Contribution of the glottal flow residual in affect-related voice transformation

This paper explores the contribution of the glottal flow residual in affect-related voice transformation. This signal, which is defined as the difference between the output of the inverse filter estimating the glottal flow signal and the modelled source signal, was analysed using multiple regression analysis. Results show that the strength of the residual varies as a function of the source parameters and this variation is frequency dependent: low frequency energy in the residual is mainly determined by the glottal excitation strength, whereas mid to high frequencies are more influenced by the glottal pulse shape. A method for modelling the residual is presented, which enables modifications based on the changes in source parameters used for voice transformation. This method makes it possible to use the residual as part of the voice source signal when transforming the voice quality in expressive speech synthesis. The result of a listening test, involving the transformation of a neutral voice to an angry or a sad voice, shows that including the glottal flow residual can improve the perceived naturalness of the synthesis. However, the fact that the transformed utterances are still relatively degraded indicates that other factors also need to be considered.