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Brickwall
All the above filters intentionally violate a rule
of digital technology - at half the sample rate
the filter should already have maximum stop-
band attenuation, i.e. at 44.1 kHz sample rate
at an audio frequency of 22.05 kHz. But in
practice there are no disadvantages with a
slightly higher filter (24 kHz). However, the fil-
ter is further out of the audible range (good for
the ears), and easier to implement in hardware
(good for the chip manufacturers) - that's why
most filters are implemented this way. As an
alternative, the Sharp filter can also be se-
lected with a matching stopband attenuation -
under the name Brickwall. The measurements
show that the filter is absolutely the same, but
starts a bit earlier, because the peak is slightly reduced in level. So the filter is not - as Brickwall
suggests - steeper, but simply shifted a bit downwards. See also chapter 34.15.
NOS (Non-Oversampling, SuperSlow)
The AKM DAC of the ADI-2 Pro includes another filter called Super Slow, whose impulse re-
sponse looks perfect. But checking the output signal with an Oscilloscope reveals steps that are
more typical for so called Non-OverSampling (NOS) devices. That's why this filter is called NOS
within the DAC filter menu.
Note that there is no audible distortion, the steps equal high frequency harmonics that are mostly
higher than 20 kHz. Please also note that Slow and especially NOS filters cause much more
aliasing into the audio band and out-of-band noise than Sharp filters.
Since the NOS filter is very popular in Hi-Fi and audiophile circles, but generally missing in ESS
DACs, RME has emulated the filter of the AKM DAC, and loads the appropriate filter coefficients
into the ESS chip in real time when NOS is selected. Miracle of technology...
User's Guide ADI-2/4 Pro SE – v 1.0
89
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