Improving Naturalness and Intelligibility of Helium Speech, Using Vocoder Techniques

Abstract

Previous analysis of helium speech has shown that the peculiar characteristic of this speech is due primarily to changes in formant frequencies of the speaker. Even though some change does take place in the fundamental pitch frequency of the speaker, this effect is small and usually can be neglected. Computation of the resonant frequencies of the vocal tract as a function of the gas mixture occupying the tract gives a reasonable estimate of the changes that take place in the formant frequencies. A modified self-excited channel vocoder was designed to restore approximately the normal values of the talker's formant frequencies while preserving his fundamental pitch frequency. This “formant-restoring-vocoder” (FRV) separates the spectral energy of the helium speech into a number of narrow bands that then amplitude-modulate lower-frequency pitch harmonics derived directly from the hedium speech. The lower-frequency harmonics are separated by bandpass filters that are proportionately narrower and lower in frequency than the spectral-energy separating filters. Helium speech from SEALAB II was processed by an FRV simulated on a digital computer. Results of several simulations indicated that considerable improvement in naturalness and intelligibility of helium speech can be achieved.