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24 CHAPTER 4
Introduction
HP the high pass filter which
works in the opposite manner of
the low pass filter: It suppresses
the lower frequencies in a signal
and lets the higher frequencies
pass.
BP the band pass filter which
suppresses both ends of the tonal
spectrum and allows only a narrowly
defined bandwidth of the
original sound to pass.
BS The band stop filter, band
reject filter or notch filter which
works in the opposite manner of
the bandpass filter. It allows all of
the frequencies of a signal except
for a narrow frequency band
around the cutoff to pass. The
term “notch” is fairly descriptive;
you might say this filter chops a
notch out of the sound spectrum.
Now activate the different operating
modes and rotate the CUTOFF
pot to get a feel for the way each
filtering option works.
Along with the CUTOFF pot, the
RESONANCE pot is the most important
control feature of a filter. The
filter resonance increases the volume
of the frequencies located
near the cutoff frequencies and
suppresses the more remote frequencies.
This sound shaping feature
has a striking effect -
especially when used in conjunction
with the low pass filter: it produces
a nasal or honking type of
tone which increases as you turn
the resonance up. Experiment by
varying the RESONANCE setting in
the different operating modes in
conjunction with different CUTOFF
settings. You will find the effect
that the RESONANCE pot achieves
is markedly different for the band
stop filter in comparison to the
effect it has on the other filter
types: as the resonance increases,
the bandwidth of the notch
decreases; in other words more
frequencies on both sides of the
filter frequency are allowed to
pass.