The second block in Fig. 2 assembles the units according to the list of targets set by the front-end. It is this block that is responsible for the innovation towards much more natural sounding synthetic speech. Then the selected units are fed into a back-end speech synthesizer that generates the speech waveform for presentation to the listener. In a sense, the desire for more natural-sounding synthetic voices that is driving this work was a natural extension of the earlier desire to achieve high intelligibility. We have started a new era in synthesis, where, under certain conditions, listeners cannot say with certainty whether the speech they are listening to was recorded from a live talker, or is being synthesized.
Many dimensions come to play.
One important aspect is the ever-increasing power and storage capacity of computers. This has direct effect on the size of the voice inventory we can store and work with. Other important aspects include the fact that efficient search techniques are now available that allow searching potentially millions of available sound units in real time for the optimal sequence that make up a target utterance. Finally, we now have automatic labelers that speed up labeling a voice database phonetically and prosodically. It is important to note that both, the automatic labelers and the optimal search strategies borrow heavily from speech recognition.