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The phonetic component
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the parameter value assigned to the time of the segment boundary. These con-
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stants are determined by rules that involve features of the current phonetic segment
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PHOCUR, the previous phonetic segment PHOLAS, and the next phonetic seg-
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ment PHONEX. In some cases, the rules have to examine features of segments
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further from the current segment, but this is rare. For example, in pin, the time of
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voicing onset in the vowel preceded by the voiceless plosive pp is delayed by
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about 50 msec, unless the segment preceding the voiceless plosive is an ss, as in
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spin. The variable control parameters are listed later in Table 11-3.
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11.1.3 History of formant synthesis-by-rule
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As originally demonstrated by John Holmes, successful imitation of a natural ut-
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terance depends primarily on matching observed short-term spectra. This tech-
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nique succeeds, in part, because it reproduces all of the potential cues present in
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the spectrum, even though we may not know which cues are most important. The
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speech perception apparatus appears to be aware of any and all (perceptually
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discriminable) regularities present in the acoustic signal generated by the speech
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production apparatus, and these regularities should be included in synthetic stimuli
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if possible.
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There have been a number of previous efforts to specify general strategies for
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formant synthesis-by-rule (see, e.g., Holmes et al, 1964; Mattingly, 1968a;
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Rabiner, 1968a; Coker et al., 1973; Klatt, 1972, 1976a). However, examination of
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these publications suggests that consonant-vowel intelligibility is nowhere near as
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high as in listening to natural speech. For example, Rabiner (1968a) estimated that
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consonants in his synthetic consonant-vowel nonsense stimuli were 85 percent in-
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telligible to phonetically trained listeners, but that natural tokens of the same syll-
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ables were about 99 percent intelligible. Other rule programs, apparently, perform
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no better, although relevant evaluative data are generally not available.
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Why isn’t intelligibility higher? Each rule system attempts to make ap-
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propriate generalizations and simplifications concerning the form and content of
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rules for consonant-vowel synthesis. Have the wrong generalizations been made?
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The results described below in Section 11.2 suggest that this conjecture is true.
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11.2 “Synthesis-by-analysis” of consonant-vowel syllables
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11.2.1 Analysis of CV syllables
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The data base that was recorded and analyzed in order to develop new consonant-
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vowel synthesis rules consists of speech samples obtained from six talkers who
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were native to a single midwestern dialect region -- three males and three females
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(Klatt, 1979b). The intent was to use the data from all six talkers to establish the
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form of the synthesis rules, but the actual parameter values inserted in the rules
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109
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