Alesis ModFX Reference Manual page 39

3
Using the Philtre
Band Pass
The Band Pass filter has two 2-pole, 12 dB per octave slopes: one
rolling off high frequencies above the center, and another rolling
off low frequencies below the center. It's like a high pass filter
combined with a low pass filter, with the corners set to let the
midrange through. The distance between the two slopes is set by
the [REGEN] knob: the band lets most of the audible frequency
spectrum through at the lowest [REGEN] setting, and narrows
considerably at full clockwise rotation (see page 30 for an
illustration of this).
When using modulation, a Band Pass filter is often used when you
want an effect similar to Low Pass, but you don't want the high
end to disappear completely as the filter center moves towards the
low end. Acoustically, it mimics the effect of sound being pointed
in different directions, toward and away from the listener, as in a
rotary speaker or the "helicopter" effect.
When [STEEP] is on, the Band Pass filter slopes increase to 8-
pole, or 48 dB per octave. This steep slope isn't as obvious as in
other filters, however, if the band is set relatively wide.
Band Stop
The Band Stop filter is the reverse image of the Band Pass filter.
Instead of preserving the frequencies in the midrange near the
center, it cuts them off, so only the edges are heard. The distance
between the two slopes is set by the [REGEN] knob: the band is
fairly wide at the lowest [REGEN] setting, and becomes a notch
filter at full clockwise rotation (see page 31 for an illustration of
this). When you modulate a Band Stop filter, the sound will
alternate between cutting out the low mids, and the upper mids,
while leaving the extreme high and low frequencies (if the
PHREEQ knob is at the center).
When [STEEP] is on, the Band Stop filter slopes increase to 8-
pole, or 48 dB per octave.
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