User's Manual

User’s Guide ADI-2 DAC v2.2 © RME

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 DAC. 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, Crossfeed), 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 2 channels

5-band parametric EQ for 2 channels

Standard phase functions for 2 channels

Crossfeed for 2 channels

30-Band bi-quad bandpass filter spectral analyzer

Peak Level meters for all channels

Display rendering

Volume control on 2 channels

Several controller-like functions, like volume ramp-up, mute, signal routing control etc.

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:

¾ At sample rates 705.6 kHz and up Crossfeed or EQ can be active, not both at the same

time. Bass/Treble and Loudness are not available.

The high sample rates available in the ADI-2 DAC also exceed the capabilites of the digital

input. Both AES and SPDIF are limited to 192 kHz, and there is no way around it. Therefore all

higher sample rates are only usable analog and with USB. 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 DAC is

seamless 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 DAC.

Even more extreme is DSD Direct. If activated (SETUP, Options, Phones / DSD), the DSD

signal 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 DAC intentionally deactivates the headphone outputs in

DSD Direct mode – the analog signal is only available at the rear outputs.