Word Boundary Identification from Phoneme Sequence Constraints in Automatic Continuous Speech Recognition

Word Boundary Identification fro m Phoneme Sequence Constraints in Automatic Continuous Speech Recognition Jo~atha~t HARRINGTON Gordon WATSON Maggie COOPER The Centre for Speech Technology Research, University of Edinburgh, 80 South Bridge, Edinburgh EH1 1HN, Scotland. Abstract Th.s paper explores the extent to which phoneme sequence constralnt'~ can be used to identify word boundaries in coutinnous speech recog~fition. The input consists of phonemic transcriptions (without word boundaries indicated) of 145 utterances produced by 1 ~e speaker. The constraints are derived by matching the complete set of 3 phonente sequences that can occur' across word boundaries to entries in large lexicons containing both citation and reduced h)rm pronunciations. Phonemic assimilatoxy adjushnents across word boundaries are alse taken into account. The results show that around 37% of all word boundaries can be correctly identified t rom a knowledge of .~uch phoneme sequence contraints alone, and ~hat this figure rises to 45% when a knowledge of one- and two-phoneme words and all legal, word-initial and word-final, two-pitoneloe sequences are taken into account. The possibility of including :~uch constraints in tim architecture of a corrtinuous speech reeogniser is discussed. I. Introduction TiLe identification of word boundaries fl'om continuous speech by human listeners depends, in part, on an interaction between :prosodic, syntactic and semantic processing, Since, however, ~his interaction is difficult to model in machines and since some prosodic variables, such as sentence stress patterns, are difficult to extract automatically from the acoustic waveform, the identification of word bmmdaries must often be accomplished by different hinds of processing in continuous ,~peeciL reeognisers: one possibility~ discussed in Lamel & Zue (1984) and explored in this paper, depends on the incorporation of a knowledge of' phoneme sequence constraints, Phoneme sequence constraints are based on a knowledge of phoneme sequences which do not occur ! word-internally: for' example, since there are no words which end in/m g/~ at~d since/m g l/does not occur word-internally, a word boundary must occur after/m/ (Lamel & Zue, 1984). Harrington, Johnson & Cooper (1987) showed that word boundary CVC sequences are often excluded word-internally in monomorphemic words if the pre- and post-vocalic consonants are similar: thus,/s N V N/(N == nasal),/C l V l/,/f V p/,/g V ld,/z V ,iJ,/sh V sh/ are all exchtded, or are at least extremely rare, word-internally in British English Received Pronunciation (top). In the study discussed b.~low, we extend the investigations of Lamel & Zue i (1984) anti 1 larrington et al. (1987) by developing an algorithm for the autmmttic identification o~" word boundaries from such sequences in a continuous speech recogniser. In the Alvey Demonstrator continuous speech recogniser being developed at the Centre for Speech Technology Research (CSTR), Edinburgh University (Figure 1), the identification of word boundaries from a string of phonemes is accomplished by a chart-parsing s~rategy which matches the lexicon from left-to-right against a string of phonemic symbols that are themselve~ derived from the phonetic processing of the acoustic-waveform. In this system, only cmnplete parsings of the phonemic units are passed to higher' levels for …

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