of Microprocessors

SoUND MODIFICATION METHODS 63
The most dramatic variable filter effect, however, is produced by
sweeping the center frequency of a bandpass filter. For moderate bandwidths
(around 1/4 to 1/2 octave), the effect is similar to that of an opening and
closing hat mute used with a trumpet or trombone. Such variable bandpass
filters, are popularly called "wah-wah" boxes due to their distinctive sound.
If the bandwidth is made smaller yet, only one harmonic will be
emphasized by the narrow peak, and the others will be greatly attenuated.
Varying the center frequency in this case gives the effect of a distinct scale of
sine wave notes as each harmonic is approached and passed by. White noise
passed through a variable narrow-band filter is given a definite although airy
pitch that is easily controlled.
Several tunable bandpass filters may also be used to provide a number of
independently movable formants. In fact, the voiced portions of human
speech (vowels and dipthongs) may be simulated with a harmonically rich
tone and two to (our variable bandpass filters depending on the degree of
naturalness desired. Each possible steady vowel corresponds to a particular
steady-state combination of formant frequencies, while each dipthong corresponds
to a particular pattern of formant frequency transitions. Of course, a
good speech synthesizer requires more than this, although the formant
generator and control means for producing accurate formant frequency variation
constitute the majority of the circuitry.
Variable notch filters produce a somewhat more subtle effect. Using
white noise as an input, a frequency sweeping notch filter will create a sound
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