Unlimited Vocabulary Grapheme to Phoneme Conversion for Korean TTS

Unlimited Vocabulary Grapheme to Phoneme Conversion for Korean TTS Byeongchang Kim and WonI1 Lee and Geunbae Lee and Jong-Hyeok Lee Department of Computer Science & Engineering Pohang University of Science & Technology Pohang, Korea {bckim, bdragon, gblee, jhlee)@postech.ac.kr Abstract 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 detec- tion, morpheme to phoneme conversion and phoneme connectivity check. The morpheme normalization is to replace non-Korean symbols into standard Korean graphemes. The phrase-break detector assigns phrase breaks using part-of-speech (POS) information. In the morpheme-to-phoneme conversion module, each morpheme in the phrase is converted into phonetic patterns by looking up the morpheme phonetic pat- tern 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 grammaticality checking of the adjacent two phonetic morphemes. In the experiments with a corpus of 4,973 sentences, we achieved 99.9% of the grapheme- to-phoneme conversion performance and 97.5% of the sentence conversion performance. The full Korean TTS system is now being imple- mented using this conversion method. 1 Introduction During the past few years, remarkable improve- ments have been made for high-quality text- to-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 mapping the spelling of words to their phonetic symbols. Nevertheless, the function in some al- phabetic languages needs some linguistic knowl- edge, especially morphological and phonologi- cal, but often also semantic knowledge. In this paper, we present a new grapheme-to- phoneme conversion method for unlimited vo- cabulary Korean TTS. The conversion method is divided into mainly four modules. Each mod- ule has its own linguistic knowledge. Phrase- break detection module assigns phrase breaks onto part-of-speech sequences using morpho- logical knowledge. Word-boundaries before and after phrase breaks should not be co- articulated. So, accurate phrase-break assign- ments are essential in high quality TTS sys- tems. In the morpheme-to-phoneme conver- sion module, boundary graphemes of each mor- pheme in the phrase are converted to phonemes by applying phonetic patterns which contain possible phonological changes in the boundaries of morphemes. The patterns are designed us- ing 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 conver- sions, phoneme connectivity table supports the grammaticality of the adjacency of two phonetic morphemes. This grammaticality comes from Korean phonology rules. This paper is organized as follows. Section 2 briefly explains the characteristics of spoken Ko- rean for general readers. Section 3 and 4 in- troduces our grapheme-to-phoneme conversion method based on morphological and phonolog- ical knowledge of Korean. Section 5 shows experiment results to demonstrate the perfor- mance and Section 6 draws some conclusions. 675 2 Features of …

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