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57 lines
1.7 KiB
57 lines
1.7 KiB
12
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The Klatt formant synthesizer
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12.1 Overview
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The final two modules of the MITalk system (CWTRAN and COEWAYV) simulate
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a formant synthesizer (Klatt, 1980). Figure 12-1 shows the interface between these
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modules and the hardware which produces the actual speech. Synthesizer control
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parameter data are specified every 5 msec by rules contained in the phonetic com-
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ponent described in Chapter 11. There are 39 control parameters that specify the
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actions of the software synthesizer, of which only 20 are varied as a function of
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time.
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- LOUDSPEAKER
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4.9 kHz
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LOW-PASS
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FILTER
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CWTRAN
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COEWAV
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D/A
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CONVERTER
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A A
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! !
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1
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CONTROL WAVEFORM
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PARAMETERS SAMPLES
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(1 SET/5 ms) (10,000/s
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12 BITS/SAMPLE)
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Figure 12-1: Interface between synthesizer software and hardware
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The Klatt formant synthesizer consists of two logically distinct modules,
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CWTRAN and COEWAYV. The first modcle, CWTRAN, accepts control
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parameter data such as formant freqhéncies, formant bandwidths, and fundamental
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frequency (all specified in Hz), as well as source amplitudes and amplitudes of
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each parallel formant (specified in dB) and derives a set of difference-equation
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coefficients for each digital formant resonator and a set of linear amplitude coef-
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ficients. The second synthesizer subroutine, COEWAYV, accepts as input this coef-
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ficient and amplitude array for a 5 msec frame and computes the next 5 msec
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chunk of waveform.
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Considerable system speed improvement can be obtained by implementing
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the final module, COEWAYV, as a hardware digital filter. A TTL implementation
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has been constructed (Miranker, 1978). Whether the software subroutine or the
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VTM is used, output waveforrn samples are played through a digital-to-analog
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converter, analog low-pass filter, and loudspeaker.
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