RME Audio ADI-2 Pro, User Manual

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User’s Guide ADI-2/4 Pro SE – v 1.0
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34.23 Digital Volume Control
The ADI-2/4 Pro SE deliberately avoids an analog level adjustment by means of a potentiometer.
Its digital level control in TotalMix technology surpasses an analog one in practically every con-
ceivable point. Typical disadvantages of setting with potentiometers:

Synchronicity errors lead to panoramic shifts and significant volume deviations left / right, in
particular near the end points of the adjustment range.

In the middle setting range, there is an increased crosstalk and changes in the frequency
response. Changes in the frequency response also occur at the end regions of the adjustment
path.

The setting range for optimum volume adjustment is often too small, or at the lower or upper
end of the potentiometer's adjustment range.

Non-reproducible settings (except 0 and 11).

Higher THD/THD+N. A point well known to measurement technicians. As soon as an analog
potentiometer is in the signal path, the unstable contact between wiper and resistive track
causes noise, which contains both THD (distortion) and N (noise), even in the stationary state.
Thus the -110 dB of a DAC quickly gets reduced to for example -80 or -70 dB.
Special volume ICs, which activate different resistance values by means of numerous electronic
switches, avoid some of the above mentioned points. Unfortunately, even the best of these ICs
do not achieve either THD or dynamics of the DACs used in the ADI-2/4 Pro SE, thus would affect
its analog output signal.
However, none of this is an issue with RME's digital volume control!
In fact an analog volume control has a (theoretical) advantage in only one point, namely the max-
imum signal to noise ratio at a higher level reduction. In reality, current circuitry overturns the
theory, and the SNR at the output of such a device is no better than that of a digitally controlled
one. This is even more true the better the DA converter works and the less noise it has - just like
the ADI-2/4 Pro SE, which provides the maximum noise ratio over a wide level range of 20 dB,
thanks to its four reference levels realized in the analog domain.
The most often cited issue of a digital volume control is an alleged loss of resolution at higher
attenuation. An example: 117 dB dynamic roughly equals 19 bit resolution. A volume attenuation
of 48 dB (8 bit) leaves 11 bit of resolution. Such a simple, but important details omitting argumen-
tation, usually ends with: the music must sound distorted in quieter parts, and the signal to noise
ratio is down to a useless 69 dB.
The former is simply wrong, the latter irrelevant in practice. Indeed there is a reduced signal to
noise ratio, but it doesn't matter, as the noise was not audible before (below the hearing thresh-
old), and is still not audible after lowering the level. And the reduced SNR also applies to devices
with potentiometers, since the potentiometer is never placed at the output, but in the middle of
the circuit, followed by further electronics which also add some basic noise.
The quality of the ADI-2/4 Pro SE's digital volume control is best shown by measurements. Hard
times coming up for convinced supporters of the analog control, because here it is very clear that
the disadvantages of a digital volume adjustment, such as roughness and distortions at higher
attenuation, simply do not exist - at least with RME.