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8
Cambridge Audio
Filters continued
Minimum phase filter frequency response
20
-20
1
\
-160
·160
·200
0 .1
0.2
0.3
0.4
0 .5
0.6
0.7
0.8
0.9
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
'
~
AAA
, n n •
~vv
v
vv
20
40
60
60
100
120
140
160
180
Time (samples)
Steep filter frequency response
20
,_~,_--,---~---r---r---r---,---,---,--~
\
'
-20
t----+---+---+--+--1-+---+---+----+-----i----
Steep Iter
-
\
~ ~--~--~--~--~~~--~--~--~--~--
m
$
~--+---+---4---~--~+--+---+---+--~----
'0
:-
Li ear pha e filter
~
$
~--+---+---4---~--~~
·
--~--+---+---4----
~
·100
~--+---+---4---~--~-'--~--+---+---4---
~
~
·UO
t----+---+---+---+--~~-+---1-----+----+--~
-140
t=t=tt=t=1MiiM-
·160
11•
\ I
1
f
'
!
·180
~--+---+---+---~--+--+---+---+--~-----!
-200
l----+---_j_----1-----+-----l-----'-----l-----1------1------1
0.1
0.2
0.3
0.4
0 .6
0.6
0.7
0.8
0.9
frequency/FS (Input sampling rate)
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
~
.A
I
A
~
IV\ IV
20
40
60
80
100
uo
140
160
160
Time (samples)
Note: Ail filters exhibit the same ultimate roil-<>ff of approximately 140dB.
The following table shows the filter stop band attenuation for 44.1kHz
material as an example:
Linear Phase
Minimum Phase
Steep
Roil-off at 20kHz
-0.1dB
-O.ldB
-2dB
Roil-<>ff at 22kHz
-10dB
-10dB
-82dB
Ultimate roil-off
140dB
140dB
140dB
We encourage you to experiment with the filters to determine which sound
best
to your ears and using your source equipment/programme material.
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