of Microprocessors

34 MUSICAL ApPLICATIONS OF MICROPROCESSORS
moment a spectral envelope like the one that was shown in Fig. I-8e.
Disregarding the exact shape of the bell-shaped curve, it should be obvious
that three parameters can adequately describe the spectrum. First, there is
the width and height of the peak on the curve, and finally the frequency at
which the peak is highest. In a typical application, the width and height of
the curve are related. Also, since only the relative height of one portion of the
curve with respect to another is important, the absolute height parameter is
usually eliminated. This leaves just the width and center frequency as variables.
Note that for definitely pitched, periodic sound waveforms the spectrum
curve being considered is really the envelope of the individual harmonic
amplitudes.
It turns out that manipulation of these two variables is sufficient to
create very interesting dynamic spectrum changes. In fact, if the width
variable is set to a reasonable constant value such as Y3 octave at the 6-dB
down (with respect to the peak) points, then varying just the center
frequency is almost as interesting. This in fact is the principle behind the
"wah-wah" sound effect for guitars that became popular years ago.
Other methods for changing or distorting the spectrum under the
influence of a small number of parameters exist and will be covered in more
detail later.
Simultaneous Sounds
The preceding should serve as a brief introduction to the fundamental
parameters of a single, isolated sound. Most interesting music, however,
contains numerous simultaneous sounds. One common use for simultaneous
sounds is chords and harmony. Another application is rhythm accompaniment.
Sometimes quantities of sound are used simply as a kind of acoustical
background for a simpler hut more prominent foreground. The physics and
fundamental parameters of each component sound remain unaltered, however.
The real frontier in synthesis, after adequate control of the basic.
parameters of sound is accomplished, is applying this degree of cOfitrol to
numerous sounds, all simultaneously, and all under the direction of a single
composer/performer. Extensive use ofmicroprocessors in synthesis will be the
final stride toward reaching this goal.
History of Electronic Sound Synthesis
Sound and music synthesis by electronic means has a long and interesting
history. Although only the most significant milestones can he briefly
described here, the effort is certainly worthwhile, since the ongoing evolution
of synthesis techniques and equipment is far from complete. Without
exception, significant developments in sound synthesis closely followed significant
developments in electronic and computer technology. Often, how-