The use of Diphone Variants in Optimal Text Selection for Finnish Unit Selection Speech Synthesis

The speech quality of a unit selection speech synthesizer depends highly on the database. This paper describes an approach for sentence selection for Finnish speech database recordings aiming at optimal coverage. The main idea is to define the diphone in a slightly different way: to distinguish diphones consisting of different allophones and also different linguistic positions, i.e. intraand inter-syllabic diphones. We call these diphone variants. We evaluated if diphone variants become included in text selection for TTS prompt design without separate optimization and coarsely verified their acoustic dissimilarity. With the same number of sentences (292) that fulfill the traditionally determined diphone coverage completely, 66% more allophonic and inter/intra-syllabic contexts were missing with the conventional method compared to the proposed approach. We also describe how the approach inspired the synthesis process to reduce computational load.