081 – VCAs/DCAs
You can assign input channels to VCAs or Subgroups, but they function differently.
Written by Scott Adamson
Most professional audio consoles feature voltage controlled amplifiers, abbreviated on the control surface as VCAs. This refers to the way they work in analog consoles. Digital consoles have DCAs, meaning digital controlled amplifiers, but because the term is so ingrained in our audio lexicon, they’re often still labelled as VCAs.
Check out this video from our Live Sound Essentials course for a run through:
A VCA is a way to control multiple faders with a single fader. This makes it easy to manage a whole set of inputs at once. For instance, if all drum channels are assigned to a single VCA, that fader will turn the entire drum mix up or down. If the fader is turned all the way down, it would be like turning down each of those individual drum channels.
So what’s the difference between this and a subgroup? There’s an important distinction!
When channels are assigned to a subgroup, those signals are actually sent through an output buss which can then be routed to the main mix or sent directly out of the console. Because it’s an actual audio path, EQ and dynamics processors can be inserted on a subgroup, as well. (I love Distressors on a drum subgroup.)
VCAs, however, are not an actual audio buss. They can’t be routed anywhere and nothing can be inserted on them. A VCA is simply an amplification circuit whose gain level is modified by an external voltage.
Many consoles have 8 or 10 VCAs. This is incredibly helpful because all the various instrument groups or parts of a mix can be controlled with one set of faders. One VCA each could be assigned for drums, bass, guitar, strings, horns, and so on. Moving 10 faders is a lot more streamlined than 35.
If the backup vocals are sitting in exactly the right place as a group, but need to be turned off for a song, the VCA can be turned down instead of turning down any number of individual channels. If the balance between drums and guitar changes from song to song, two VCAs can be adjusted instead of all the individual drum and guitar faders. You get the idea!
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Digital consoles optimize space by having banks of faders. This means that even though there may be only 16 faders on the surface of the console, those faders can switch among different inputs or outputs. By pressing a button, an engineer can move from channels 1-16, 17-32, 33-48 and so on. This is a simple concept, but it can get needlessly confusing during a fast-paced show. By setting up a mix on VCAs, it won’t be as necessary to push buttons and move among the various fader banks.
VCAs control relative volume. This means, if the VCA is at unity gain, it’s not going to move your input fader to unity, as well. It just means that it won’t be cutting or boosting anything from the relative position of the input fader. If the VCA is moved down to -3, all of the assigned faders will drop by 3dB. So then, if an input channel is at 0, it will actually sound like -3. If another channel is already at -6, it will then sound like -9.
Finally, VCAs control mutes. Just like adjusting the fader level, muting a VCA will mute all of the channels assigned to it. I love to use this for vocal FX. Rather than quickly flipping through the fader banks to mute the individual FX at the end of each song before an artist starts talking to the audience, I just assign the FX returns to a VCA and mute it as needed.
