CHOICE OF FILTER ORDER IN LPC ANALYSIS OF VOWELS
Gautam K. Vallabha, Betty Tuller · 2004
In studies of vowel perception and production, the formants of the signal are usually estimated using Linear Predictive Coding (LPC) analysis. The order of the LPC filter is typically estimated by starting with a heuristic value (e.g. the sampling frequency in kHz) and adjusting it in a trialand-error manner. This method is adequate when analyzing a few sounds, but it is cumbersome for studies involving large vowel corpora (and error-prone, since under- and over-estimation of the filter order can both lead to inaccurate formant estimates; Vallabha & Tuller (2002), Speech Comm. 38(1-2), 141-160). We present a “reflection coefficient cutoff” (RCC) heuristic that can be used to determine quickly the “best” filter order for either a corpus of vowels or for a single vowel. The heuristic is as follows: (1) Pick a set of representative analysis frames from the vowel corpus and compute their reflection coefficients. (2) Choose the “best” filter order to be the smallest order past which a majority of the reflection coefficients are less than a pre-defined cutoff. The cutoff value was determined empirically, but we show that it is related to general model selection criteria such as the Minimum Description Length. In addition, we show that the heuristic can reliably mark the distinction between different sampling rates and male versus female speakers so that the filter order can be adjusted appropriately.