of Microprocessors

MUSIC SYNTHESIS PRINCIPLES 31
frequencies are emphasized (or the higher ones eliminated) the sound becomes
a roar, or in an extreme case, a rumble. If the high frequencies
predominate, a hiss is produced.
The middle frequencies can also be emphasized as in Fig. I-8e. If a
wide range of middle frequencies is emphasized, the sound is only altered
slightly. However, if a sufficiently narrow range is strengthened, a vague
sense of pitch is produced. The apparent frequency of such a sound is normally
near the center of the group of emphasized frequencies. The narrower
the range of frequencies that are emphasized, the more definite the pitch
sensation. If the range is very narrow, such as a few hertz, a clearly pitched
but wavering tone is heard. If the waveform of such a tone is examined over
only a few cycles, it may even appear to be a pure, repeating, sine wave!
Multiple groups of emphasized frequencies are also possible with a clearly
different audible effect. In fact, any spectrum envelope shape is possible.
Parameter Variation
In review, then, all steady sounds can be described by three fundamental
parameters: frequency if the waveform repeats, overall amplitude, and
relative harmonic amplitudes or spectrum shape. The audible equivalents of
these parameters are pitch, loudness, and timbre, respectively, with perhaps
a limited degree of interaction among them.
What about unsteady sounds? All real sounds are unsteady to some
extent with many useful musical sounds being particularly so. Basically a
changing sound is a steady sound whose parameters change with time. Such
action is frequently referred to as dynamic variation of sound parameters.
Thus, changing sounds can be described by noting how the parameters vary
with time.
Some terms that are often used in discussing parameter variation behavior
are steady state and transition. If a parameter is changing only part of
the time, then those times when it is not changing are called steady states.
Usually a steady state is not an absolute cessation of change but instead a
period of relatively little change. The transitions are those periods when
movement from one steady state to another takes place. An infinite variety of
transition shapes are possible from a direct, linear change from one steady
state to another to a variety of different curves. Often it is the speed and form
of the transitions that have the greatest influence on the overall audible
impact of a sound.
Frequency Variation
Dynamic variation of frequency is perhaps the most fundamental. A
simple one-voice melody is really a series of relatively long steady states with
essentially instantaneous transitions between them. If the frequency transitions
become fairly long, the audible effect is that of a glide from note to
note.