Phonetically enriched labeling in unit selection TTS synthesis

Unit selection techniques have improved the quality of textto-speech (TTS) synthesis. However, mistakes which had been less noticeable previously in poorer quality synthetic speech become very noticeable in more natural-sounding synthetic speech. Many problems appear to be caused by mismatches between phones requested by the TTS frontend and phones selected from the labeled speech inventory. Given the input text and the added information predicted by the TTS front-end, finding the optimal units from a speech inventory database still remains a challenge in unit selection TTS synthesis. Consonants in American English affect intelligibility of speech synthesis and they are realized differently depending on their position in the syllable. Pre-vocalic plosives must have a release burst before the vowel begins while postvocalic consonants may or may not be released. When a post-vocalic consonant is chosen to synthesize a pre-vocalic consonant, it may cause problems such as missing consonants, consonant confusion or word-boundary confusion. In this paper, a new phone labeling method which differentiates pre-vocalic and post-vocalic consonants is proposed. The proposed phone labeling method leads unit selection to choose contextually accurate phone units and minimizes unit selection errors caused by lack of specification in TTS front-end transcriptions and phone labels in the speech inventory. In a listening test the TTS voices labeled with the pre-vocalic / post-vocalic distinction were rated significantly higher (+0.33) compared to reference voices that did not use this distinction.