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Estimations of glottal waves and vocal-tract area functions from speech signals Deng, Hui Qun

Abstract

This study estimates glottal waves and vocal-tract area functions (VTAFs) from vowel sounds. Traditional estimations assume that glottal waves are zero over closed glottal phases, and that glottises and lips are terminated with constant impedances. In reality, these assumptions are invalid: glottal waves can hardly be zero due to common incomplete glottal closures and acoustic disturbances during vocal-fold collisions; glottal impedances are time-varying during phonation; lip radiation impedances are frequency-dependent. Consequently, traditional estimations yield biased and distorted estimates. In this study, a method which for the first time obtains unbiased vocal-tract filter (VTF) estimates from sustained vowel sounds over closed glottal phases is developed. It assumes that glottal waves for such sounds are periodically stationary random processes, allowing non-zero glottal waves to exist over closed glottal phases. A new method for detecting glottal phases using vowel sounds is also developed. The effects of glottal and lip terminal impedances on VTF estimates are modeled realistically using high-pass, and low-pass filters, respectively. The VTF estimates are used to obtain glottal waves from the vowel sounds. Moreover, a new method for deriving VTAFs from the VTF estimates over closed glottal phases is developed. It eliminates the distortion effects of lip radiation impedances on the VTAF estimates, assuming the glottises are completely closed. Effects of glottal losses on the estimates obtained using our methods are investigated. It is shown that estimates from large-lip-opening vowel sounds are less affected by glottal losses than those from small-lip-opening vowel sounds. Theoretically, to enable the elimination of the degrading effects of glottal losses on the estimates, lip-opening areas must be known. Glottal phases, glottal waves and VTAFs estimated using our methods from vowel sounds produced by male and female subjects contain detailed information. The obtained glottal phases were validated using electroglottograph signals. The obtained glottal waves increase during rapid vocal-fold collisions, and decrease or even increase during vocal-fold parting. The differences in glottal waveforms of different genders are explained by their physiological differences in larynxes. The VTAFs obtained from large-lip-opening vowel /a/ sounds of these subjects are very similar to the VTAF measured from an unknown subject's magnetic resonance image. Such detailed results cannot be obtained using traditional methods.

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