Systematic errors in the formant analysis of steady-state vowels
The locations of formants in a speech signal are usually estimated by computing the linear predictive coefficients (LPC) over a sliding window and finding the peaks in the spectrum of the resulting LP filter. The peak locations are estimated either by root-solving or by computing a coarse spectrum and finding its maxima. We discuss four sources of systematic error in this analysis: (1) quantization of the speech signal due to the fundamental frequency, (2) incorrect order for the LP filter, (3) exclusive reliance upon root-solving, and (4) the three-point parabolic interpolation used to compensate for the coarse spectrum. We show that the expected error due to F0 quantization is ?10% of F0, and that the other three sources can independently skew the final formant estimates by 1080 Hz. We also show that errors due to incorrect filter order are related to systematic differences between speakers and phonetic classes, and that root-solving is especially error-prone for low formants or when formants are close to each other. We discuss methods for avoiding these errors and improving the accuracy of formant estimation, and give a heuristic for estimating the optimal filter order of a steady-state signal.
vowels lpc formant estimation speech analysis