22
C.
FILTERS
Ahh… the Moog filter – the sound that started it all… Filters are used for
transforming the character of an audio signal. Filters are used to modify a sound by
stopping some frequencies and allowing others to pass through. An important term
regarding filters to understand is “Cutoff Frequency”. This is a frequency at which
frequencies begin to be rejected. There are different types of filters. Some of the
most common and most musically useful are lowpass, highpass, and bandpass. A
lowpass filter behaves as its name indicates: it allows low frequencies through and
rejects high frequencies. A highpass is filter does the opposite. It passes all
frequencies above the cutoff and rejects the frequencies below the cutoff.
A bandpass comes from a lowpass and highpass filter put together. The lowpass
filter defines a maximum frequency that will pass through, and the highpass filter
defines a minimum frequency that will pass through. What’s left is a band of
frequencies that will pass through the filters, hence the name, bandpass.
Another aspect of filters is their cutoff slope. This refers to how many dB per
octave frequencies are rolled off beyond the cutoff frequency. The number of
“poles” in a filter determines the cutoff slope. Each pole adds 6dB to the cutoff
slope. Therefore a 1-pole filter rolls off frequencies at 6db/octave, a 2-pole filter has
a 12dB/octave cutoff slope, 3 poles makes for a 18dB/octave cutoff slope, and a 4
pole filter has a 24dB/octave cutoff slope. The classic Moog filter is a 24dB/octave
lowpass filter.
With the Resonance control, a peak is created at the cutoff frequency (fig. 19).
