Quasi-periodic instability in a linear prediction analysis of voiced speech

A significant semiperiodic fluctuation of the vocal tract area functions derived by linear prediction of the speech waveform has been noted during apparently stationary voiced segments of speech. In one example some values of the area function varied over a range of 9:1 over a few pitch periods. The phenomenon is attributed to "beating" of the pitch period and the time interval between successive computations which causes variations of the time relationship between glottal pulse and analysis window. This is supported by the fact that no fluctuations occur in the area function derived from natural or synthetic speech when the computation interval is equal to the pitch period. Any slight difference between the two leads to significant pulsations, however. A simple theoretical model is used to show how the positioning of the analysis window can influence area function estimates. The problem can be largely overcome by using longer time windows (greater than 2.5 pitch periods), or alternatively, by averaging the area functions over several adjacent intervals