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From text to speech: The MITalk system
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BW2=100 BW2=50
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20 Uniform tube 20 \ BW2=200
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10 10 /
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0 0
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-10
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1 2 3 4 5 0 1 2 3 4 5
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) (b)
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0
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(a
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o
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o
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Transfer function |T(f)| (dB)
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(O}
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o
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30
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20 ' F1=500 20 ‘
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10 10 / ”\ 3
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0 0 T
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-10 250 -10 2
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-20 -20
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0 1 2 3 4 5 0 1 2 3 4 5
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Frequency (kHz) Frequency (kHz)
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(c) (d)
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Figure 12-11: Effect of parameter changes on the vocal tract transfer function
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where formant frequencies are set to 500, 1500, 2500, 3500, and
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4500 Hz and formant bandwidths are set to be equal at 100 Hz. This
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corresponds to a vocal tract having a uniform cross-sectional area, a
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closed glottis, open lips (and a nonrealistic set of bandwidth values),
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as shown in part (a) of Figure 12-11.
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2. The amplitude of a formant peak is inversely proportional to its
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bandwidth. If a formant bandwidth is doubled, that formant peak is
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reduced in amplitude by 6 dB. If the bandwidth is halved, the peak is
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increased by 6 dB, as shown in part (b) of Figure 12-11.
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3. The amplitude of a formant peak is proportional to formant fre-
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quency. If a formant frequency is doubled, that formant peak is in-
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creased by 6 dB, as shown in part (c) of Figure 12-11. (This is true
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of T(f), but not of the resulting speech output spectrum since the
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glottal source spectrum falls off at about -12 dB/octave of frequency
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increase, and the radiation characteristic imposes a +6 dB/octave
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spectral tilt resulting in a net change in formant amplitude of +6 -12
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+6 =0dB.)
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4. Changes to a formant frequency also affect the amplitudes of higher
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formant peaks by a factor proportional to frequency squared. For ex-
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146
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)
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