Finite State Processing Of Tone Systems

It is suggested in this paper that t w o l e v e l morphology theory (Kay, Koskenniemi) can be extended to include morphological tone. This extension treats phonological features as I/O tapes for Finite State Transducers in a parallel sequential incrementation (PSI) architecture; phonological processes (e.g. assimilation) are seen as variants of an elementary unification operation over feature tapes (linear unification phonology, LUP). The phenomena analysed are tone terracing with tone-spread ing (horizontal assimilation), downstep, upstep, downdrift, upsweep in two West African languages, Tem (Togo) and Baule (C6te d'Ivoire). It is shown that an FST acccount leads to more insightful definitions of the basic phenomena than other approaches (e.g. phonological rules or metrical systems). 1. Descriptive context The topic of this paper is tone sandhi in two West African tone languages and suitable formal models for it. The languages investigated are Tern (Gur/Voltaic family, Togo) and Baule (Akan family, C6te d'Ivoire). Tone languages of other types, in particular the S ino-T ibe tan languages, will not be discussed. The specific concern of this paper is with the way in which certain quite w e l l k n o w n morphophonological (lexical) tone patterns are realized in sequence in terms of phonetic pitch patterns. There are three interacting factors involved: i. t o n e t e x t association rules; ii. t one sandh i rules; iii. phonetic interpretation rules. T o n e t e x t association rules are concerned with the association of tones with syllables (primary associations and a form of tone spreading) as well as floating tones and compound tones. Floating tones are not associated with syllables, but are postulated to explain appparent irregularities in phonetic patterning in terms of regular tone sandhi properties. The tone sandhi rules define how tones affect their neighbours. The example to be treated here is a kind of tonal assimilation known as tonal spreading in which low tones are phonetically raised following a high tone or, more frequently, high tones are lowered after a low tone, either to the level of the low tone (total downstep) or to a mid level (partial downstep). The newly defined tone is then the reference point for following tones. The latter kind of assimilation produces a characteristic perceptual, and experimentally measurable, effect known as tone terracing. Tone sequences are realized at a fairly high level at the beginning of a sequence, and at certain w e l l defined points the whole pitch register appears "to be downstepped to a new level. The process may be iterated several times. It is often represented in the literature in the following way (partial downstep); it can be seen that a later high tone may be as high as or lower than an earlier low tone: h h h l l h h l l h h In particular, it will be seen that the two terraced tone languages, Tem and Baule, involve similar processes in detail and have similar basic FST architectures, but differ systematically at certain wel l -def ined points involving sandhi generality, and scope of sandhi context.