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Department of Linguistics

MU-Talk: Grapheme-to-phoneme Word Stress Rules

Robert Mannell

GTP rules tend to poorly handle word stress. In the string of phonemes output by the present GTP module, word stress is only indicated by the presence of full vowels or by the schwa vowel. GTP rules tend to greatly under-produce reduced vowels because only the most complex rules deal with orthographic strings that consist of more than one syllable. Single syllable rules are effectively monosyllabic word rules and single syllable words in citation form generally contain fully realised vowels rather than reduced vowels (schwa).

Word stress rules require a word-stress post-processor module which applies vowel reduction rules once the syllabic structure of the word and the vowel in each syllable is known. Additionally, word stress rules require knowledge of whether a word is a noun or a verb (etc.) for the application of certain rules. Also, in continuous speech, function word reduction rules also need to be applied. Words generated by the GTP rule system are unknown words, so part-of-speech and function/content words status would be unknown (although such words will always be assumed to be content words as the set of function words is a closed set and all such words are in the dictionary).

In the present system, the word-stress post-processor is currently absent. It is intended that such a module will apply vowel reduction rules to the strings output by the GTP rules. Since many rules are probabilistic, this proposed module will often produce more than one version of a word, with different vowels reduced in each version. Note that the GTP rules produce only one pronunciation, but that the word-stress post processor may produce more than one pronunciation differing only in stress patterns.

Ultimately, it is hoped that an algorithm based on stress rule probability will select the most likely word pronunciation. In cases where the probability of two alternative reduced pronunciations are approximately equal, it may be necessary to select the original unreduced form. The unreduced form is more likely to be understandable than a form that has had the wrong vowels reduced to schwa.