Synthesis of Spoken Messages from Semantic Representations (Semantic-Representation-to-Speech System)

Synthesis of Spoken Messages from Semantic Representations (Semantlc-Representat Ion--to-Speech System) Laurence DANL05, Eric LAPORTE Laboratoire d'Automatlque Documentalre et Ltngulstlque Universlte Paris 7 2, place Jussleu 75251 PARIS CEDEX 05 Franqoise EMERARD Centre National d'Etudes des Telecommunications 22301 LANNION CEDEX Abstract A semantic-representation-to-speech system communicates orally the information given in a seman- tic representation. Such a system must Integrate a text generation module, a phonetic conversion module, a pro- sodic module and a speech synthesizer We will see how the syntactic information elaborated by the text genera- tlon module is used for both phonetic conversion and prosody, so as to produce the data that must be supplied to the speech synthesizer, namely a phonetic chain Including prosodic Information. Introduction A spoken message can be produced either to utter a written text (text-to-speech system), or to communi- cate orally the information given in a semantic repre- sentation (semantic-representation-to-speech system). In both cases, the speech synthesizer must be provided with a phonetic chain including prosodic Information In order to reconstruct the acoustic signal. As we will recall in 1., syntactic knowledge is necessary to com- pure the phonetic transcription of a written text and to include prosodlc Information in It. Hence a text-to- speech system must Include a parsing module to get this syntactic knowledge. On the other hand, a semantic- representation-to-speech system can take advantage of the syntactic information elaborated when expressing the semantic representation in natural language. There- fore, we design a semantic-representation-to-speech system that generates directly from the semantic representation a phonetic string with prosodic markers, without a written stage. Our system has been designed for French but It could be extended to other languages. 1. tn French, semantic features ore needed to distinguish only o few non-homophonic homographs, mostly technical words, I. Knowledge needed In a text-to-speech system !.1. Spelling-to-sound conversion The first problem encountered in synthesizing speech from written text is that of spelling-to-sound conversion. Certain languages are much easier than others in this respect. For example, about 50 rules are sufficient for tl~e conversion of written Spanish into phonetic symbols, with a virtually zero error rate (,Santos & Nombela 1982). For other languages, such as French or English, the problem is much greater A phoneme does not generally correspond to only one grapheme, and the reverse ls also true For instance, the word o/seau is pronounced /wazo/ : none of its gra- phemes Is pronounced as would be expected (ie. /o/ for o, /i/for /~ /s/ for 5, schwa for e, /a/for a and/y/ for u). Spelling-to-sound conversion is further compli- cated by the existence of' non-homophonic homographs, ie. words spelled the same but pronounced differently. The distinction between two homographs requires to know their grammatical categories (record Is pro- nounced ['reko:d] If it is a noun and [rl'ko:d] if It is a verb), their Inflexional features (read Is pronounced [ri:d] in the infinitive form and [red] in the preterite), or their semantic features (lead Is pronounced tied] when It is a noun or a verb related …

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