User Manual

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User’s Guide ADI-2 Pro - v 1.91
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/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). A single sample impulse includes
frequencies higher than half the sample rate
(indeed it is an invalid, the Nyquist border violating
signal). Therefore it is not possible to capture it at
44.1 kHz sample rate without added rounding or
pre/post ringing. SD Sharp generates the expected
post ringing.
SD Slow shows lower amplitude (the filter's treble
decrease), but only a slight rounding, and nearly no
ringing.
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
post and pre-ringing, Slow a near perfect impulse
response.
Both Short Delay filters are IIR type (so called
minimum phase), the other two are FIR type. FIR is
phase-linear over the whole frequency range. The
advantage of IIR is the very low latency of a few
samples only, which is welcome in a studio
monitoring situation.
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.