There is a lot of confusion about different kinds of volume control not only implemented in our HiFiBerry products, but in general. This article should help to clarify it a bit. Note that this articles defines, what we mean when we talk about different kinds of volume control. As there are no standardized definitions for these terms, other persons might use different definitions.
Hardware volume control
This is any kind of volume control that is directly implemented in hardware, this means it is part of a circuit or even a chip. It might still be possible to control the volume by software if the circuit or chip has a digital control interface.
Software volume control
This is any kind of software that can be used to control the volume of a circuit that has no hardware volume control or it should not be used for some reason. It can be implemented in the playback application itself or in an operating system driver.
Analog volume control
Volume is reduced or increased after digital-to-analog conversion. This is usually done by some kind of variable resistor. It can be a classic potentiometer, but also a chip that provides a variable attenuation.
Digital volume control
The output volume is reduced or increased before digital-to-analog conversion. This means the input data will be multiplied or divided by a specific value to increase or decrease the output volume.