Obviously, also, the unit selection algorithm must run in a fraction of real time on a standard processor. In respect to quality, there are two good explanations why the method of unit-selection synthesis is capable of producing customer quality or even natural quality speech synthesis. Another important issue related to quality of the synthetic speech is the choice of the voice talent one records. Unit Selection As outlined so far, Unit-Selection Synthesis will run too slowly on any given computer. In practice, however, we need to be able to run many channels of TTS on a single processor.
Several options exist to speed up unit selection.
The effort for computing the full target costs even for a smaller set of candidates, however, may still be too large, because every potential choice must be evaluated, given the sequence of phones we want to synthesize in their context. This set turns out to be significantly smaller than the set of all units of a particular unit type, and, as such, leads to a speed-up by a factor of two, with no loss of synthesis quality. More speed can be gained by attacking the join cost computation. Note that for an NxN transition, N2 join costs need to be computed. Since the join costs depend only on the database, but not on the current utterance, we can precompute them off-line.