Modification of pitch using DCT in the source domain

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 in the positions of the harmonics between the pitch modified signal and the LP spectrum introduce gain variations, which is more pronounced in the case of female speech [16]. This is minimised by modifying the radii of the poles of the filter to smoothen the peaky linear predictive spectrum before 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. KeywordsLinear Prediction, Concatenative synthesis, residual signal, resampling, 3dB bandwidth.