An audio subgroup is absolutely essential in certain circumstances.
For example, groups of inputs can be grouped, submixed, and sent to a recording device in situations where there are insufficient tracks available to record each channel from the respective direct outputs.
In distributed loudspeaker systems, the audio groups can be mixed into the matrix output system to feed separate loudspeaker clusters or locations.
Most importantly, where circumstances do not allow individual signal processing channels to be used on each input channel, outboard devices can be inserted into groups of inputs.
Thus a single compressor can be applied to multiple choir vocal microphones, or to an entire drumkit.
But audio subgroups also have disadvantages when compared to VCA groups.
Whereas it would require two audio subgroup channels to control input channels with stereo panning (one subgroup panned to the left, the other to the right), or three for LCR panning, a single VCA group fader can control such inputs.
That’s because, in a VCA console, the VCA is located in the channel circuit before the panpot. Panning, routing, and pre-fade auxiliary sends are therefore unaffected by the VCA.
Controlling stereo or LCR groups using VCA group faders has the additional benefit of freeing-up audio subgroup faders.
There are a number of mixing consoles available that allow the audio group faders to be used to control auxiliary send levels and create stage monitor mixes with fader control and insert capabilities. The more audio group faders available, the more monitor sends that can be set up on a front of house console doing double-duty as a monitor desk.
VCA groups can also be overlapping, with inputs assigned to more than one group. This allows sophisticated control of instruments and voices.
For example, a VCA group could be created for all of the choir microphones, with a second VCA group controlling only those mics used by the soloists, allowing the overall vocal level to be controlled with one fader and the balance of the solo voices against that group with another.
In the same way, instruments can be grouped. The drums and percussion might be on one VCA group fader while the entire rhythm section—drums, percussion, bass, and piano—is on a separate fader.
For further sophistication, if different groups of instruments and voices must be made more or less prominent in different pieces of music, they can be grouped accordingly and controlled on separate VCA faders.
To offer a simple example, a solo vocal mic, piano, bass, and drums may be in one VCA group, with a different solo vocal mic, guitar, bass, and drums on another VCA group fader.