dCS 900E / 902E
User Manual
Manual for Software Versions 1.3x to 1.5x
dCS
Ltd
12
th
June 2000
Manual part no: DOC0029021E1
Page 30
Document No: OS-MA-D0002-902.1E1
Contact
dCS
on + 44 1799 531 999
email to: [email protected]
(inside the UK r 44 with 0)
web site: www.dcsltd.co.uk
dCS 900 / 902
T
ECHNICAL
I
NFORMATION
Anti Alias Filtering
The
dCS 900
&
902
offer a choice of 4 anti-alias filters on most sample rates.
These filters affect the ultrasonic part of the spectrum - 20 kHz upwards.
The unit is an ADC, with an output data rate set by the interface standard used.
The bandwidth of the input stages and oversampling converter used is high, and
so any signals that that are in the input signal, up to a MHz or so, will be aliased
3
back into the output signal if they are not removed by filtering. The demands on
this anti-alias filter can be quite severe at the lower (“normal”) sample rates - it
must pass signals in the audio band (0-20 kHz) unimpaired, but it must prevent
aliasing about Fs/2. This can result in a very sharp filter, and it is an unavoidable
mathematical result that sharp filters have a poor, ringing, transient response.
One effect of the ringing is to spread the energy in a transient over a significant
period of time (it can be up to 1 ms). This seems to affect the stereo image that
the ear would otherwise form.
One can trade off filter roll-off, and energy smear - more relaxed roll-off gives
less energy smear, but it may allow some of the signals in the input to alias
irrevocably into the output data. Once a signal has aliased, it cannot be
corrected. However, as far as the ear is concerned, this may not matter. The
ear can tell the frequency of a signal - up to a point. As the frequency rises, the
accuracy with which the ear can tell what the frequency is decreases, and above
a limit, all the ear can tell is that there is a signal, and it is above ... kHz. It can
tell no more. So - it may be that some degree of aliasing is acceptable to the
ear.
The filters that we have included give increasingly good energy smear
performance, and consequently have increasingly relaxed roll off.
FiLT1
gives
the sharpest roll off, with no aliasing, but the worst energy smear. Then as the
number increases the smear decreases, but the aliasing increases. Try them, to
see which you prefer.
You may find that for different material, different filters are appropriate - and you
may find that for different stages in the recording and mastering process,
different filters are appropriate.
The
dCS 900
&
902
use linear phase FIR filters to avoid the limit cycle problems
that come with many IIR filters. Linear phase gives filters a symmetrical transient
response before and after a transient (“pre-ringing”). The passband may or may
not have a ripple
4
, depending on the filter being used. The stop band is typically
below –110 dB and can be as low as –130 dB.
3
See, for example “Principles of Digital Audio”, 3rd Edition, by Ken C Pohlmann (McGraw-Hill Inc, 1995)
4
Filters always have some ripple. For “zero ripple” filters this is in the
µ
dB to pdB region.
