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Church Sound: Understanding Mixing Console VCAs And VCA Groups
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Audio and VCA groups are not exclusive, either.

Where processing is to be inserted an audio group is essential.

But that exact same group of inputs can also be assigned to a VCA fader to provide overall level control.

VCA grouping also allows the balance between input sources and effects returns to be maintained more easily.

A reverb, for example, inserted via an audio group and applied to several input sources, will maintain its return level even though that audio group fader is pulled down.

But a VCA group can also include the channel into which the reverb is returned, maintaining the balance between effected input sources and effect return.

If a single effects device is shared by two groups, VCA control allows the post-fade aux sends for the individual groups of inputs to also be attenuated. When the first group fader level is reduced, the balance between source and effect for that particular group is again maintained, while the second group’s effect return level is unaffected.

The benefits of VCA group faders also apply to muting. VCA group faders have associated mute switches, thereby acting as an additional layer of mute groups. How they are implemented may differ; on some consoles it is a master on/off switch while on others the VCA group mute switch turns any assigned input channel mute on/off.

Using “soft"switching for VCA group and mute assignments also opens up the possibility for console manufacturers to incorporate those settings into ‘snapshot’ memory automation using MIDI, onboard memory, or RS232 to an outboard controller.

In practical use, and leading on from the discussion on console gain structure in the previous issue of this publication, each input channel VCA fader should be set at the ‘0’ position. That represents 0V, in other words, no change to the audio level through the VCA.

Console manufacturers design VCA consoles such that 0 dB unity gain is passed through at 0V. That allows multiple input channels at 0V to be summed to a VCA group without producing a gain change.
The 0 dB/0V position is usually marked clearly on the channel fader and VCA group fader faceplate. On some consoles there are even LED indicators that show that the fader is at or close to 0.

Lowering the VCA group fader by 10 dB lowers each input level while retaining the relative balance. An input set at 0 dB now becomes -10 dB, an input at -10 dB goes to -20 dB, and a channel set at -20 dB now produces -30 dB. 

The flip side of this is that where a single channel is assigned to multiple VCA groups the output level is the sum of the VCA group levels. With the input channel set at -5dB, group 1 at -10 dB, group 2 at +5 dB, and group 3 at 0 dB, the resulting output is -10 dB.

One “gotcha” for those new to the concept: Be aware that if any one VCA group fader to which an input channel is assigned is all the way down at infinity then that input channel will not be audible.

As mentioned previously, pre-fade aux sends are unaffected by changes at the channel VCA. Monitor feeds from a front of house console generated on aux sends will not therefore alter level in response to input channel VCA level changes.

A single VCA fader could be set up to control all of the choir and instrument inputs, allowing the operator to reduce the level or mute all of the performers while a single pulpit microphone is open. Such a fader, controlling all or a majority of the inputs, is referred to as a “grand master.”

If the monitor feeds are set to pre-fader, then there is the possibility of noise—a cough from one of the choir members, for example—being heard through the onstage speakers.

Setting the monitor sends to post-fade will prevent this from happening, as it will not only eliminate input channel feeds to the main loudspeakers, but also to the monitor system when the grand master VCA fader is pulled down.

This is especially useful if wireless microphone systems are in use, so as to avoid unwanted “backstage” or handling noise to enter the monitor system.

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