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The Klatt formant synthesizer
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P(f) = S()"T(f)"F(f)
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SOUND SOURCE
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VOICING
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ASPIRATION
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FRICATION
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VOCAL TRACT
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TRANSFER
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SOURCE FUNCTION LIP RADIATED
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VOLUME T(f) VOLUME SOUND
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VELOCITY VELOCITY PRESSURE
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S(f) u(®) | P(f)
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Figure 12-2: Components of the output spectrum of a speech sound
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RADIATION
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CHARACTERISTIC
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R(f)
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domain. This is actually how a waveform is generated in the computer. The syn-
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thesizer includes ‘components to simulate the generation of several different kinds
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of sound sources (described in Section 12.1.10), components to simulate the vocal
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tract transfer function (Figure 12-3), and a component to simulate sound radiation
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from the head (Figure 12-14).
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12.1.3 Cascade vs. parallel
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A number of hardware and software speech synthesizers have been described
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(Dudley et al., 1939; Cooper et al., 1951; Lawrence, 1953; Stevens et al., 1955;
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Fant, 1959; Fant and Martony, 1962; Flanagan et al., 1962; Holmes et al., 1964;
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Epstein, 1965; Tomlinson, 1966; Scott ef al., 1966; Liljencrants, 1968; Rabiner et
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al., 1971a; Klatt, 1972; Holmes, 1973). They employ different configurations to
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achieve what is hopefully the same result: high-quality approximation to human
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speech. A few of the synthesizers have stability and calibration problems, and a
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few have design deficiencies that make it impossible to synthesize a good voiced
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fricative, but many others have an excellent design. Of the best synthesizers that
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have been proposed, two general configurations are common.
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In one type of configuration, called a parallel formant synthesizer (see e.g.
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Lawrence, 1953; Holmes, 1973), the formant resonators that simulate the transfer
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function of the vocal tract are connected in parallel, as shown in the lower portion
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of Figure 12-3. Each formant resonator is preceded by an amplitude control that
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determines the relative amplitude of a spectral peak (formant) in the output
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spectrum for both voiced and voiceless speech sounds. In the second type of con-
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figuration, called a cascade formant synthesizer (see e.g. Fant, 1959; Klatt, 1972),
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sonorants are synthesized using a set of formant resonators connected in cascade,
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as shown in the upper part of Figure 12-3.
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The advantage of the cascade connection is that the relative amplitudes of for-
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mant peaks for vowels come out just right (Fant, 1956) without the need for in-
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125
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