User's Manual

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User’s Guide ADI-2 DAC v2.2 © RME
31.14 Digital Volume Control
The ADI-2 DAC deliberately avoids an analog level adjustment by means of a potentiometer. Its
digital version surpasses an analog one in practically every conceivable 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 DAC, 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
maximum 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 con-
trolled one. This is even more true the better the DA converter works and the less noise it has -
just like the ADI-2 DAC, 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 attenua-
tion of 48 dB (8 bit) leaves 11 bit of resolution. Such a simple, but important details omitting
argumentation, usually ends with: the music must sound distorted in quieter parts, and the sig-
nal 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 DAC'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.