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From text to speech: The MITalk system
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We have argued in Chapters 2-6 that in order to transform English text to
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speech, one must first try to derive an underlying abstract linguistic representation
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for the text. There are at least two reasons why a direct approach is suboptimal: 1)
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rules for pronouncing words must take into consideration morphemic structure
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(e.g. consider the pronunciation of the th of outhouse) and syntactic structure (e.g.
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there exist many noun-verb ambiguities in English such as perm’it - p’ermit), and
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2) sentence duration pattern, and fundamental frequency contour depend, to a
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major extent, on the syntactic structure of the sentence.
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There are currently several text-to-speech systems under development in the
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United States (Nye et al., 1973; Kurzweil, 1976; Caldwell, 1979; Morris, 1979)
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and elsewhere (Carlson and Granstrom, 1976). The simplest approach is to devise
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a set of heuristic letter-to-sound rules and then create an exceptions dictionary for
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frequently occurring words that are processed incorrectly by the letter-to-sound
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rules (Kurzweil, 1976). The exceptions dictionary is then augmented by function
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words that are useful for parsing strategies. The phonetic representation for a sen-
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tence that is derived in this way serves as the input to a synthesis-by-rule device
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such as Votrax (Gagnon, 1978) or a software synthesis-by-rule program.
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The MITalk system represents a more ambitious approach of generalized
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morphemic analysis, so as to do a better job of figuring out the pronunciation of
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words and to better assign parts of speech to each word, and thereby compute
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phrase and clause boundaries with greater accuracy. The real question is whether
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current algorithms are good enough to make automatic text-to-speech output ac-
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ceptable to the user. There is clear indication that motivated users (such as the
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blind) benefit from these devices after a period of acclimation, but considerable
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concentration is required.
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80
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