DSP Functions
Filters
14-16
Two-pole Allpass Filter (2POLE ALLPASS)
Using 2POLE ALLPASS is very similar to using ALPASS, with two differences. First, the phase
shift is -180 degrees for partials at the center frequency, approaching 0 degrees for partials at
low frequencies, and approaching -360 degrees for partials at high frequencies.
Second, since this is a two-stage function, there’s an additional control input page (F2 WID)
which controls the filter width. The parameters on this page affect the frequency range,
measured in octaves, where most of the phase shifting occurs. Small values cause a drop from 0
to -360 in the phase shift to occur near the center frequency, while large values spread the drop
in the phase shift over a broader frequency range. Small values tend to affect just a few partials,
leaving others mostly untouched. The affected partials seem to become detached from the
others, creating the illusion of an additional sound source.
If you leave the center frequency constant and assign an LFO to vary the width, partials with
frequencies above the center will shift their pitches in the opposite direction of partials below
the center frequency.
-360
-340
-320
-300
-280
-260
-240
-220
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
10 100 1000 10000 100000
Frequency in Hertz
Phase in degrees
Width = 2 octaves;
Cutoff frequency
from C 4 to C 10
C 4
C 10
-360
-340
-320
-300
-280
-260
-240
-220
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
10 100 1000 10000 100000
Frequency in Hertz
Cutoff frequency = C 6;
width from .1 to 4 octaves
4.1
Phase in degrees
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