Modification of pitch using DCT in the source domain

Abstract

In this paper, we propose a novel algorithm for pitch modification. The linear prediction residual is obtained from pitch synchronous frames by inverse filtering the speech signal. Then the discrete cosine transform (DCT) of these residual frames is taken. Based on the desired factor of pitch modification, the dimension of the DCT coefficients of the residual is modified by truncating or zero padding, and then the inverse discrete cosine transform is obtained. This period modified residual signal is then forward filtered to obtain the pitch modified speech. The mismatch between the positions of the harmonics of the pitch modified signal and the LP spectrum of the original signal introduce gain variations, which is more pronounced in the case of female speech [Proc. Int. Conf. on Acoust. Speech and Signal Process. (1997) 1623]. This is minimised by modifying the radii of the poles of the filter to broaden the otherwise peaky linear predictive spectrum. The modified LP coefficients are used for both inverse and forward filtering. This pitch modification scheme is used in our Concatenative Speech synthesis system for Kannada. The technique has also been successfully applied to creating interrogative sentences from affirmative sentences. The modified speech has been evaluated in terms of intelligibility, distortion and speaker identity. Results indicate that our scheme results in acceptable speech in terms of all these parameters for pitch change factors required for our speech synthesis work.