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From text to speech: The MITalk system
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Harvard and Haskins sentences, performance improved on the second half of the
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test relative to the first half. Although the differences were small, amounting to
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only about 2 percent improvement in each case, the result was very reliable (p <
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.01) across subjects in both cases.
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The performance levels obtained with the Haskins semantically anomalous
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sentences are very similar to those reported earlier by Nye and Gaitenby (1974),
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and more recently by Ingeman (1978) using the same sentences with the Haskins
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synthesizer and text-to-speech system. Nye and Gaitenby (1974) reported an
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average error rate of 22 percent for synthetic speech and five percent for com-
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parable natural speech. However, Nye and Gaitenby used both naive and ex-
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perienced listeners as subjects, and found rather large differences in performance
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between the two groups, as we noted above. This result is presumably due to
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familiarity and practice listening to the output of the synthesizer. We suspect that
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if the experienced subjects were eliminated from the Nye and Gaitenby analyses,
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performance would be lower than the original value reported and would therefore
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differ somewhat more from the present findings. Nevertheless, the error rate for
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these anomalous sentences produced with natural speech is still lower than the cor-
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responding synthetic versions, although it is not clear at the present time how
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much of the difference could be due to listener familiarity with the quality of the
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synthetic speech.
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13.3.3 Conclusions
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The results of the two word-recognition tests indicate moderate to excellent levels
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of performance with naive listeners depending on the particular test format used
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and the type of information available to the subject. In one sense, the results of
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these two tests can be thought of as approximations to upper and lower bounds on
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the accuracy of word-recognition performance with the current text-to-speech sys-
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tem. On the one hand, the Harvard test sentences provide some indication of how
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word recognition might proceed when both semantic and syntactic information is
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available to a listener under normal conditions. On the other hand, the Haskins
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anomalous sentences direct the subjects’ attention specifically to the perceptual in-
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put and therefore provide a rough estimate of the quality of the acoustic-phonetic
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information and sentence analysis routines available for word recognition in the
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absence of contextual constraints. Of course, in normal listening situations, and
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presumably in cases where a text-to-speech system such as the present one might
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be implemented, the complete neutralization of such contextual effects on intel-
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ligibility would be extremely unlikely. Nevertheless, a more detailed analysis of
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the word-recognition errors in the Haskins anomalous sentence test might provide
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160
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