Very fast unit selection using Viterbi search with zero-concatenation-cost chains

This paper introduces a very fast heuristic search algorithm for unit-selection speech synthesis. The algorithm modifies commonly used Viterbi search framework by introducing zero-concatenation-cost (ZCC) chains of unit candidates that immediately neighbored in a source speech corpus. ZCC chains are preferred as they represent perfect speech segment concatenations (so there is no need to compute concatenation costs inside the chains) unless a so-called target specification is violated. The number of ZCC chains is reduced based on statistics calculated upon the synthesis of a large number of utterances. ZCC chains are then combined with single unit candidates to fill possible gaps in the sequence of candidates. The proposed method reduces the computational load of a unit selection system up to hundreds of times. According to listening tests, the quality of synthetic speech was not deteriorated.