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Enhancement of Alaryngeal Speech Using Transformation Functions

In normal speech generation, air stream from the lungs acts as dc energy source. This dc flow is changed to ac flow by the vocal chords in the larynx or stoppage / constriction in the vocal tract. The vocal tract also provides the spectral shaping for the resulting speech. Persons who suffer from the diseases such as throat cancer or injury require the removal of vocal chords. Such persons use artificial larynx to generate the speech as an alternative to vocal folds. The quality of the alaryngeal speech so produced is very annoying and difficult to interpret the meaning. The objective of this research is to investigate the use of transformation functions obtained from the harmonic plus noise model (HNM) parameters of the alaryngeal and natural speech for the enhancement of quality and intelligibility. For this study, we have used natural vowels and alaryngeal vowels uttered by using artificial larynx. For establishing a mapping from the acoustical space of the alaryngeal speech to the natural speech, harmonic analysis of the vowels was carried out. The harmonic magnitude of the alaryngeal vowels and the corresponding harmonic magnitudes of the natural vowels were plotted. A linear relation was established for each harmonic to estimate the mapping function from alaryngeal space to the natural one. The analysis of the transformation functions shows that these vary across vowels and speakers. It means any alaryngeal vowel can be transformed towards natural vowel by using these transformation functions. This functions may be used for enhancing the naturalness and intelligibility of the alaryngeal speech. Keywords— Alaryngeal Speech; artificial larynx; noise model; style; HNM; transformation functions