TEXT TO SPEECH SYNTHESIS SYSTEM AND METHOD USING CONTEXT DEPENDENT VOWEL ALLOPHONES

A text-to-speech conversion system converts specified text strings into corresponding strings of consonant and vowel phonemes. A parameter generator converts the phonemes into formant parameters, and a formant synthesizer uses the formant parameters to generate a synthetic speech waveform. A library of vowel allophones are stored, each stored vowel allophone being represented by formant parameters for four formants. The vowel allophone library includes a context index for associating each said vowel allophone with one or more pairs of phonemes preceding and following the corresponding vowel phoneme in a phoneme string. When synthesizing speech, a vowel allophone generator uses the vowel allophone library to provide formant parameters representative of a specified vowel phoneme. The vowel allophone generator coacts with the context index to select the proper vowel allophone, as determined by the phonemes preceding and following the specified vowel phoneme. As a result, the synthesized pronunciation of vowel phonemes is improved by using vowel allophone formant parameters which correspond to the context of the vowel phonemes. The formant data for large sets of vowel allophones is efficiently stored using code books of formant parameters selected using vector quantization methods. The formant parameters for each vowel allophone are specified, in part, by indices pointing to formant parameters in the code books.