A two-level drive - Response model of non-stationary speech signals

The transmission protocol of voiced speech is hypothesized to be based on a fundamental drive process, which synchronizes the vocal tract excitation on the transmitter side and evokes the pitch perception on the receiver side. A band limited fundamental drive is extracted from a voice specific subband decomposition of the speech signal. When the near periodic drive is used as fundamental drive of a two-level drive-response model, a more or less aperiodic voiced excitation can be reconstructed as a more or less aperiodic trajectory on a low dimensional continuous synchronization manifold (surface) described by speaker and phoneme specific coupling functions. In the case of vowels and nasals the excitation can be described by a univariate coupling function, which depends on the momentary phase of the fundamental drive. In the case of other voiced consonants the coupling function may as well depend on a delayed fundamental phase with a phoneme specific time delay. The delay may exceed the length of the analysis window. The resulting long range correlation cannot be analysed or synthesized by models assuming stationary excitation.