When a formant is modified based on the pole of the AR model, a problem arises due to the interaction between the pole to be modified and other poles such as the apparent elimination of the formant, if the frequency of the considered pole simply is modified. To solve this problem, the method proposed in this paper uses the spectral intensity at the pole frequency as a new feature parameter and the spectral intensity at the pole after the frequency modification is corrected to the desired value by iterative processings.
To examine the effectiveness of the proposed method, an experiment was conducted in which a phoneme in a word is replaced into the same phoneme in a different environment, and then the formant frequency is modified by the proposed method in order to restore the original speech. The speeches before and after the modification are compared in terms of the cepstral distance (CD) from the original speech, and it is verified that the proposed method is effective in the modification of the formant. An attempt is made to solve the formant discontinuity problem at the concatenation point in waveform based speech synthesis-by-rule and to modify the formant frequency accompanying the modification of the fundamental frequency using the proposed method. It was verified as a result of experiment that the speech after formant modification is better than the speech before modification, indicating that the proposed method is useful in improving the quality of the synthesized speech.