Filter Curves, Orders, And Slopes; Modulating The Filter Frequency - Alesis ModFX Reference Manual

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Using the Philtre

Filter curves, orders, and slopes

When you look at a graphic display of their frequency response,
every filter has a slope—gentle or steep, like the slope of a hill. The
passband—the frequency range that the filter lets through—looks
like a flat plateau. The frequency at the edge of that plateau, just
as the response starts to go downhill, is called the corner frequency.
Then, the frequency response goes down at a particular slope,
measured in decibels per octave. The slope is determined by the
number of components or poles in the filter. A simple single-pole
filter, the gentlest kind, rolls off frequencies at a rate of 6
dB/octave above its corner frequency; a four-pole filter is a much
steeper 24 dB/octave.
In the Philtre, the PHREEQ control determines the corner
frequency, and the STEEP switch allows you to quadruple the
slope. The REGEN control increases gain at the corner
frequency, further changing the filter characteristics. You hear
this at high settings as the filter beginning to "whistle" as the
harmonics near the corner frequency all begin to peak together
due to phase addition; this is also called resonance or "Q".

Modulating the filter frequency

All the rest of the controls on the right-hand side of the Philtre are
designed to modulate or dynamically change the filters' corner
frequencies. If the DEPTH control is at minimum, and you move
the PHREEQ control around by hand, you'll hear the effect of
changing the corner frequencies. The Modulation section of the
Philtre essentially does the same thing: it moves the frequency of
the filter around dynamically, under control of an LFO (Low
Frequency Oscillator). Not only does the Philtre give you a lot of
modulation options, it can automatically synchronize the
movement of the filter frequency with the speed of the music
using the Tempo Sync features.
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