Speech Technol. Lab., Panasonic Technologies, Inc., 3888 State St., Santa Barbara, CA 93105 and Dept. of Linguistics, Univ. of California at Santa Barbara, Santa Barbara
Panasonic Technologies, Inc., Santa Barbara, CA 93105
Distinguishing between the voiceless fricatives /f/ and /(theta)/ is a difficult problem in natural and synthetic speech. In a previous experiment using natural stimuli [K. Hata et al., Proc. ICSLP 327--330 (1994)], it was found that adding vowel transitions increased identification for /f/ at least 15% in comparison with frication-only stimuli. However, with vowel transitions, the identification of /(theta)/ failed to show significant improvement. The purpose of the current study was to investigate, with an improved procedure, significant cues for /(theta)/ which we can use in our synthesizer. Six monosyllabic nonsense words (e.g., /fiyk/, /(theta)ayk/) were recorded. Segments of approximately 30-ms duration from different locations of /(theta)/ and its following vowel were spliced into f-initial words. Eight subjects were asked to identify each stimulus as ``th,'' ``f'' or ``indistinguishable.'' In the /iy/ context, /f/-initial stimuli spliced with fricative-vowel transitions from /(theta)/ were perceived as /(theta)/ 55% of the time, while stimuli involving other vowel contexts and other splices tended to be perceived as /f/. This implies that a cue for /(theta)/ resides in this transition when followed by a high, front vowel, but that the cue is rather obscure in the 0--5 kHz region when other vowels follow.