User Guide

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 modify a sound by stopping some frequencies and allowing
others to pass through. An important term regarding filters 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 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).