Glottal source modeling for voice conversion

Abstract This paper describes recent advances in glottal source modeling for speech synthesis. In particular two procedures for modeling the glottal excitation waveform are described and applied to voice conversion. One model uses a polynomial to represent the glottal excitation waveform for one pitch period. The coefficients of the polynomial model form a vector that is used to design a glottal excitation code book with 32 entries for voiced excitation. The codebook is designed and trained using two sentences spoken by different speakers. Speech is synthesized using a quantized glottal excitation waveform for one speaker as the excitation for a glottal excitation linear predictive (GELP) synthesizer designed using tract parameters obtained from the speech of another speaker. Our implementation of the LP synthesizer is patterned after both a pitch-excited LP speech synthesizer and a code excited linear predictive (CELP) speech coder. In addition to the glottal excitation codebook, we use a stochastic codebook with 256 entries for unvoiced noise excitation. Analysis techniques are described for constructing both codebooks. The GELP synthesizer, which resynthesizes speech with high quality, provides the speech scientist with a simple speech synthesis procedure that uses established analysis techniques, that is able to reproduce all speech sounds, and yet also has an excitation model waveform that is related to the derivative of the glottal flow and the integral of the residue. Another approach uses the LF glottal volume-velocity waveform to model the characteristics of three voice types: modal, breathy, and vocal fry (creaky). We then convert a modal voice to sound like a breathy or vocal fry voice using the vocal tract characteristics for modal voice and the glottal volume-velocity waveform model for breathy and vocal fry voices as the excitation.