of Microprocessors

572 MUSICAL ApPLICATIONS OF MICROPROCESSORS
OSCILLATOR
-+-J[rJ
FILTER
(EXCITATION
(SYSTEM
FUNCTION)
FUNCTION)
OUTPUT
FREJENCY
WAVEFORM
REJONSE
SHAPE
(A)
FREQUENCY
SPECTRUM OF OSCILLATOR
FREQUENCY
AMPLlTUOE RESPONSE OF FILTER
SPECTRUM OF OUTPUT
(8)
Fig. 16--14. Natural sound modeling. (A) Simple model of natural sound process.
(B) Spectral interpretation of simple model.
ternatively, the excitation function can be white noise, which does invalidate
the pitch parameter but has no effect on anything else. The system function
may be either fixed or variable depending on the sound being modeled.
The range of natural sounds to which this model is applicable is actually
very large. Most wind instruments, such as the bassoon, and bowed
string instruments, such as the violin, are well described by this model. The
human voice is a prime example. In these examples, a harmonic-rich excitation
function is generated by a vibrating element such as a reed, sticky
string, or flapping folds of flesh. Resonators in the instruments, such as the
folded tube of the bassoon, wood panels with odd-shaped cutouts, or oral and
nasal cavities filter the excitation function before it actually escapes into the
air. Musical instruments usually have fixed resonators (notable exceptions are
muted brass instruments), whereas the human voice depends on a highly
variable resonator for its expression. All of these resonators may have a
number of distinct resonant frequencies (peaks in the amplitude response)