Phonological and phonetic coarticulation
Phonological aspects of coarticulation (allophonic variation) are accounted for by means of feature sharing.
Both vowels and consonants have vocalic place features. Vowels and glides are fully specified for voc features, but:
- /l/ is "clear" in onset, "dark" in coda
- /r/ is unspecified for rounding
Voc features which are unspecified in the lexicon, receive a value through feature spreading:
- in YorkTalk, spreading of voc features is defined in the phrase structure rules on a case-by-case basis
- in IPOX, spreading of voc features is defined by means of templates
- the following templates are used to guarantee that (within a syllable) voc features are "percolated upwards" along the left and right edges of a constituent:
[left=A, voc=A, right=A].
[left=A right=B] --> [left=A, right=B].
[left=A right=B] --> [left=A], [right=B].
- the following templates are used to share voc features between strong and weak nodes:
 --> ([spread=A] / [left=A]).
 --> ([right=A] \ [spread=A]).
- the following templates assign the feature spread the status of a head-feature:
[spread=A] --> ( / [spread=A]).
[spread=A] --> ([spread=A] \ ).
- the following constraints are added to the lexicon:
voc:[grv=A, height=close, -rnd]] --> "l".
voc:[+grv, height=mid, rnd=A], spread:[rnd=A]] --> "r".
cons:[-cnt, ... voc=A, spread=A ...] ---> ...
cons:[-son, ... voc=A, spread=A ...] ---> ...
In the exponency rules different values may be used for various synthesis parameters, depending on the voc features associated with a node. Thus, very subtle differences in phonetic implementation can be accounted for.
The following examples illustrate allophonic variants of /s/ in onset, with a "clear" or "dark" spectrum, depending on whether /s/ coarticulates with the vowel or with the glide: