Some users asked why the HiFiBerry DAC does not support volume control. It is true that the card itself does not come with the volume control widget that can be used by “amixer”. However, most music playback software has its own volume control. How does this work? Basically, the incoming audio samples will be scaled by dividing them by a given value. Dividing by 2 gives an output voltage reduction of 6 dB.
The following picture shows what happens. At the left side is the original, unscaled signal and on the right side a signal that has been reduced by factor 3 (which is roughly a 9db voltage reduction).
With volume control in the playback software, the software will do this scaling and feed the data on the right side to the DAC.
But many people tell me, that is bad! Why is it bad? Because you will lose some resolution. Reducing the volume by 50% means that you will lose 1 bit. The resulting resolution will be only 23 bits now. If you decrease the volume further, you will lose more bits of resolution.
Is it really that bad? Remember, that most music that is available today is still recorded with 44.1kHz and 16bit. That means you can “lose” 8 bit with volume control on the DAC without really loosing anything from your music.
There is another downside of digital volume control: signal-to-noise ratio will decrease. That might be an issue in some areas. But many modern DACs have the noise floor at -110dB or even lower. If this increases to e.g. -90dB it is still a good value.
But some DACs have a “hardware” volume control. Having a mixer control in Linux means, that your software does not have to scale down the samples. The DAC will reduce the output volume. But how does it do this? Just have a look at the datasheets of the chips used on these sound cards. You will notice, that almost all chips with integrated volume control also use digital volume control, that means reduce the volume by dividing the digital data by some number. This is not better than doing it purely in software.
Summary: Software volume control might have an impact on the audible performance, but with modern 24bit DACs, it often works quite well. Not every “hardware” volume control is really hardware. In many cases, it means that the software scaling is done inside a chip, but works exactly like a software volume control.