8. A system comprising: a processor; and a computer-readable storage medium having instructions stored which, when executed by the processor, result in the processor performing operations comprising: combining a first database of first text-to-speech voice units and a second database of second text-to-speech voice units, the first text-to-speech voice units originating from a different speaker than the second text-to-speech voice units, to yield a combined database; storing the combined database as if the combined database were from a single speaker; determining a selection policy based on a current emotional context; selecting from the combined database, based on the selection policy, voice units of a phonetic category for a synthetic voice, to yield selected voice units, wherein the selected voice units comprise a first voice unit from the first text-to-speech voice units and a second voice unit from the second text-to-speech voice units; and synthesizing speech based on the selected voice units.
10. The system of claim 8 , wherein the selection policy defines, for a particular phonetic category, from which text-to-speech voice to select the voice units.