Integrating Linguistic and Performance-Based Constraints for Assigning Phrase Breaks

Integrating Linguistic and Performance-Based Constraints for Assigning Phrase Breaks Michaela Atterer Institute of Natural Language Processing University of Stuttgart, Germany atterer@ims.uni-stuttgart.de Ewan Klein Division of Informatics University of Edinburgh, UK, and Edify Internat. Development Centre, UK ewan@cogsci.ed.ac.uk Abstract The mapping between syntactic structure and prosodic structure is a widely discussed topic in linguistics. In this work we use insights gained from research on syntax-to-prosody mapping in or- der to develop a computational model which assigns prosodic structure to unrestricted text. The result- ing structure is intended to help a text-to-speech (TTS) system to predict phrase breaks. In addition to linguistic constraints, the model also incorporates a performance-oriented parameter which approxi- mates the effect of speaking rate. The model is rule- based rather than probabilistic, and does not require training. We present the model and implementa- tions for both English and German, and give eval- uation results for both implementations. We then examine how far the approach can account for the different break patterns which are associated with slow, normal and fast speech rates. 1 Introduction Normal spoken language is not delivered in an un- interrupted monotone; prosodic cues such as pauses or boundary tones greatly help the listener to un- derstand an utterance. Most text-to-speech systems use statistical models to find the appropriate loca- tions for prosodic phrase breaks. In this work we use insights gained from the linguistics literature to develop a computational model which assigns prosodic structure to unrestricted text. We start by briefly reviewing the relationship be- tween syntactic and prosodic structure. Figure 1 shows an example of the right-branching syntactic structure that is standardly assigned to English sen- tences. Figure 2 shows a much flatter tree which corresponds to widely accepted views of the same sentence’s prosodic structure. According to the lat- ter, the Utterance level is partitioned into intona- a little girl who didn’t like big dogs He would tease N AP V NP N V DetAP I NP IP N’ N’ I’ I VP NP NP VP I’ IP Figure 1: Syntactic structure of a sentence. a little girl who didn’t like big dogs He would tease φ φ φ φ I−phrase I−phrase Utterance Figure 2: Prosodic structure of a sentence. tional (I-) phrases,1 which in turn are partitioned into phonological ( � -) phrases. (We ignore lower levels of representation such as prosodic words and syllables for the purposes of this paper.) In their investigation of the syntax-prosody map- ping, Nespor and Vogel (1986) define � -phrases as consisting of a lexical head (e.g., a verb, noun or adjective) together with all the material on its non- recursive side up until the next head.2 In the ex- 1Intonational phrases are phonologically defined as units which are associated with a characteristic intonational contour; in particular, an I-phrase is marked by the presence of a major pitch accent. The boundary of an I-phrase is canonically man- ifested as a perceptible pause, accompanied by a local fall or rise in F � (fundamental frequency); it can …

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