Behind The Mixer

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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 for grouping channels together.  Each works differently, of course, because some days things have to be complicated.

Why group channels?

There are three main groupings used in mixing:

  1. Aux send groups. Useful for sending specific channels to a recording device or to hallway speakers or, more likely, to an effects unit for reverb on all vocalists.
  2. Volume groups.  I place singers in a group and instruments, by type, in other groups.  Such grouping provides an easy way to pull back instruments or push vocals to meet the needs of the moment. 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 their multiple microphones.
  3. Mute groups. I love mute groups.  The music ends and the pastor walks out so you’ve got seconds before they start talking.  Instead of hunting around the board to mute the band’s channels, you can mute all of their channels at once.  This way, you avoid broadcasting the pop of the unplugged guitar or other such sounds.  A mute group mutes all of the channels assigned to the mute group.  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

There are three types of groupings; subgroups, VCAs, and DCAs.

The “normal” subgroup control works the simplest way by acting only at the grouped level.  Take four channels and send their signals to a mix bus that sums the signals.  Moving the fader up or down is increasing or decreasing the volume of that summed sound.

The VCA (voltage-controlled amplifier) works by referencing the source channels.  Take four channels and send their signal to one fader.  Every time you increase that group fader volume, 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.

The DCA (digital-controlled amplifier) works much like it sounds; instead of altering the actual signals from the channels, the signals are processed in the same way, which then leads to increased or decreased volume.

Is there an advantage between group types?

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 works the same.

Standing in front of your mixer, you’re 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.

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Comments

  1. Your comments on Mute Groups has at last explained to me what would make life easier for our A&H Qu24 mixer for operators on the desk with our Music Group. This leads me to look at if there are other areas that could be in separate Mute Groups. So a good article Chris.

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  2. Groups also allow the addition of processing across the group whereas vca/dcas lack that capacity. Agentle compressor over a vocal group will pull all the vocals together dynamically.

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  4. Chris,
    I guess my question is do you have to have a DCA group to go along side of a subgroup? I am a sound engineer at my church and collectively we are learning on a Behringer x32. I saw a video where the instructor put a subgroup together let’s say choir, but also had a DCA group for choir also. I understand that with the subgroup you can add things like compression, where as with a DCA you cannot, but since they both give you volume adjustments do you need both? Thank you for all of your work to try to help folks like me understand the various facets of audio and live sound at church!

    Reply

    • DCA’s are helpful for routing to house speakers or subs. For example, you might have a DCA just for kick drum and bass and it’s routed to the sub so if you want more of those in the sub output then up raise the DCA. Think of it as having subgroups whenever it’s helpful for a service or a song but DCA’s are set with the same channels all the time.

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    • I know this topic is over a year old, but having just found it, another reason I can put forth to use a DCA *and* a subgroup is this:

      Suppose you have maybe a horn section, and you want to run them as a group through one very cool outboard FX unit – so you want a subgroup, obviously. But then you find that the trumpets are just *really* getting into the groove and pushing harder as the song progresses, so the t-bones and saxes are following their lead – not enough to scare their respective headamps, but enough to collectively start clipping on the bus, on their way to the group insert/fader. If all you had was the subgroup, you’d have to start tweeking all of those individual horn channels down to keep from clipping the bus, while keeping your mix intact. But, if all those horn input channels are also assigned to a VCA/DCA, then pulling that 1 fader back a bit keeps the whole section mix stable, but drops all of the output levels at the point where they leave their channels – *before* they are sent collectively to the group bus. This keeps the mix balanced, allows 1-fader control over the total level being passed to the bus, *and* still gives subgroup-style insert and processing capabilities.

      So, while you don’t need to assign to a subgroup *and* a VCA/DCA , there are some cases where it could be an option to consider.

      As Chris has said in other places, the sound engineer has to know his/her instrument – the mixer – inside and out. A lot of that intimate knowledge comes from knowing the internal signal chain including: the points at which controls affect it, from whence taps are pulled, and in some cases how and where the chain itself can be modified (e.g., the ability to shift the EQ block from input to output in many in-line studio consoles).

      Good stuff, Chris, thank you!

      Reply

  6. Thank you!
    Just switched to a digital mixer at our church. Despite my Recording degree from UofM 17 years ago, some of these internal concepts are just confounding, especially when you’re trying to train non-engineers up alongside.

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  7. Hi Chris,
    So would it be safe to say that the difference between a VCA and a DCA is that the VCA uses an analog control voltage whereas a DCA uses a staircase control signal, whose resolution is based on the length of the word? (not the pastor’s) :)

    Reply

  8. 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

    Reply

  11. 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.

    Reply