Mixing consoles: Part 4

In addition to sub-groups, some larger consoles, like this Midas Heritage 1000, also feature VCA groups, which function in a similar way to groups but don’t actually output an audio signal — just a control voltage that determines the relative amplitude of the channels assigned to it.

Not all consoles are equipped with groups (or sub-groups), but most — even many compact budget mixers — do have a least one pair of sub-groups in addition to the main output. The idea of a sub-group is to mix together a selected subset of channel signals, either to afford easier control within a complex mix, or to provide a separate, mixed output independent of the main mix. For example, you might want to combine the outputs of all the backing vocalists’ channels or a large keyboard rig to a single fader, or gather all the drum mic channels together and control the overall drum mix level on a single fader.

The advantage of this approach is that the relative balance of the source channels, as set on the channel faders, remains constant, with the sub-group fader affecting only the overall level. It’s simply a way of helping to make mixing a complex balance a bit easier and more practical — especially when working on a large desk, since the sub-group controls are usually in the centre, where they fall comfortably to hand (and are therefore easier to control than having to stretch to reach a channel right at one end of the console). Sub-grouping is also useful when it is necessary to gather a sub-set of channels together for processing, such as EQ, dynamics or delays, or for setting up a recording feed independently of the main outputs.

Different consoles provide different sub-group facilities, of course. The simplest option is a number of separate mono sub-groups — anything from two to eight, or even 16 — the output(s) of which can be panned and routed into the main output mix bus. Nothing more than a basic mixing function.

Some consoles provide a combination of both mono and stereo sub-groups — the latter being useful when combining something like a drum set, where you might want to pan the individual channel mics into a stereo image and control the overall level on a single (stereo) fader rather than two linked mono sub-group faders. It can still be done with mono sub-groups, of course, by panning and routing the groups hard left and right and linking their faders together mechanically with a plastic clip or tape, although this is obviously less convenient.

Sub-groups can be very useful if, for example, you want to control the level of drums within a mix, but without changing the relative levels of different drum mics.

Most consoles equip the audio sub-groups with similar equalisation facilities to the channels, making it easy to provide overall tonal tweaking on the complete sub-mix. Virtually all provide access to the aux sends as well, making it very easy to send foldback of the complete drum sub-mix, say, using the one group aux send control instead of six channel controls. But thinking about aux sends and sub-groups, there is a potential trap to be aware of here. If you set up post-fader aux sends from the channels, they obviously remain active if you close the group fader (because the channel faders are still open) — and that may be undesirable in some situations. For example, if you are sending backing vocals to a reverb unit using the channels’ post-fader auxes, then although closing the group fader will kill the direct backing vocals in the mix, the reverb return will continue to provide their ghostly presence! A simply way around this is to return the vocal reverb into the group as well, of course, but that’s not always ideal and other situations might not be so easily solved. In general, if you’re sub-grouping channel signals then it is often safer to use the group’s aux sends rather than the channel aux sends — particularly for post-fader effects.

Sub-groups are normally provided with direct outputs, so that you can use them to provide independent outputs of a subset of channels from the console. For that reason, most consoles include independent metering and monitoring (PFL/AFL) for the sub-group channels too, so you can see (and hear) what’s being sent from those outputs. Larger consoles will also provide group inserts for external processors like compressors, and some can also accept line inputs to the groups, making them useful as extra line input channels or effects return channels.

There are countless ways of using groups and group outputs, limited really only by your imagination and the circumstances. In live-sound applications, sub-groups are sometimes used to feed separate sections of PA — maybe stage fills, a centre cluster carrying only the vocals, perhaps, or surround towers, for example. And in recording applications they are often used to route several source channels to a single recording track, or to gather a number of channels together for common processing, such as ducking any music channel under a voice-over.

In broadcast circles, audio sub-groups are often used to provide ‘clean feeds’ (also known as ‘mix-minuses’), which provide a complete mix except for one (or more) selected sources. When working with an outside broadcast, for example, the OB needs to receive the output of the destination studio so that the remote presenters can hear other contributors to the programme — but they won’t want to hear themselves coming back because of the potential for delays and howlrounds. So the return feed from the studio to the OB would be arranged to be ‘clean’ of the OB itself, or the signal would be a complete mix ‘minus’ the OB signal. That kind of thing is easy to set up with a sub-group, as you simply allocate all the local channels to that group in addition to the main outputs, but route the OB signal only to the main output.

Some larger and more sophisticated consoles provide VCAs as well as, or instead of, conventional audio sub-groups. VCA is short for ‘voltage-controlled amplifier’, and with this system each channel has a VCA in the channel’s signal path instead of a normal audio fader. There is still a channel fader, of course, but rather than affecting the audio signal directly, this now sends a control voltage that determines the gain of the VCA to raise or reduce the channel’s signal level as required.

