Audio Mixing Consoles: Defining Auxes, Groups, VCAs, Matrixes

Take a closer look at how each of these four systems work as a part of audio mixing consoles.

For the novice sound engineer, it can be pretty intimidating to walk up to a large analog or digital console and try to figure out the routing of audio signals.

It wouldn't be so bad if the only place the sound had to go was to the main loudspeakers in the room. Occasionally that's the case, but most of the time, we're also sending different mixes to monitors for musicians; to lobby and cry room speakers; and to various recordings.

What makes aux mixes so cool is that they don't affect the house mix at all…

Moreover, we have several options for how we can group and control our signals. Let's define what Auxes, Groups, VCAs and Matrix mixes are, and then in my next article, I'll talk about when you'd use each of them.

Auxiliary Mixes

Often simply abbreviated as auxes, an auxiliary mix is pretty much what it sounds like; another, alternate mix using the same set of input signals that you are working with in the house mix. Each aux mix will have an individual level control on each channel, as well as a master aux mix level. In this way, aux mixes are very much like the faders; turning up the aux level for channel 1 adds more of whatever is in channel 1 to that aux mix.

As an example, let's say you are using Aux 1 for your worship leader's monitor, and that their vocal mic is in channel 1. If she wants more of her voice in her monitor, you turn up the Aux 1 send in channel 1, thus raising the level of her voice in her monitor.

What makes aux mixes so cool is that they don't affect the house mix at all, and you can just as easily lower or eliminate that signal from Aux 2. Thus, each aux mix is another complete mixing layer for your input signals, albeit without individual EQ adjustments for each of those sends.

Most consoles/mixers that we use for live sound will have at least four, and often six auxes. Of course, larger consoles gain more auxes, and it's not uncommon to see digital consoles with 16, 24 or more aux mixes (many times, in stereo).

Aux sends (the points you send from each channel) can either be pre-fader or post-fader. That simply means that the fader (which controls your house mix) will either have no effect on the aux send (pre-fader) or it will have an effect (post-fader).

In other words, if Aux 1 is pre-fader, you can turn the channel 1 fader down all the way to off, and still have signal coming from the monitor connected to Aux 1. If it's post-fader, turning down the fader on channel 1 will also turn down the signal in Aux1.

Some consoles allow you to switch each aux to be either pre- or post-fader (and some provide additional options), while other consoles allow you to switch them in pairs, and still others will give you 4-6 pre-fader auxes, and 2-4 post-fader auxes. Or sometimes the first few are fixed at pre-fader, while the last few are switchable.

Remember the manual that came with your console? Now's a good time to check it out if you're not sure what your console does.

Groups

Groups have somewhat fallen out of favor of late as digital consoles have become more popular. It's a shame, really, as groups are incredibly useful.

Sometimes called subgroups, a group is simply a way to collect a bunch of channels together and do something with them. Strictly speaking, the main left and right (L&R) mix is a group (as is a mono or center mix if your console has those).

A sub-group then, is a way to collect some channels for processing on the way to the main group (or not, depending on your needs).

To use a group, you simply assign the channel to the group you want to use. Some consoles will give you say, 4 groups, with three switches on each channel to use for assigning; Group 1-2, Group 3-4 and L&R.

Assigning a channel to Group 1-2 will send that channel's output to the summing mixers known as Groups 1 & 2. In these consoles, group assignments follow the pan knob. So if you want to assign a channel to just Group 1, you would pan hard left. Panning hard right puts that channel in Group 2 only.

We'll talk about why you would want to do that in the next post. But for now, just know that you can usually assign the sub-groups to the main mix as well.

Thus, you may have a collection of channels (let's say the drums) all going to Groups 1-2, then have Groups 1-2 going to the main L&R mix. Would you also assign the individual drum channels to the main L&R mix? Maybe, maybe not; it depends on what you're doing.

Now let's tackle VCAs, and then most confusing off all, the Matrix (don't worryKeanu Reeves will not be making an appearance).

VCAs

When I first mixed on a console with VCAs I thought, "This is awesome! I never have to use groups again!" I was right. And I was wrong.

The truth is VCAs and Groups, while they can be used to the same effect, are really two different animals, and both have their applications. Again, we'll save the applications for my next article. Let's figure out what a VCA is.

VCA is an abbreviation for Voltage Controlled Amplifier. Without getting super-technical, think of a VCA as a remote control for your fader. That might not seem very useful at first, considering you already have the fader right there. Where it gets fun is that you can assign multiple channels to a VCA, and you can assign a channel to multiple VCAs. More on that later.

Here's how a VCA works (at least functionallylook it up if you want to know how they work electrically): Let's say you have your lead vocalchannel 1assigned to VCA 1. And let's say you have channel 1 sitting at Unity level (sometimes known as 0).

The fader is not adding or subtracting gain from the signal. If VCA 1 is also at Unity, there is no gain change. But turn VCA 1 down to -10, and the signal on channel 1 drops 10 dB, even though the fader hasn't moved.

Move the fader for channel 1 down to -10, and the signal is now down to -20 dB. If you leave the fader at -10, and put the VCA back to 0, the signal is at -10 dB. Push the VCA up to +10, and the signal is back to 0 dB.

Digital consoles sometimes call these faders DCAs (for Digitally Controlled Amplifiers), or even Control Groups. No matter, they basically do the same thing.

The important thing to remember is that VCAs aren't better than groups; they're different. I use bothto very different effectsevery week. Given the choice, I would always have a healthy supply of both Groups and VCAs on my consoles (which is one reason I really like mixing on DiGiCo consoles).

Matrix Mixes

The modern matrix mix can be a confusing beast. Back in the days of analog consoles, the matrix operated much like aux mixes do, but instead of being fed by individual channels, they were fed by groups.

Thus, to create a matrix mix, you would assign your channels to groups, then build a matrix mix from your groups (including the L&R mix).

Some mixers gave you a few matrix mixes, others gave you a lot. Matrix mixes are defined by how many inputs and outputs they have; to wit, a 16x12 matrix has 12 individual mixes being fed by 16 sources. On an analog desk, this would probably mean the console has 14 auxes plus L&R, and you can create 12 mixes of those groups.

With the advent of digital consoles, there is no real reason why individual channels can't also be fed into the matrix mixes. Some companies treat their Matrixes like Auxes (Yamaha and Roland, for example). You can assign any or all channels to a matrix mix and use it just like another aux. The difference is that you can also assign auxes or the L&R mix to the matrix, so it's really a hybrid.

Other digital consoles let you assign groups to the matrix, plus a limited number of input channels. Still others let you pick a fixed number (say 16) of whatever inputs you want input channels, auxes, groups, L&R and mix them into 12 or so mixes.

Matrix mixes can seem confusing, but hopefully once we get to the practical applications, it will start to make more sense. For now, know that they are incredibly useful for feeding ancillary rooms or destinations (recording for example), or even running your main PA depending on the complexity of your PA system.

So that sums up how each of these four mixing systems work. Maybe next time I'll talk about actually applying them.

(Mike Sessler has been involved with church sound and live production for more than 25 years, and is the author of the Church Tech Arts blog. Based in Nashville, he serves as project lead for CCI Solutions, which provides design-build production solutions for churches and other facilities.)

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