Analysis of Vocal Tract Constrictions using Zero Frequency Filtering

This work proposes evidence using zero frequency filtering (ZFF) that gives an approximate measure of vocal tract constriction in terms of the low frequency component present in the speech signal. The vocal tract is completely closed in the case of voice bars and nasals and is wide open for low vowels. Intermediate cases are for high vowels, semivowels, laterals, voiced fricatives and other sounds. Vocal tract constriction affects the spectrum by reducing the first formant and attenuating the amplitude of the spectrum. The attenuation is relatively high in higher frequencies resulting in an increase in the low frequency component. The proposed method exploits the sinusoid like nature of ZFF signal (ZFFS) to obtain the evidence. Epoch synchronous analysis is performed and the ZFFS between successive epochs is compared with the corresponding speech segment using a cosine kernel. The low frequency dominant voiced regions match closely with ZFFS as compared to other regions and hence give higher value. This evidence when used as a feature gives relatively higher performance for the constricted phones in an HMM-based phoneme recognizer.