From VCAs to DCAs to Spill Groups – History and Evolution

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Whether you refer to them as VCAs, DCAs, or spill groups, these controls are a must on any mixing console with a high channel count. If you’re not familiar, a VCA or DCA is a single volume control on a fader that can control the relative volume of any mix channel or buss master assigned to it. They’re common on large format digital mixers like the Soundraft Vi Series, and make handling complex mixing tasks like orchestras and musical theatre possible. So why are they referred to by three names? Let’s get historical…

In the glory days of large-format analogue mixers for concert sound, there were four big issues – real estate, available processing, noise floor,  and control. Anyone who was behind a top-of-the-line 64 channel desk back in those days (or had to help lift one) will remember how vast they were – three metres long, and weighing over 100 kg. They took up a lot of space on any given show, and so did the huge outboard racks that sat next to them. It wasn’t practical for the engineer to run up and down the board to make adjustments, and even with a big budget, outboard was limited, so a solution was needed.

Subgroups were the first tool to address the practical issue of the size of the board and dole out the outboard efficiently. With eight subgroups available in the centre of the board, an engineer could assign overall volume control over groups of instruments, and thanks to TRS insert points, insert outboard compressors and gates over those busses. Handy, but groups had two problems – they were a physical analogue circuit which added noise to the signal path, and if you wanted to maintain stereo panning from your channels, you had to use two groups. But at least the engineer could stand in the middle of the console and keep levels where they needed to be.

Enter the VCA – Voltage Controlled Attenuator (some say Amplifier, but I find that name inaccurate). An ingenious solution – an overall volume control for any mix channel assigned to it that wasn’t an audio circuit. Engineers could now assign any channel to a VCA, and with one VCA fader control the volume relative to where that channel’s fader was sitting. For example, if you had three channels assigned to the VCA, one at -6dB, one at -3dB and one at 0dB, and moved the VCA master down -3dB, those channel’s outputs would now be at -9dB, -6dB and -3dB respectively. Vitally, the VCA didn’t affect panning.

When simple automation of large format analogue consoles such as primitive scene memories became available, VCAs really came into their own. Now you could change VCA assignments (and mute group assignments) on a scene-by-scene basis. This was a huge leap forward for technicians working on complicated tasks such as radio mic’d musicals with multiple cast member entrances and exits. Modern musical mixing simply wouldn’t be possible without it.

Soundraft Vi Series large format digital mixers

When digital consoles started making inroads into live production, particularly in theatre, the VCA functionality followed. But it wasn’t an analogue attenuator circuit anymore, it was a function of DSP. So now we had the DCA – Digital Controlled Attenuator. At first , DCAs were only available on top-of-the-line consoles. It seems counter intuitive for what is just a volume control, but DCAs actually take up a surprising amount of DSP. The DCA has made it’s way down the pecking order of live desks, but it’s still a feature that’s not available on most ‘budget’ desks.

The DCA has all the advantages of the VCA – it doesn’t take up a buss that could be used for something else, it puts volume control of complex channel assignments in one fader, and can change from scene to scene. It maintains panning, which when digital consoles started doing 5.1 surround and more, became even more important.

As digital console footprints became smaller, and manufacturers started making high-channel count desks with less physical faders and more ‘layers’, DCAs once again came to the functional rescue. Some manufacturers added functionality where a DCA master could be selected on a fader on the left hand-side of a desk, and at the press of a button,  the channels assigned to it would ‘spill’ across the available faders to the right of the master. This enabled engineers to see at a glance all channels assigned to the DCA, make adjustments quickly, then jump back to their preferred control layout.

Each manufacturer uses their own nomenclature for ‘spill’, but ‘spill’ has caught on as the generic name for the function. So that’s how we have three generation of sound techs referring to the same thing by three different names – VCAs, DCAs and Spill Groups. And to head off any arguments – it doesn’t matter which name you use – you’re all right!

Noisegate is an Australian based collective of working musicians, producers, DJ’s, and live audio professionals.

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