Filter Section - Moog Stage Edition User Manual

B – The Filter Section
Filters are used for adjusting the timbre of an audio signal. Filters modify sound by attenuating some fre-
quencies while allowing others to pass through unaffected. An important term to understand regarding
fi lters is "Cutoff Frequency". This is the point at which frequencies begin to be rejected. The LP features
a lowpass fi lter, which behaves as its name indicates: it allows low frequencies to pass and rejects high
frequencies.
resonant peak passes through the overtones of the sound being fi ltered, those overtones are reinforced.
This gives the fi lter a character that can sound vocal, buzzy or zappy,
depending on how it's used. When the resonance is turned up past
about 3 o'clock on the analog edit dial, the fi lter begins to self-oscillate
at the cutoff frequency, producing a sine wave tone.
Other fi lter section controls include Keyboard Amount (KB AMOUNT),
Envelope Generator Amount (EGR AMNT) and OVERLOAD.
The KB AMOUNT parameter allows you to set the degree to which the fi lter cutoff frequency tracks the
note that is played. For example, when the KB AMOUNT is set to it's highest level (100%), the fi lter cut-
off will rise by an octave each time you play an octave higher on the keyboard. This setting allows you to
maintain a consistent fi lter tone regardless of whether you play low or high on the keyboard. When the KB
AMOUNT is set to zero, the fi lter cutoff remains at the value determined by the analog edit control, regard-
less of where you play on the keyboard. This can make the sound less bright as you play higher up on the
keyboard. The KB AMOUNT parameter includes the keyboard GLIDE control signal, which allows the fi lter
cutoff to follow the glide of the notes being played (when GLIDE is switched ON). Using the KB AMOUNT
control, you can adjust the amount of Glide that will affect the fi lter cutoff.
The EGR AMNT parameter allows you to set the degree to which the Filter EG affects the fi lter cutoff
frequency. The EGR AMNT parameter is bi-polar, meaning the Filter EG can affect the fi lter cutoff either in
a positive or negative way. A positive amount will cause the Filter EG to raise the cutoff frequency, while a
negative amount will cause the Filter EG to lower the cutoff.
Another important aspect of fi lters is the cutoff slope. The cutoff slope
defi nes how well the fi lter rejects signals above the cutoff frequency.
The cutoff slope is measured in decibels per octave (dB/Octave), and is
specifi ed as a multiple of six. A 6dB/Octave slope (the simplest pos-
sible fi lter design) exhibits a gentle roll-off with a minimum rejection of
frequencies above the cutoff. By comparison, a 12dB/Octave slope is
twice as steep, and rejects frequencies above the cutoff twice as fast.
An 18dB/Octave slope is steeper still, with a corresponding frequency
rejection. Finally, a 24dB/Octave slope provides the steepest rejection
of frequencies above the cutoff point. The classic Moog fi lter is a 24dB/
Octave lowpass fi lter.
The Moog fi lter also features a parameter called Resonance. This
parameter adds a resonant peak at the cutoff frequency. When the
LP User's Manual - The Components
Page 13