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INTRODUCTION

To facilitate human-machine communication, there is an increasing need for computers to adapt to human users. This means of interaction should be pleasant, easy to learn, and reliable. Since some computer users cannot type or read, the fact that speech is universal in all cultures and is the common basis for linguistic expression means that it is especially well suited as the fabric for communication between humans and computer-based applications. Moreover, in an increasingly computerized society, speech provides a welcome humanizing influence. Dialogues between humans and computers require both the ability to recognize and understand utterances and the means to generate synthetic speech that is intelligible and natural to human listeners. In this paper the process of converting text to speech is considered as the means for converting text-based messages in computer-readable form to synthetic speech. Both text and speech are physically observable surface realizations of language, and many attempts have been made to perform text-to-speech conversion by simply recognizing letter strings that could then be mapped onto intervals of speech. Unfortunately, due to the distributed way in which linguistic information is encoded in speech, it has not been possible to establish a comprehensive system utilizing these correspondences. Instead, it has been necessary to first analyze the text into an underlying abstract linguistic structure that is common to both text and speech surface realizations. Once this structure is obtained, it can be used to drive the speech synthesis process in order to produce the desired output acoustic signal. Thus, text-to-speech conversion is an analysis-synthesis system. The analysis phase must detect and describe language patterns that are implicit in the input text and that are built from a set of abstract linguistic objects and a relational system among them. It is inherently linguistic in nature and provides the abstract basis for computing a speech waveform consistent with the constraints of the human vocal apparatus. The nature of this linguistic processing is the focus of this paper, together with its interface to the signal processing composition process that produces the desired speech waveform.

As in many systems, the complexity of the relationship between the text input and the speech output forces levels of intermediate representation. Thus, the overall conversion process is broken up through the utilization of two distinct hierarchies. One of these is structural in nature and is concerned with the means to compose bigger constructs from smaller ones (e.g., sentences are composed of words). The second hierarchy is an arrangement of different abstractions that pro-



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