of Microprocessors

62 MUSICAL ApPLICATIONS OF MICROPROCESSORS
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FREQUENCY LOG SCALE
Fig. 2--7. Combining two filters to get a better response curve
a "conch shell" effect on the sound. Often the essential character of the tone
of certain musical instruments may be simulated with a simple fixed filter
and simple electronically generated waveforms. A double-humped bandpass
response with humps at 1,200 Hz and 2,700 Hz, for example, resembles
the resonance characteristics of an English horn. Sawtooth waveforms played
through such a filter setup resemble the sound of an English horn. Such
resonant peaks are called formants.
Vanable Filters
Inexpensive filters with easily variable parameters that cover a wide
range are actually a relatively recent development. The parameter most easily
varied is the cutoff frequency or center frequency. For bandpass filters, the
bandwidth is also easily varied. The cutoff slope, on the other hand, is a
function of filter circuit topology rather than component values so as a result
is difficult to control. Variable-slope filter circuits, however, are under investigation.
Dynamic variation of the cutoff frequency of low- or high-pass filters
provides a definite audible effect. The upper harmonics of many percussion
instruments, for example, decay faster than the lower harmonics do. The
same effect may be simulated with a constant sound spectrum and a low-pass
filter whose cutoff frequency is lowered during the course of the note. The
opposite effect may be created by using a high-pass filter to increase high
harmonic content as a note progresses.