With a VCA in place, it becomes possible to control the channel signal level remotely, and that’s what VCA groups are used for. Each input channel can be assigned to a VCA fader, to have its level controlled by that fader in addition to the channel’s own fader — the control voltages from both are combined to determine the overall level. So a balance can be set up on the channel faders as usual, but the overall level of selected parts of that mix can then be fine-tuned with the allocated VCA fader as well.

In terms of pure level control this arrangement works in exactly the same way as a conventional sub-group — but the important difference is that the source channels are no longer being combined into a composite mixed signal in the ‘group’ itself — VCA groups don’t generate an audio output signal at all. The individual channel signals remain wholly independent of each other and can be routed to different sub-groups or outputs as required. However, since there is no physical audio output from a VCA group, there can be no audio metering either.

VCA grouping is a very flexible and useful facility, although the cost of installing VCAs in every channel and audio sub-group means it tends to be restricted to high-end analogue consoles. Digital consoles are usually able to provide similar functionality.

While there is a potential downside in using VCAs (they can introduce more noise and distortion than a simple audio fader), the VCA approach potentially removes a load of electronics from the signal path too, since an audio sub-group may no longer be necessary to control multiple channels (avoiding a mix amp, fader, and fader gain amp, for starters), and that will reduce the noise and distortion figures slightly to compensate for the VCA itself. To be fair, though, modern VCAs perform and measure extremely well, so the technical disadvantage is minimal.

There are several other advantages to VCA groups. For example, when working with a stereo source, a pair of mono channels could be controlled from a single VCA fader, instead of a stereo audio sub-group. And since the VCA group is actually a remote control for the channel fader, post-fader aux sends work exactly as expected (avoiding the potential problem we have already seen with audio sub-group working). In a theatre or broadcast application it’s often useful to allocate one VCA to control the lead singer or presenter level, so that you always have direct control under your fingertip, while selecting the appropriate singer or presenter for that scene or sequence with the channel faders.

On some larger desks, the VCA groups can also be applied to the real audio sub-groups, making it possible to sub-group related audio sources and then control a subset of those audio groups with a single VCA group fader. Consider a large string section on stage, for example, where you might submix all the first violins in one audio subgroup, the second violins in another, and the ‘cellos and basses in a third — and then control the complete string section level with a single VCA group fader. It’s also usually possible to assign each channel to more than one VCA group, so that the string section might be assigned to VCA 1, while VCA 2 controls the string section and all the other orchestral backing sections — allowing the entire orchestra to be balanced against the band. Once again, the limitations here are your own imagination and how complicated you are prepared to make the routing and control configurations!

Matrix outputs, as seen on this Yamaha IM8 mixer, work in a similar way to aux sends, but the output signals are derived from the sub-groups and main outputs, rather than individual channels.

Large-scale, live-sound applications often have very complex signal-routing requirements, as do broadcast applications and some theatre situations, and to satisfy those diverse requirements several mixer manufacturers incorporate an additional audio sub-grouping facility, which is generically referred to as the matrix. The name describes what it does fairly well — it is usually configured as a matrix or grid of rotary gain controls and forms another independent mixer within the main console.

The matrix system works in much the same way as aux sends from the channels, but normally the matrix inputs are derived from the audio sub-groups and main outputs. The outputs are referred to as matrix outputs, and the number will vary with console, but four is common and some larger desks might have eight. So in a four-output matrix section, there will be four knobs associated with each sub-group and main output source — one to route that source’s signal to each matrix output — plus master level controls for each matrix output, along with an AFL facility and maybe a mute switch.

These individual matrix outputs would typically be sent to separate loudspeaker clusters, or any other destination that requires a specialised blend of signals — maybe a backstage monitor feed, a mix for the MD’s headphones, a foyer speaker mix, or whatever. Essentially, it provides additional and independent main outputs. In a console without a matrix facility, the audio sub-groups feed the main outputs, and so there is therefore only one possible version of that main mix. The matrix system enables the audio sub-groups to be mixed together at different relative levels to feed different matrix outputs, effectively providing multiple main outputs. Matrix systems are also very useful when generating foldback mixes from the FOH console. Yet again, the flexibility and usefulness of this system is limited only by the imagination.

Next month I’ll take a close look at the kinds of monitoring facilities to be found in different consoles, and why different applications have different monitoring requirements. I’ll also discuss digital consoles and look at the commonalities and differences between those and their analogue forebears. 0

Mixing consoles: Part 4

Tech Notes