Dsp Limitations - RME Audio ADI-2 Pro User Manual

8.8. DSP Limitations

There is never enough DSP power – no matter how much you add (frustrated developer).
That is true even for the ADI-2 Pro. Although being equipped with a quite capable 2.17 Giga
FLOPS DSP chip, plus using the FPGA to perform further calculations (RME's virtual DSP for
mixing/routing, level meters, filtering), 768 kHz sample rate takes its toll. The calculation power
available at 48 kHz is divided by 16 (!) then. Even at 384 kHz it is just 1/8 of that at 48 kHz. The
DSP in the ADI- 2 Pro performs:
Bass/Treble and Loudness for 6 channels
5-band parametric EQ for 6 channels
Standard phase functions for 6 channels
Crossfeed for 4 channels
30-Band bi-quad bandpass filter spectral analyzer
Peak Level meters for all channels
Display rendering
Volume control on 4 channels
Several controller-like functions, like volume ramp-up, mute, signal routing control etc.
Balanced Phones mode control
DSD to PCM conversion (for level meters)
At 48 kHz that is no big deal, at 192 kHz it already needs efficient coding and a better DSP chip.
But at 768 kHz you need a DSP with 4 times the power of the 'better' one. Therefore there is no
way around disabling some functions at higher sample rates. Fortunately those limitations have
only small impact in real-world usage:
•
At sample rates 352.8 kHz and up the Bass, Treble and Loudness function is deactivated.
The number of available EQ channels is reduced to 2 (1 x stereo). EQ can still be used with
Analog Input, Main Output 1/2 or Phones Out 3/4, but only one of these.
•
At sample rates 705.6 kHz and up Crossfeed or EQ (1 x stereo) can be active, not both at
the same time.
The high sample rates available in the ADI-2 Pro also exceed the capabilites of the digital I/Os.
Both AES and SPDIF are limited to 192 kHz, and there is no way around it (except a special,
one channel SMUX mode, see chapter 14.1.2, Setup Clock). Therefore all higher sample rates
are only usable analog and in USB mode. And in iOS mode when using an iPad/iPhone with an
app that supports such high sample rates (Neutron, Onkyo HF-Player etc.).
DSD comes with its own limitations. DSD is a 1 bit stream of data that can not be processed
digitally. There is no Bass, Treble, Loudness, EQ etc. possible at all. The volume control is no
longer done by the DSP, but the DAC chip, which converts DSD into PCM to be able to offer
level (volume) modification. You won't notice that, volume operation at the ADI-2 Pro is seam-
less and behaves identical in any mode. The DSP now performs an additional DSD to PCM
conversion, to be able to show the audio signal on the level meters and the Analyzer – a unique
feature of the ADI-2 Pro.
Even more extreme is DSD Direct. If activated (SETUP, Options, Device Mode), the DSD sig-
nal is not converted to PCM within the DAC, therefore there is no volume control at all – except
for the analog reference levels, which can be used to set the coarse output level/volume. Left
with no volume control, the ADI-2 Pro intentionally deactivates the headphone output 1/2 in
DSD Direct mode – the analog signal is only available at the rear outputs. Phones Out 3/4 con-
tinues to work as it is independent and uses normal DSD mode or PCM, according to what
source signal it receives.
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User's Guide ADI-2 Pro © RME