Alesis ModFX Reference Manual page 33

In BAND STOP mode, with [REGEN] set to minimum, the band
that is being attenuated is relatively wide. As you increase the
[REGEN] setting, the stop band gets narrower. At the maximum
setting, with the addition of filter resonance, the Band Pass filter
essentially becomes a narrow notch filter with a high "Q", like a
parametric EQ set to its narrowest setting:
Band Stop: Low [REGEN]
• To make the band stop filter narrower, turn [REGEN]
clockwise.
Using the Philtre
Band Stop: High [REGEN]
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