When a church is first planted, chances are the worship team will be small and the audio mix will be easy to manage. However, as the worship team grows so will the complexity of the band and the intricacy of the mix. Accordingly, the church will need to upgrade their audio console to accommodate this growth. As the capacity of the audio console increases, features become available to help the audio engineer manage more complex mixes.
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| Figure 1: The audio signal path of traditional audio groups. |
Many of you are familiar with audio groups – this is usually the first tool to appear on audio consoles to help manage a mix. By allowing the engineer to assign channels to a group instead of the main mix, channels can be grouped together and the group’s level controlled by one fader, enabling the group of backup vocalists or drum kit to be manipulated by a single group fader. Figure 1 shows the signal path of channels assigned to audio groups. The next tool is typically the mute group. Mute groups enable multiple audio channels to be muted and unmuted with the press of a single button. With four mute groups available, the engineer could assign the instrumentalists to mute group one, the lead vocalist(s) to mute group two, background vocalists to mute group three, and the drama team to mute group four. Thus, instead of having to unmute numerous individual channels when the band is ready to play, the same thing can be accomplished with the push of just a few buttons.
VCAs are the next feature to help manage a mix. VCA is an acronym for voltage controlled amplifier, the electronic concept that makes the feature possible. VCAs are another way of grouping audio channels together to manipulate them as a group, allowing you to adjust the level of that group as a whole with one fader. There are some subtle yet key differences between VCAs and audio groups that are important to understand. This difference starts in the individual channel strip.
In a non-VCA console, the audio level of the channel is controlled directly by the channel fader. The audio signal fl ows literally through the potentiometer that is the fader - the higher the level of the fader, the less resistance the potentiometer presents to the audio signal, thus the more signal passes through that fader. Ditto with standard audio groups – the audio signal literally passes through the group fader. One of the negatives of this design is that potentiometers can introduce noise into the signal. In a live sound environment, you wouldn’t likely notice this, as the typical hiss of the amplifiers and the room’s HVAC noise would drown out any introduced noise. However, in a post-production environment, this can make a difference.
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| Figure 2: The audio and control signal paths of VCA audio groups. |
In a VCA-equipped console, the audio signal does not pass through the channel fader’s potentiometer. Instead, each channel strip contains a VCA. A VCA is an amplifier (a device that can raise and lower the level of the audio signal) that is controlled by voltage. Functionally, it accepts an input audio signal and an input control voltage. It then emits the input audio signal at the level indicated by the control voltage (refer to Figure 2). The channel fader merely affects the level of the control voltage applied to the VCA. As VCAs introduce less noise than potentiometers, the audio signal coming out of the channel will be cleaner.
Once you have channel faders implemented via VCAs, it’s a small matter to enable more than just the channel fader to set the control voltage on the VCA. Enter in the VCA group. VCA groups are similar to audio groups in that they control the levels of groups of channels. But unlike audio groups, they do so by controlling the level of the individual channels via the VCA control voltage. The control voltages from the channel fader and any VCA groups to which the channel is assigned are summed and the resulting control voltage is applied to the channel’s VCA. If a channel assigned to a VCA group has its channel fader set to +5 dB and the VCA group’s fader set to -10 dB, the resulting attenuation level of that channel would be -5 dB.
This has several distinct advantages. The first, and least interesting for live sound, is the reduced noise by eliminating two faders from the audio signal path. Second, instead of altering the level of the combined signal after the individual channel signals have been mixed together, it’s altering the level of the channel itself. “Same difference,” you may be thinking.
But not so.
Let’s say you want to fade out all the background vocalists at the end of an a capella number. If you do so via an audio group, the combined mix of vocalists will fade out, but what about that post-fader feed to your reverb unit that’s adding a rich ambiance to the mix? If you’re using that reverb for instruments in other songs as well, you are probably not routing it through the vocal audio group. Thus, as you fade out the vocals, the reverb remains strong and takes over the mix unless you fade it out separately.
In the same scenario using a VCA group, what would happen? As you pull back the VCA group fader to fade out the vocals, it is the output levels of the individual channels that are decreasing, not the combined mix of the channels. Thus, all post-fader aux sends are decreasing as well, reducing proportionally the signal to the reverb unit – exactly as if you were pulling back the faders of the individual channels.
If your console implements both standard audio groups as well as VCA groups, you have a whole extra level of control available to you. I like to use the audio groups as my course-granularity level of grouping. I’d use audio groups one and two as a stereo group for the instrumentalists, three and four as a stereo group for background vocalists. I would then use VCA groups one, two and three to group tenors, altos and sopranos, effectively sub-dividing the vocal audio group. I’d use VCA group eight to group the drum kit inputs together, and VCA group seven to group any other related instruments (such as a horn section) together. This provides an enormous level of control over the mix, enabling macrolevel adjustments of the entire band, or micro-level adjustments over the horns making it easy to bring out a horn-section solo instrumental in a song.
In the July/August 2004 issue of Church Production Magazine I discussed automating VCA and mute group assignments using snapshots; if your console supports this feature you might want to look at that article to learn about additional fl exibility that can be obtained combining VCAs and snapshots.
Jim Kumorek is the managing editor of Church Production Magazine.
