Rule-based lexical modelling of foreign-accented pronunciation variants

Rule-based lexical modelling of foreign-accented pronunciation variants Stefan Schaden Institute of Communication Acoustics Ruhr-Universitat Bochum D-44780 Bochum, Germany schaden@ika.rub.de Abstract This paper describes a novel approach to generate potential foreign-accented pho- netic transcriptions using phonological re- write rules. For each pair of a native lan- guage (Li) and a target language (L2), a set of postlexical rules is designed to trans- form canonical phonetic dictionaries of L2 into adapted dictionaries for native Li speakers. Some general considerations on the design of such a rule-based system are presented. 1 Introduction Pronunciation dictionaries are a crucial component of speech recognition and speech synthesis sys- tems, as they form the link between the acoustic and symbolic level of automatic speech and lan- guage processing. Typically, each entry in a lexi- con is assigned a phonetic transcription that repre- sents its canonical form, i.e. its standard pronunciation in the language the system is de- signed for. Canonical lexicons, however, have the general drawback that every marked deviation from the standard form will lead to a mismatch between lexicon transcription and actual pronunciation. In This study was carried out at the Institute of Communication Acoustics, Ruhr-University Bochum (Prof. J. Blauert, PD U. Jekosch). It is funded by the Deutsche Forschungsgemein- schaft (DFG). Automatic Speech Recognition (ASR), this may cause a significant decline of the recognition per- formance. In recent years, a number of approaches to com- pensate for this mismatch by various lexical adap- tation techniques have been proposed (for an over- view see Strik, 2001), e.g. by adding alternative pronunciation variants to the lexicon, by generat- ing these variants using phonological rules, or by building pronunciation networks. Usually these techniques are applied to model frequently occur- ring stylistic variations such as within-word or cross-word assimilations or elisions in informal speech. It is the aim of our current research to extend the lexicon adaptation approach from intra-lingual variation to the domain of foreign-accented pro- nunciation. Non-native speakers frequently pro- duce variants that deviate markedly from the ca- nonical form. They are characterized by phenomena such as changes in allophonic realiza- tions, phoneme shifts, word stress shifts, or alter- nations in syllable structure caused by epenthesis or deletion of speech sounds. A primary (though not the only) source of these mispronunciations is a transfer of phonetic elements and rules from the speaker's native language onto the target language. The idea to model these errors by lexicon adap- tation is based on the assumption that for each lan- guage direction — i.e. a pair of a native language (Li) and a target language (L2) — a number of characteristic pronunciation errors can be identi- fied. Although there is a considerable range of in- ter-individual variation even for speakers with the same native language background (due to variables 159 such as L2 proficiency, age, education, dialectal origin, etc.), it is assumed that common mispro- nunciations can be formulated as rewrite rules to generate prototypical interlanguage transcriptions. Currently, the languages investigated are Ger- man (GER), English (ENG), and French …

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