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34.11 AD Impulse Responses
On the AD side the ADI-2/4 Pro SE offers
four oversampling filters, shown to the right
from top to bottom: Short Delay Sharp,
Short Delay Slow, Sharp and Slow. Basi-
cally these behave and operate exactly the
same way as the DAC filters described in
the next chapter. SD Sharp and Sharp offer
the most linear frequency response and
highest suppression of mirroring (aliasing)
at high frequency input signals. SD Slow
and Slow try to combine a high aliasing
suppression with an optimal impulse re-
sponse, but start to act early within the
higher audible range at standard sample
rates. See measurements in chapter 34.13.
Note: SD Sharp/SD Slow each have nearly
the same frequency response as the shown
Sharp/Slow.
The pictures show the impulse response of
all four filters at 44.1 kHz. The source signal
is an analog single sample impulse (see
picture NOS in chapter 34.12). It includes
frequencies higher than half the sample
rate that must be removed from the aliasing
filter of the ADC. Therefore it is not possible
to digitize such a signal at 44.1 kHz sample
rate without added rounding or pre/post
ringing.
SD Sharp and SD Slow are IIR filters (also
called minimum phase), which are not
phase-linear and mostly cause post ringing.
The advantage of IIR is the very low latency
of a few samples only, which is welcome in
a studio monitoring situation.
Sharp and Slow are FIR filters that cause
post and pre-ringing. FIR in general has a
higher latency, but is phase-linear over the
whole frequency range. Slow shows a
lower amplitude - the filter's treble decrease
in the audible range, but only very low pre
and post ringing.
In Preamp mode the analog signal is AD-,
then DA-converted. In factory default state
this happens at 192 kHz sample rate. The
same impulse is recorded and played back
in much better quality, as the quadrupled
sample rate samples the 44.1 kHz single
sample impulse at least four times, and the
filters operate at a much higher frequency
as well. Therefore the pre- and post-ringing
is four times 'quicker', and as such only a
quarter in length. Still Slow offers the most
perfect impulse response.
User's Guide ADI-2/4 Pro SE – v 1.0
87
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