Vowel normalization by frequency warping
Hiroshi Matsumoto, H. Wakita · The Journal of the Acoustical Society of America · 1978
Normalizations of formant frequencies have frequently been used to eliminate inter-speaker differences in vowel identification. However, estimation of formant frequencies become difficult under certain circumstances. This paper proposes a new approach to vowel normalization based on a frequency warped spectral matching. A frequency normalized distance between test and reference spectra is defined on the basis of the minimum mean square difference over all possible choices of frequency warping functions. The warping functions are constrained so that they mostly take account of linear scaling for the differences in vocal-tract lengths and of slight nonlinear differences. The slope of the warping function is restricted by an assumed range of the vocal-tract length. After adaptively eliminating the individual glottal characteristics, the spectral distance is computed by means of dynamic programing. A preliminary experiment indicates a substantial improvement in identifying female steady-state vowels when male vowels are used as references. The vocal-tract length can also be estimated from the slope of the warping function.