From Wikipedia....
In sound mixing consoles[edit]
Some
mixing consoles come equipped with VCAs in each channel for
console automation. The
fader, which traditionally controls the audio signal directly, becomes a DC control voltage for the VCA. The maximum voltage available to a fader can be controlled by one or more master faders called
VCA groups. The VCA master fader then controls the overall level of all of the channels assigned to it.
[2] Typically VCA groups are used to control various parts of the mix;
vocals,
guitars,
drums or
percussion. The VCA master fader allows a portion of a mix to be raised or lowered without affecting the blend of the instruments in that part of the mix.
A benefit of VCA sub-group is that since it is directly affecting the gain level of each channel, changes to a VCA sub-group level affect not only the channel level, but also all of the levels sent to any post fader mixes. With traditional audio sub-groups, the sub-group master fader only affects the level going into the main mix and does not affect the level going to the post fader mixes. Consider the case of an
instrumentfeeding a sub-group and a post fader mix. If you completely lower the sub-group master fader, you would no longer hear the instrument itself, but you would still hear it as part of the post fader mix, perhaps a
reverb or
chorus effect.
[3]VCA mixers are known to last longer than non- VCA mixers. Because the VCA controls the audio level instead of the physical fader, decay of the fader mechanism over time does not cause a degradation in audio quality.
Digital variable-gain amplifier[edit]
A
digitally controlled amplifier (DCA) is a variable-gain amplifier that is digitally controlled.
The digitally controlled amplifier uses a stepped approach giving the circuit graduated increments of gain selection. This can be done in several fashions, but certain elements remain in any design.
At its most basic form, a toggle switch strapped across the feedback resistor can provide two discrete gain settings. While this is not a computer controlled function, it describes the core function. With eight switches and eight resistors in the feedback loop, each switch can enable a particular resistor to control the feedback of the amplifier. If each switch was converted to a relay, a microcontroller could be used to activate the relays to attain the desired amount of gain.
Relays can be replaced with Field Effect Transistors of an appropriate type to reduce the mechanical nature of the design. Other devices such as the CD4053 bi-directional CMOS integrated circuit can serve well as the switching function.
Sounds to me as if VCA and DCA are nothing more than discrete methods of achieving a ganged (grouped) fader automation function. (I think we can look past a dated notion of a physical attenuator directly controlling a channel-amplifier's gain, since they don't exist in a DAW anyway.) Maybe 'VCA' sounds more 'professional'...I dunno.