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Application Notes
30
Audio Precision 01/19/01 16:00:021st order allpass @ 300, 1K, 10K
phase.at1
Color Line Style Thick Data Axis
Green Solid 1 Anlr.Phase Left
Yellow Solid 1 Anlr.Phase Right
Yellow Solid 1 Anlr.Phase Left
-150
+150
-100
-50
+0
+50
+100
d
e
g
-150
+150
-100
-50
+0
+50
+100
d
e
g
20 20k50 100 200 500 1k 2k 5k 10k
Hz
Response of the All Pass Filter for 1
st
Order phase
shift at frequencies of 300Hz, 1kHz and 10kHz
The response of the filter for a 2
nd
order shift is shown below with the Q control set to 1, at
frequencies of 300Hz, 1kHz, and 10kHz, and also Q set to 6 (max) and 0.4 (min) at 1kHz. As can be
seen, the filter response now tends from 360 at low frequencies to 0 at high frequencies, and Q
controls the rate at which the phase changes (i.e. the slope) around the transition point. With low Q,
the phase changes gradually across the whole frequency range. With high Q, the phase changes
rapidly in the transition area, and is unchanging at 360°/0° over the remainder of the frequency
spectrum. Hence, the 2
nd
order all-pass provides the user with the additional control of shaping the
phase shift ‘window’.
A
udio Precisio
n
01
/
19
/
01 16:09:1
1
2nd order allpass
Q
=1
@
300, 1K, 10K +
Q
=6
Q
=0.4
@ 1K
p
hase.at1
Color Line Style Thick Data Axis
Green Solid 1 Anlr.Phase Left
Yellow Solid 1 Anlr.Phase Right
Yellow Solid 1 Anlr.Phase Left
-200
+400
-100
+0
+100
+200
+300
d
e
g
-200
+400
-100
+0
+100
+200
+300
d
e
g
20 20k50 100 200 500 1k 2k 5k 10k
Hz
Response of the all pass filter for 2nd order phase shift with Q=1 at frequencies
of 300Hz, 1kHz and 10kHz and also with Q=6 at (max) and Q=0.4 (min) at 1kHz
Note: Both filters are all-pass with a flat amplitude response between 20Hz and 20kHz; only the
phase response changes with frequency.