A pitch pulse evolution model for linear predictive coding of speech /
Jacek Stachurski · eScholarship@McGill (McGill) · 1997
Speech coding is important in the effort to make more efficient use of digital telecommunication networks, particularly wireless systems, and to reduce the memory requirements in speech storage systems. The desire for a low-rate digital representation of speech is often contrary to the demand for a high quality speech reconstruction. In this thesis we present a new speech compression technique designed for near toll quality speech coding at bit rates as low as 4 kb/s. In low-rate speech coding based on linear prediction (LP), poor modelling of the LP excitation for voiced, quasi-periodic segments contributes to the degradation of the quality of the reconstructed speech. In this dissertation, we present a new speech coding method designed for improved modelling of the LP excitation. Conceptually, the LP excitation is decomposed into a series of underlying pitch pulses and a simultaneous unvoiced noise-like signal. The underlying pitch pulses are estimated from noisy observations, i.e. the pitch pulses extracted from the LP residual. Since the pulses change little from one time instant to another, we call our representation the Pitch Pulse Evolution (PPE) model. The PPE model provides a framework to analyze and effectively control the periodicity of voiced speech. We have developed a robust algorithm for extracting noisy pitch pulses from the LP residual based on error minimization with respect to a set of model pulses, and we have examined a number of methods for calculating the underlying pulses. The evolving pitch pulse waveshapes, the pulse positions, and the unvoiced signal are encoded separately. The positions and the shapes of the underlying pulses need only be coded infrequently, and the characteristics of intermediate pulses are obtained by interpolation. The software implementation of a 4 kb/s PPE coder is described. The main features of the implemented PPE coder are: a novel approach to pitch analysis; estimation of evolving pitch pulses which enables control over the pulse characteristics; and a unique coding scheme which avoids the time dilation and contraction of individual pitch pulses found in other waveform interpolation coders.