The use of allophonic variations of /a/ in automatic continuous speech recognition of French
Résumé
The acoustic characteristics of the vowels in continuous speech are affected by their duration, the stress condition, and the surrounding phonemes. The purpose of this study is to investigate the extent of the allophonic variations of the most frequent and most variable vowel in French, /a/, and the integration of such variations in an automatic speech recognition system. The preliminary corpus consists of 200 vowels /a/ extracted from 128 sentences uttered by two speakers. Using the SPIRE facilities, the sentences were digitized and segmented [Zue and Leung, J. Acoust. Soc. Am. Suppl. 1 75, S59 (1984)]. The segments labeled as SONORANT corresponding to the occurrence of /a/ were extracted. The values of F1 and F2 at the onset, offset, and middle of the segments were estimated using automatic peak tracking from the LPC spectra. Our results suggest that: (1) the MANNER of articulation of the preceding and following consonant plays a negligible role on F1 and F2 onset and offset (in contrast with other data published for English and Swedish); (2) if the PLACE of articulation of the consonants is divided into FRONT, MID, and BACK, then the difference (F2-F1) at /a/ onset allows the unique specification of the place of articulation of the preceding consonant in 86% and 57% of the cases, for SP1 and SP2, respectively, and there is no overlap between FRONT and BACK consonants; (3) (F2-F1) at /a/ offset indicates that F2 of the following vowel has to be taken into account before interpretation; (4) determination of the thresholds (for MID and BACK consonants) can be done on a few well selected words, where the effect of the context is known to be maximal. Detailed descriptions of our findings extended to the study of more speakers, and of their consequences for the coding of the permissible variations of the vowel /a/ for each word of the vocabulary will be presented. [Work supported in part by the Office of Naval Research under contract N00014-82-K-0727.]