Prosodic phrasing modeling for vietnamese TTS using syntactic information

Abstract

This research aims at modeling prosodic phrasing for improving the naturalness of Vietnamese (a tonal language) speech synthesis. The proposed phrasing model includes hypotheses on: (i) prosodic structure based on syntactic rules (ii) final lengthening linked to syllabic structures and tone types. Audio files in the analysis corpus are manually transcribed at the syllable level and perceived pauses. Text files are parsed and represented with annotated-syntax trees. Statistical treatment brings out a correlation between syntactic element boundaries and pause duration. Major breaks may appear at the end of a clause or between predicates or head elements. Other rules between grammatical phrases/words or shorter clauses may trigger minor breaks. Break levels (including ones predicted by syntactic rules) and relative positions of syllables are used to train VTed, an HMM-based Text-To-Speech (TTS) system for Vietnamese. In the synthesis phase, break levels are explicitly inserted while lengthening is applied for last syllables of prosodic phrases. Perceptive testing shows an increase of 0.34 on a 5 point MOS scale, for the new prosodic informed system (3.95/5) compared to the previous TTS system (3.61/5). In the pairwise comparison test, about 70% of the synthetic voice with the proposed model is preferred to the previous version.