Local linguistic features

[Alex S]

In the videos for unit selection, it is stated that the linguistic features are local to each target and candidate unit.
What is meant explicitly by local in this case? Is this effectively assuming that each target and candidate unit have linguistic features which are independent to others in the unit sequence (e.g. the linguistic features of d-a have independent features to a-n in the sequence /dans/)?

[Simon]

The features (used by the target cost function) of candidates do indeed have to be independent of the features of other candidates. If there was a dependency, this would violate the conditional independence assumption that the search makes (recall that it is equivalent to a Markov model – it is memoryless).

Now, the features of target units will of course depend on the features of other target units. That’s not a problem – we are not searching over different sequences of target units (they are fixed, and depend only on the input text).

Also, the features of candidate units do depend on their neighbours within the sentence that they were extracted from. Again, that is a constant and not something that we are not searching over.

[Author]

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