LPC voicing periodicity correction

Abstract

The autocorrelation method of linear predictive coding (LPC) analysis matches the first p samples of the autocorrelation function of the impulse response of an all‐pole filter to the autocorrelation function computed from a speech waveform. The autocorrelation function of the speech waveform is influenced by the periodicity of the voice source, as well as by the waveform window. This influence, when incorporated into the autocorrelation function of the all‐pole filter, results in errors in formant frequency and bandwidth values that become important at the fundamental frequencies typical of female speech. The use of an analysis‐by‐synthesis technique is proposed, where the autocorrelation function of the output waveform of an LPC synthesizer is matched to the speech data autocorrelation function. The all‐pole filter determined by LPC analysis provides a match when the synthesizer is operated at an extremely low fundamental frequency. The synthesizer fundamental frequency is increased in small steps and corrections are made to the all‐pole filter at each step to preserve the autocorrelation function match. This process continues until the correct value of the fundamental frequency is reached. [Work supported by NIH Grant NS 21516‐3.]