Identifying Signal Flow Differentiation Between VCA’s, DCA’s, and Subgroups

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Identifying Signal Flow Differentiation Between VCA's, DCA's, and Subgroups

A subgroup, by any other name….works differently.

Analog and digital mixers allow for controlling multiple channels as if they were one.  These controls are commonly known as subgroups…but not really…maybe in the generic sense…but then you might say VCA’s if that’s what your board uses…because subgroups are different. [Sigh]  There are several methods in which channels can be grouped together.  And each works differently, of course, ’cause some days things just have to be complicated.  Maybe I’m just in one of those moods…

Why group channels?

Channels can be grouped for a variety of reasons;

  • Aux send groups. For example, you might only want specific channels to go to a recording device or to the nursery or to the hallway speakers or, more likely, to an effects unit where, say, you have reverb on all your vocalists.
  • Volume Channels.  I’ve mentioned this one a lot on this web site.  I will place my singers in a group and my instruments in another group.  Such grouping gives you an easy way to pull back instruments for a more intimate feel or whatever you want to do. Using such volume groups, the blend of the sounds within a group doesn’t change, only their overall volume. Drums are ideal for grouping with multiple microphones.
  • Mute groups. For example, you might have the band in a mute group so when the music ends, you can mute all of their channels at once.  This way, you avoid broadcasting the pop of the unplugged guitar or other such sounds you’d rather everyone not hear.  In this case, activating a mute group mutes all of the assigned channels.  Mute groups are different than the other groups in how they work but since I’m talking grouping, I thought I’d mention it.

How group volume controls work differently

The normal” subgroup control can be considered the lazy* person’s way of controlling multiple signals.  Take four channels and send their signals to a summing amp.  When you move that group fader up and down, you are increasing or decreasing the volume of that summed sound.  *I’m not against lazy; there is a time and a place for everything.

The VCA (voltage-controlled amplifier) is the busy person’s way of controlling multiple signals.  Take four channels and send their signal to one fader.  Every time you increase the volume of that group fader, the group control sends a message back to those channels so each channel increases the volume coming out of it.  A VCA is more like a remote control.

And if that’s not enough, let’s add in the DCA. The DCA (digital-controlled amplifier) works much like it sounds; instead of altering the actual signals from the channels, they are processed in the same way which then leads to increased or decreased volume.

Is there an advantage between subgroup types?

Glad you asked.  I guess I am in one of those moods.

The “normal” subgroup is good for adjusting the volume of the grouped channels and sending that level out to the main mixer faders.  That being noted, the volume levels of those individual channels stay the same as far as their signal being sent to other locations such as a post fader mix like a reverb unit.  Lowering the normal subgroup fader, you could still hear those channels going through your reverb unit.  In which case, you’re back to using more than one control for working with a group of channels.

The benefit of the VCA/DCA group is that when you lower the volume on the VCA channel, it’s lowering the output of each channel.  Therefore, any post fader mixes, like the reverb units mentioned, are equally affected.  You are preserving the dry/wet balance of the reverb effect.

The Take Away

The first time I started throwing around the word “subgroup,” a fellow tech said to me, “but VCA’s work differently.”  It was like they opened up a whole new side of the technical realm – just because it looks the same doesn’t mean it does the same.

Standing in front of your mixer, you are in control of a gigantic musical instrument.  Like any great musician, you must know how each part of your instrument (mixer) works.  Subgroups, VCA’s, and DCA’s control the volume of your channels at different points.  Knowing how your mixer works with grouped channels, you know what other work you may or may not have to do for controlling your sound.

Next week, you’ll learn a new way of setting channel assignments on your mixer for better workflow management.  It’s one I learned  just weeks ago.

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Comments

  1. Neil Millward says

    Hi Chris,
    I am the sen, sound tech at a Baptist church in Scotland. i am really enjoying your site and insights.
    A quick question… How do DCA’s work?

    I have read loads about them, and we are looking at a new console that has them. I know what they do, but am looking for the physical details….

    What happens if you mute them?
    How do you turn the signal down if the DCA is low?
    Does it disengage when you mute it?

    Cheers

    Neil

  2. Chris Skelnik says

    Hey Chris, thanks for all you do to help the Tech community! I wanted to add a couple of things to your excellent post. First, I think it’s better to say that a subgroup is “summing buss” than a “summing amp”. The other thing about subgroups, when compared to VCAs/DCAs, is that subgroups function as another channel on the mixer. So, with subgroups, you can route them to different places (like you mentioned), and since they are acting as a channel, you can apply other processing (EQ, compression, etc) to the subgroup.

    • says

      Chris, thanks for your kind words and thanks for your notes on subgroups. Great point on a subgroup being another channel on the mixer. Regarding the amp/buss, wouldn’t it be a subgroup mix buss with a summing amp?

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