User's Manual

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User’s Guide ADI-2 Pro FS - v 2.5
34.10 AD Impulse Responses
On the AD side the ADI-2 Pro offers four filters: Short Delay Sharp, Short Delay Slow, Sharp
and Slow. Basically these behave and operate exactly the same way as the filters already de-
scribed for the DAC. 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 response, but start to act early
within the higher audible range at standard sample rates. See measurements in chapter 34.7.
Note: SD Sharp/SD Slow each have the same frequency response as the shown Sharp/Slow.
In the picture to the right the impulse responses of
filters Short Delay Sharp (left) and Short Delay Slow
(right) are shown, at 44.1 kHz. The source signal is
an analog single sample impulse (see picture NOS in
chapter 34.9). This one 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.
In the picture to the right the impulse responses of
filters Sharp (left) and Slow (right) are shown, again
at 44.1 kHz. Sharp generates the expected, Slow a
near perfect impulse response.
Both 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.
SD Slow and Slow work best at 88.2/96 kHz,
because the decrease in the high frequency area
then happens outside the audible range. At the same
time the impulse response is near perfect – caused
by both the filter as well as the doubled sample rate.
In Preamp mode the analog signal is AD-, then DA-converted. In factory default state this hap-
pens 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 and NOS
offer the most perfect impulse responses.