Spectral modification for concatenative speech synthesis

Concatenative synthesis can produce high-quality speech but is limited to the allophonic variations and voice types that were captured in the database. It would be desirable to modify speech units to remove formant discontinuities and to create new speaking styles, such as hypo- or hyper-articulated speech. Unfortunately, manipulating the spectral structure often leads to degraded speech quality. We investigate two speech modification strategies, one based on inverse filtering and the other on sinusoidal modeling, and we explain their merits and shortcomings for changing the spectral envelope in speech. We then propose a method which uses sinusoidal modeling and represents the complex sinusoidal amplitudes by an all-pole model. The all-pole model approximates the sinusoidal spectrum well, both in the amplitude and in the phase domain. We use the sinusoidal+all-pole model to control the spectral envelope in recorded speech. High-quality modified speech is generated from the model using sinusoidal synthesis. A perceptual test was conducted, which shows that the model was effective at changing vowel identities and was preferable over residual excited LPC.