20. The non-transitory computer-readable storage device of claim 15 , wherein the concatenation cost comprises a weighted sum of subcosts across phones.
15. A non-transitory computer-readable storage device having instructions stored which, when executed by a computing device, cause the computing device to perform operations comprising: synthesizing speech from a text; identifying an acoustic unit sequential pair in the speech; searching for a concatenation cost for the acoustic unit sequential pair in a database using a hash table for the database; and when the concatenation cost is not found in the database, assigning a default value as the concatenation cost for the acoustic unit sequential pair.