Unlimited Vocabulary Grapheme to Phoneme Conversion for Korean TTS

This paper describes a grapheme-to-phoneme conversion method using phoneme connectivity and CCV conversion rules. The method consists of mainly four modules including morpheme normalization, phrase-break detection, morpheme to phoneme conversion and phoneme connectivity check. The morpheme normalization is to replace non-Korean symt)ols into standard Korean graphemes. The phrase-break detector assigns phrase breaks using part-of-speech (POS) information. In tile mort)heme-to-phoneme conversion module, each morpheme in the phrase is converted into phonetic t)atterns by looking up the morpheme phonetic pattern dictionary which contains candidate phonological changes in boundaries of the morphemes. Graphemes within a morpheme are grouped into CCV patterns and converted into phonemes by the CCV conversion rules. The phoneme connectivity table supports grarnmaticality checking of the adjacent two phonetic morphemes. In tile experiments with a cortms of 4,973 sentences, we achieved 99.9% of the graphemeto-phoneme conversion performance and 97.5% of the sentence conversion performance. The full Korean TTS system is now being implemented using this conversion method. 1 I n t r o d u c t i o n During the past few years, remarkable improvements have been made for high-quality textto-speech systems (van Santen et al., 1997). One of the enduring problems in developing high-quality text-to-speech system is accurate grapheme-to-phoneme conversion (Divay and Vitale, 1997). it can be described as a function mat)ping the si)elling of words to their t)honetic symbols. Nevertheless, tile fimction in some alphabetic languages needs some linguistic knowledge, especially morphological and phonological, but often also semantic knowledge. In this paper, we present a new grapheme-tophoneme conversion Inethod for mflimited vocabulary Korean TTS. The conversion method is divided into mainly tbur modules. Each module has its own linguistic knowledge. Phrasebreak detection module assigns phrase breaks onto part-of-speech sequences using mort)hological knowledge. Word-boundaries before and alter phrase breaks should not be coarticulated. So, accurate phrase-break assignments are essential in high quality TTS systems. In the morpheme-to-phoneme conversion module, boundary graphemes of each morpheme in the phrase are converted to phonemes by applying phonetic patterns which contain possible phonological changes in the boundaries of nmrphemes. The patterns arc designed using morphological and phonotactic knowledge. Graphemes within a morpheme are converted into phonemes by CCV (consonant consonant vowel) conversion rules which are automatically extracted from a corpus. After all the conversions, phoneme connectivity table supports the grammaticality of the adjacency of two phonetic morphemes. This grammaticality comes from Korean phonology rules. This pat)er is organized as tbllows. Section 2 briefly explains the characteristics of spoken Korean for general readers. Section 3 and 4 introduces our graphcme-to-phoneme conversion method based on morphological and phonological knowledge of Korean. Section 5 shows experiment results to demonstrate the performance and Section 6 draws sonm conclusions.