separating a received voice code;
• estimating a plurality of formants in said vocal tract characteristic;
• calculating a plurality of amplification ratios, each corresponding to each of the plurality of estimated formants, for the vocal tract characteristic based on the plurality of estimated formants;
• emphasizing the vocal tract characteristic based on the calculated plurality of amplification ratios;
• and
• outputting a voice signal by synthesizing the modified vocal tract characteristic modified by the emphasizing step and the vocal source signal obtained from the voice code, wherein
• said estimating step includes estimating a plurality of pairs, each having a formant frequency and a formant amplitude at said formant frequency,
• each of the plurality of pairs corresponds to each of the plurality of estimated formants,
• said calculating step includes calculating a constant amplification reference power from said vocal tract characteristic and determining the plurality of amplification ratios of the respective plurality of formants so as to match the formant amplitude of each pair of the plurality of pairs with the same constant amplification reference power, and
• said emphasizing step includes emphasizing the vocal tract characteristic by using each of the plurality of amplification ratios of each of the respective plurality of formants.