16
FILTER
OVERVIEW
The number and relative strengths of a sound’s harmonic
frequencies determine its tone color or timbre. The Sub
Phatty contains a filter for removing certain frequencies
from audio signals. Because filtering gives you control
over an audio signal’s harmonic content, it physically
alters the waveform being filtered.
The Sub Phatty has a classic Moog lowpass ladder filter
with four selectable slopes (see Hidden Parameters on
pg.26). Lowpass filters pass all frequencies up to a point
called the cutoff frequency and gradually roll off, or
attenuate, frequencies above that point. You can change
the cutoff manually using a knob, or you can change it
by applying a signal from a control source such as an
envelope or LFO.
Turning the cutoff all the way down closes the filter so
that nothing passes through it. Raising the cutoff opens
the filter. As you turn the
CUTOFF
knob clockwise from
its lowest position, first you’ll hear only the audio signal’s
lowest frequencies, and then the timbre will grow gradually
brighter. The filter envelope, in combination with the
CUTOFF
knob’s setting, is the filter’s primary control source.
Another characteristic of the Sub Phatty’s filter is resonance. Resonance increases the level of audio
frequencies closest to the cutoff frequency by making the filter roll off frequencies less gradually. It
regenerates those frequencies by feeding them back to the filter. Turning up the resonance emphasizes
harmonics closest to the cutoff frequency and exaggerates any changes to the cutoff frequency.
FILTER CONTROLS
CUTOFF:
Use this knob to change the filter’s cutoff frequency. Its lowest setting is 20Hz, which
effectively closes the filter and doesn’t allow any audio to pass through. Its highest setting is 20kHz,
which opens the filter completely and allows all audio to pass through.
RESONANCE:
Use this knob to control how much signal is routed from the filter’s output back to
its input. Turning it clockwise increases the resonance, causing a peak in amplitude at the cutoff
frequency. Settings above 7 cause the filter to self-oscillate.
MULTIDRIVE
: MultiDrive is the Sub Phatty’s distortion processor, offering effects ranging from
asymmetrical, tube-like warmth to aggressive hard clipping, with a smooth continuous transition in
between. The
MULTIDRIVE
knob controls how hard you drive the OTA and FET stages, which are located
between the filter and the amplifier in the signal path. The higher the setting, then the more aggressive
the clipping effect. Varying amounts of MultiDrive can give your sounds a distinct tonal edge, as well as
make them more responsive to changes in filter resonance, waveform, and oscillator level.
EG AMOUNT
: Use this knob to control how much the filter envelope modulates the filter’s cutoff
frequency. In other words,
EG AMOUNT
controls the depth of the envelope generator’s effect on the filter.
Notice that the
EG AMOUNT
knob is bipolar, meaning that its control value is positive when it’s turned
up and negative when it’s turned down. Turning it clockwise from center causes the envelope to raise
the cutoff frequency from the
CUTOFF
knob’s setting. Turning it counterclockwise from center causes
the envelope to lower the cutoff frequency from the
CUTOFF
knob’s setting.
