Speech Analysis-Synthesis System Based on Homomorphic Filtering

Abstract

A digital speech analysis-synthesis system based on a recently proposed approach to the deconvolution of speech [A. V. Oppenheim and R. W. Schafer, Homomorphic Analysis of Speech, Conference on Speech Communication and Processing (1967)] will be discussed. The analyzer is based on a computation of the cepstrum considered as the inverse Fourier transform of the log magnitude of the Fourier transform. The transmitted parameters represent pitch and voiced-unvoiced information as measured from the cepstrum, and the low-time portion of the cepstrum representing an approximation to the cepstrum of the vocal-tract impulse response. In the synthesis, the low-time cepstral information is transformed to an impulse response function which is then convolved with a train of impulses during voiced portions or a noise waveform during unvoiced portions to reconstruct the speech. Since no phase information is retained in the analysis, phase must be regenerated during synthesis. Either a zero-phase or minimum-phase characteristic can be obtained by simple weighting of the cepstrum before transformation [Lincoln Lab. operated with support from the U. S. Air Force.]