Context sensitive synthesis models

We use context-sensitive models in ASR because they have lower variance than that of the general phone models. In synthesis, we use context-sensitive models for a similar reason, so that a model in a particular context generates observations appropriate to that context only. In synthesis, however, we are interested in a broader range of factors, since we have to generate the prosodic variation as well as simply the verbal, phonetic variation. 

Because of this, it is coiutnon to use a much broader notion of context . In essence we decide what linguistic features we think are required for synthesis and build a separate model for every possible combination. The issue of features in synthesis is discussed fully in Section 16.2 for now we can just assume that, in addition to phonetic context, we want to include also features such as phrasing, stress and intonation. 

The context-sensitive models are built in exactly the same way as described in Section 15.1.9, and the resultant decision tree provides a unique mapping from every possible feature combination to HMM model parameters. One significant point of note is tiiat in synthesis the extra prosodic features mean that the number of possible unique feature combinations can be many orders of magnitude larger than in ASR. Given that we may be training on fewer data than in ASR, we see that the problems arising from sparsity of data can be considerably worse. We will return to this issue in Section 16.4. 

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We first show by a counterexample that context free languages are insufficient model for organic synthesis. We then argue (alas we cannot prove, for the above reasons) that context sensitive languages are a sufficient model. 

Bradykinin stimulates natriuresis and, through stimulation of prostaglandin synthesis, inhibits the actions of antidiuretic hormone (ADH), thereby inhibiting water retention. Bradykinin further improves insulin sensitivity and cellular glucose utilization of skeletal muscle cells in experimental models. This, however, appears not to be relevant in the clinical context. 

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