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The following measurement shows a digital full-scale sine of 1 kHz, 16 bits without dither, which
is reduced in level by 40 dB. Also shown are a full-scale sine of 1 kHz with 24 bit, at 60 dB and
93.8 dB level attenuation, which is the lowest volume setting the ADI-2/4 Pro SE offers.
A high-resolution FFT makes it possible to disassemble the signal into individual frequencies, and
to identify unwanted components down to a level of -190 dBFS. The measurement shows that
the undithered 16 bit signal does not produce any distortion or other tones above -170 dBFS. So
at a volume setting of -40 dB the measurable THD is -130 dB. At 24 bit a volume setting of -60
dB also achieves -130 dB without distortion. And at a volume setting of -93.8 dB there are still -
93 dB THD measurable.
These results clearly show that distortion products of the digital volume control are not drowned
by the DAC's noise, but are not generated at all. It works perfectly even with an undithered 16 bit
signal, no detectable distortion products are produced.
If the volume control is measured at the analog output, the demonstrable THD is reduced to
around -100 dB at a volume setting of -60 dB, by the self-noise of the DAC (SNR 120 dB RMS
unweighted). In the above measurement that would be seen as straight noise floor at -160 dBFS.
The digital volume control of the ADI-2/4 Pro SE therefore works much more precisely and cleaner
than required for current top-level DACs.
In summary:
RME's digital volume control in 42 bit TotalMix technology avoids all the disadvantages of analog
level control via pots, is easy to use, offers reproducible settings, and the highest sound quality.
User's Guide ADI-2/4 Pro SE - v 1.0
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