Allophonic and Phonotactic Constraints Are Useful

Abstract

This paper argues that allophonic and phonotactic cues are a source of constraint, not a source of noise as many speech researchers have assumed in the past. These constraints are formulated so that they can be exploited with well-known parsing techniques. 1. Parsing at the Phonetic Level It is well known that phonemes have different acoustic/phonetic realizations depending on the context. 1 For example, the phoneme /t / is typically realized with a different allophone (phonetic variant) in syllable initial position than in syllable final position. In syllable initial position (e.g., Lorn), /t / is almost always released (with a strong burst of energy) and aspirated (with /h / like noise), whereas in syllable final position (e.g., ca[), /t / is often unreleased and unaspirated. It is common practice in speech research to distinguish acoustic/phonetic properties that vary a great deal with context (e.g., release and aspiration) from those that are relatively invariant to context (e.g., place, manner and voicing). 2 In the past, the emphasis has been on invariants; allophonic variation is traditionally seen as problematic for recognition. (1) "In most systems for sentence recognition, such modifications must be viewed as a kind of 'noise ' that makes it more difficult to hypothesize lexical candidates given an input phonetic transcription. To see that this must be the case, we note that each phonological rule [in an example to be presented below] results in irreversible ambiguity- the phonological rule does not have a unique inverse that could be used to recover the underlying