Phone class join costs

A significant factor is the type of phone that is being joined. In the formulations which join in the middle of phone (e.g. diphone, half-phone) we are always joining units that share the same phone across the boimdary (e g. wejoina/s-iy/toan/iy-n/). The identity of this phone has a significant impact on the quality of the join insofar as it appears that some phones are more sensitive to being chopped and recombined, whereas others are more robust. 

Syrdal and colleagues have conducted extensive formal studies of these effects [426, 428] and showed that there is a basic hierarchy of phone classes, ranked by how often listeners can detect a join. The results of this are shown in Table 16.1 and show that, regardless of the actual units involved, some phone types simply join better than others. Informally, we can think of this as a type of prior on the join cost, which is used to bias 

Tabie 16.2 Correlation of perceptual ratings of goodness of join with various acoustic measures and distance functions for a single vowel /ey/ (from Vepa etal. [472]) 

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