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dCS 904 User Manual Manual for Software Version 1.5x and 1.36
dCS Ltd June 2000
Manual part no: DOC135904 iss 2B2
Page 30
135904ma2b2.pdf file available from website
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dCS
on + 44 1799 531 999 email to: more@dcsltd.co.uk
(inside the UK replace + 44 with 0) web site: www.dcsltd.co.uk
dCS 904
T
ECHNICAL
I
NFORMATION
Anti Alias Filtering
The dCS 904 offers 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
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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 904 uses 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
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, depending on the filter being used. The stop band is typically
below –110 dB and can be as low as –130 dB.
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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 µdB to pdB region.