of Microprocessors

SOUND MODIFICATION METHODS 57
FREQUENCY
INPUT SPECTRUM
FREQUENCY
FILTER RESPONSE
FREQ-lJENCY
OUTPUT SPECTRUM
Fig. 2-5. Effect of filtering on the spectrum
The other two parameters are the two ultimate slopes of the sides of the
bell-shaped curve. As before, they are usually multiples of 6 dB per octave.
The slope just beyond the 3-dB points is usually steeper than the ultimate
slope far from the center frequency and becomes more so for higher Qs
(narrower bandwidths).
The final basic filter shape is called a band-reject (or notch) response and
is shown in Fig. 2--4D, The center frequency is, of course, the frequency of
greatest attenuation. Specification of the width parameter is not really standardized
because the exact shape of the notch varies considerably with the
filter circuit. However, one common specification is, again, the difference
between the 3-dB down points. Often, a rough attempt at specifying the
shape is made by specifying both the 3-dB and the 60-dB notch widths.
The effect of a filter on the spectrum of a sound can be easily determined
graphically as in Fig. 2-5. As a simple case, consider a sound with all
o
-5
-10
-15
~ -20
UJ -25
t.::l
:::>
~ -30
a..
~ -35
-40
-45
-50
-55
-60
5 7 9 II 12 13 17 19 21 23 25 '27 29 31
HARMONIC NUMBER
(A)
Fig. 2-6. Effect of filters on a square wave. (A) Original square wave.
,