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Filters continued
Minimum phase filter frequency response
20
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
-200
0
0.1
0.2
0.3
frequency/FS (input sampling rate)
The Minimum Phase filter is another highly regarded audio filter that
offers even lower ripple in the pass and stop bands. Unlike the Linear
Phase filter, group delay is not constant so some time-coherence is lost;
however, phase shift is low and the particular benefit with this filter is that
the impulse response exhibits no pre-ringing.
Minimum phase impulse response
0
0
20
40
60
36
0.4
0.5
0.6
0.7
0.8
0.9
80
100
120
140
160
Time (samples)
Steep filter frequency response
20
0
-20
-40
-60
-80
-100
-120
-140
-160
-180
-200
1
0
0.1
0.2
Our Steep filter is a linear phase filter that has been optimised for stop
band attenuation of close-in aliasing images. Here we have traded a little
attenuation of the very highest frequency response (for 44.1kHz sampled
material it would be -2dB at 20kHz) and a little more pre- and post-ringing
for a very steep attenuation just outside the pass band. The Steep filter
is able to attenuate aliasing at 22kHz by some 80dB.
Steep filter impulse response
0
180
0
20
40
Note: All filters exhibit the same ultimate roll-off of approximately 140dB.
The following table shows the filter stop band attenuation for 44.1kHz
material as an example:
Roll-off at 20kHz
Roll-off at 22kHz
Ultimate roll-off
We encourage you to experiment with the filters to determine which sound
best to your ears and using your source equipment/programme material.
Steep filter
Linear phase filter
0.3
0.4
0.5
0.6
0.7
frequency/FS (input sampling rate)
60
80
100
120
Time (samples)
Linear Phase
Minimum Phase
-0.1dB
-0.1dB
-10dB
-10dB
140dB
140dB
0.8
0.9
1
140
160
180
Steep
-2dB
-82dB
140dB
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