Moog Music MF-101 Lowpass Filter - Manual (Page 6)

The RESONANCE knob changes the way the filter sounds. Or, in technical words, it
changes the shape of the frequency response. When RESONANCE is near 0 the low
overtones come through evenly. When RESONANCE is turned up to around 7, the
overtones near cutoff will be boosted, creating a whistle-like or vowel-like quality. When
RESONANCE is above 8, the filter actually produces its own tone at the cutoff
frequency. That tone interacts with your instrument's tone when you play your instrument
through the filter.
The 2-POLE 4-POLE switch selects whether the signal goes through half the filter (2-
pole) or the entire filter (4-pole). 2-POLE has a bright sound, while 4-POLE has a deeper,
mellow quality.
The Envelope Follower Section
The AMOUNT knob, the SMOOTH-FAST switch, and the ENV light are all part of the
Envelope Follower section.
The AMOUNT knob determines how much the envelope signal is used to open and
close the filter. When the knob is counterclockwise, the envelope signal has no effect
on the filter. When the knob is clockwise, the envelope signal opens and closes the filter
as much as five octaves.
The SMOOTH-FAST switch determines whether or not the envelope signal is smoothed
out. Some sounds (like guitar chords) have long, rough envelopes, and they often
sound better if the envelope is smoothed out before going to the filter. Other sounds
(like snare drum) are quick and sharp, and sound really good with the switch in the Fast
position.
The ENV light shows how strong the envelope signal voltage is at any one time. The
stronger the audio input, the brighter the ENV light will be.