of Microprocessors

SOUND MODIFICAnON METHODS 61
simple circuits provide. Figure 2-7, for example, shows how two rather
sloppy low-pass shapes that can be obtained from simple circuits combine
together to give a much improved low-pass shape.
Fixed filters (those with constant parameter values) are commonly used
as equalizers in music systems. An equalizer is a filter that is intended to
compensate for nonuniform amplirude response elsewhere in a high-fidelity
sound system. For sound modification use, however, a fixed filter can be used
to change the overall tonal quality of the sound. A low-pass filter, for
example, gives a muffled effect, while a high-pass filter tends to give a thin
or tinny effect. Bandpass filters, depending on the center frequency, may have
(\ f\ f\
V
if'
(El
o
-5
-10
-15
~ -20
~ -25
~
~ -30
a..
~ -35
-40
-45
-50
-55
-60l.-J----.J--...J.....-----,J7:---------':-9---,11,....----c
1 -'::-3-c-'::15-~17=--....,,~9-721:----::2'=""3 -2::':5:-----!27::----::2~9--=:31-
HARMONiC NUMBER
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Fig.2--6. Effect of filters on a square wave (cont.). (E) Band-reject filter.