8. A system comprising: a processor; and a computer-readable storage medium having instructions stored which, when executed by the processor, cause the processor 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.
8. A system comprising: a processor; and a computer-readable storage medium having instructions stored which, when executed by the processor, cause the processor 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. 13. The system of claim 8 , wherein the concatenation cost comprises a weighted sum of subcosts across phones.