of Microprocessors

286 MUSICAL ApPLICATIONS OF MICROPROCESSORS
OUTPUT
"STRIKE"
PULSE
(A)
OVERALL
AMPLITUDE
(6)
STRIKE
Fig. 8--7. Basic percussion instrument generators. (A) Resonant percussive
sound generator. (B) Noisy percussive sound generator.
DAC may have as few as 6 bits of resolution. The combination of nonlinear
waveshaper and fixed filter produces the characteristic timbre of the instrument.
The shaper provides spectral characteristics that track the frequency of
the VCO, whereas the filter provides formants (peaks in the spectrum) that
remain constant. The VCA, its DAC, and envelope generator complete the
sound processing and produce the final output at the desired amplitude.
Additional circuitry may be added to enhance realism. One example
would be the use ofa noise generator to introduce small random variations in
frequency and amplitude. Another refinement would be interaction between
amplitude and the waveshaperlfilter to simulate muted and blaring timbres
of the instrument.
A percussive sound module might take the form of Fig. 8-7. The first
case covers resonant sources such as wood blocks and many types of pitched
drums. Basically, a sharp pulse excites one or more high Q bandpass filters to
produce damped sine waves. The second case covers instruments such as
cymbals, which consist largely of enveloped, filtered noise. A snare drum
generator would require both generator types.