Extending concatenative synthesis

We now turn to unit selection synthesis which is the dominant synthesis technique in text-to-speech today. Unit selection is the natural extension of second generation concatenative systems, and deals with the issues of how to manage large numbers of units, how to extend prosody beyond just FO and timing control, and how to alleviate the distortions caused by signal processing. 

Within one t5q)e of diphone, all variation is accountable by pitch and timing differences 

The signal processing algorithms are capable of performing all neeessary pitch and timing modifications without incurring any unnaturalness. 

One way of realising this is as a direct extension of the original diphone principle. Instead of recording and analysing one version of each diphone, we now record and analyse one version for each combination of specified features. In principle, we can keep on expanding this methodology, so that if we wish to have phrase initial, medial and final units of each diphone, or a unit for every type or variation of pitch accent, we simply design and record the data we require. 

As we use more features, we see that in practical terms the approach becomes increasingly difficult. This is because we now have to collect significantly more data and do so in just such a way as to collect exactly one of each feature value. Speakers cannot of course not utter specific diphones in isolation, and so must do so in carrier words or phrases. This has the consequence that the speaker is uttering speech in the carrier phrases that is not part of the required list of effects. If we adhere strictly to this paradigm, we should throw this extra speech away, but this seems wasteful. The unit selection approach offers a solution to both these problems, which enables us 

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