of Microprocessors

32 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Often with conventional instruments, a small but deliberate wavering
of frequency is added to the extended steady states. This wavering is called
vibrato. If the frequency parameter is plotted as a function of time on a graph,
then the vibrato shows up as a small amplitude waveform with the baseline
being the current steady state. This situation is termed frequency modulation
because one waveform, the vibrato, is modulating the frequency of another
waveform, the sound. We now have a whole new infinity of possible vibrato
frequencies, amplitudes, and shapes. Vibrato waveforms for conventional
instruments are usually around 6 Hz with an amplitude of 1% or so and a
roughly sine waveshape.
Gross alterations in the typical vibrato waveform can also have a gross
effece on the resulting sound. If the modulating wave amplitude is greatly
increased to several percent or even tens of percent, the result can be a very
boingy or spacey sound. If the modulating frequency is increased to tens or
hundreds of hertz, the sound being modulated can be completely altered.
Clangorous sounds resembling long steel pipes being struck or breaking glass
are easily synthesized simply by having one waveform frequency modulate
another. This phenomenon will be studied in greater depth later.
Amplitude Variation
Changes in amplitude are also fundamental. Again taking a one-voice
melody as an example, it is the amplitude changes that separate one note
from another, particularly when two consecutive notes are of the same frequency.
Such an amplitude change delineating a note or other sound is
frequently called an amplitude envelope or just envelope. The shape and
duration of the amplitude envelope of a note has a profound effect on the
overall perceived timbre of the note, often as important as the spectrum
itself.
Figure 1-9 shows a generalized amplitude envelope. Since they are so
important, the various transitions and steady states have been given names.
The initial steady state is, of course, zero or silence. The intermediate steady
state is called the sustain, which forms the body ofmany notes. The transition
SUSTAIN
l.LJ 1.0
a
20.8
t 0.6
~ 0.4
0.2
olL----------------=~------___
TINE
Fig. 1-9. Typical amplitude envelope shape.