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. 9. The system of claim 8 , the computer-readable storage medium having additional instructions stored which, when executed by the processor, cause the processor to perform operations comprising synthesizing future speech using the default value as the concatenation cost.