Riding the signal path from input to output

In the previous issue of Church Production Magazine, we climbed inside an audio mixer and walked the path taken by input signals. Between a mixer channel’s input jack and its level fader, there are many stages along the way where you can improve and control an individual sound—EQ, insert point, effects sends, monitor sends, bus assignment switches and more.

In this issue, we continue our journey through the output side of the common audio mixer. Once past the channel fader, an audio mixer’s output section gives you the power you need to process multiple signals and combine them in creative ways. We’ll learn how the output section operates, and offer useful tips for getting the most from the output features found on your mixer.

Output Subgroups
One of the most powerful features the output section of any audio mixer can offer is that of subgroups (also called “output busses”). Subgroups sit just downstream of the channel faders, and allow you to combine any number of channel signals together to process them as a group.

Imagine that you’ve miked your church choir with six different microphones by vocal section. You control the mic levels with six consecutive faders, and you’ve created a nicely balanced vocal mix. With all six faders assigned to the stereo bus, adjusting the overall choir level requires moving all six faders a proportional distance at the same time. Doing so without destroying your mix is all but impossible.

Assigning all the vocal mics to a pair of subgroups (instead of the main stereo output) allows you to control your overall choir level with one pair of faders. You can also mute all the choir mics with two mute buttons, or control the choir’s stereo placement with two pan knobs. Your balance of mic levels remains exactly the same until you adjust the individual channel faders. Subgroups come in just as handy for controlling a drumset, string ensemble or whole rhythm section with just a handful of faders.

Subgroups go beyond just level and panning control. Some mixers offer aux sends on subgroups, allowing you to add effects to grouped signals. Adding reverb to a choir is much easier to accomplish at the subgroups than the individual channel sends. Subgroups with pre-fader sends work well for monitors, allowing you to route a ready-made blend of input channels back to the performers.

Some mixers even offer two or three-band EQ on subgroups, for tonal changes that affect the whole group of signals. Metering is commonly added on subgroups, allowing you to spot signal level problems quickly. Most subgroups have a Solo button for listening in on that group of signals.

And that’s not all subgroups can do. Better mixers offer insert points on subgroups, which allows outboard processors to be added to combined signals. Subgroup inserts would allow compression to be added to a whole choir or drumkit at once, for example, or a single noise gate to silence a whole rhythm section. The possibilities are endless.

Finally, subgroups often include dedicated outputs. These come in extremely handy for recording, especially when you have a limited number of tracks available. Grouping like signals and recording them together allows you to reduce the number of recorded tracks while still retaining some control over levels at mixdown.

Auxiliary Outputs
Aux outputs carry the “special blend” of signals created by the channel aux send knobs. Channel sends set post-fader are usually used for effects, while those that tap the signal before the channel fader are best-suited for monitor sends.

Regardless of the application, most mixers offer some control over aux sends in the output section. The simplest control is the send level knob or fader, useful for adjusting the overall output level of the aux signal. For an effects application, the aux output level control lets you set the right signal level for the effects unit’s input. For monitoring, the aux output control will set overall monitor volume for that particular send.

Some aux outputs have metering, which allows you to spot signal problems quickly. Equally handy is a Solo button for making sure the right channels are going to the right aux outputs.

Auxiliary and Tape Returns
Most mixer output sections offer a place to return signals—usually from effects—to the mix. The simplest aux return has level and pan knobs; more capable return sections may boast full-length faders, metering, EQ, aux sends, mute/solo switches and bus assignment switches. The ability to send effects returns to a different aux output (for folding effects back into monitors, for example), is growing more common on mid-priced mixers.

Aux inputs can also be used for adding other music sources, such as a CD player or VCR, to the stereo output. Some mixers have dedicated inputs for stereo music sources (sometimes called “tape returns”) which can effectively take over the stereo output. When you route such an input to the stereo outputs, signals from the mixer channels are muted. This is a convenient (and goof-proof) way to play back music before or after a service.

Matrix
In the old days, a mixer’s stereo or aux output was the end of the line for its signals. Now many models have an output matrix, which functions like yet another mixer tapped into the console’s outputs (see Figure 1). A matrix is the ultimate tool for sending a blend of output signals to foyer speakers, a cry room, a camcorder, a remote television monitor, a radio transmitter, an audio recorder, a system for the hearing impaired or anything else.

Just like a full-blown mixer, matrices are measured by their inputs and outputs. A matrix that sends any combination of four aux sends, four subgroups and the stereo bus to six different outputs is a 10x6 matrix. Matrices as large as 20x10 are not uncommon on higher-end mixers.

Stereo Output
In most sound systems, the mixer’s stereo output carries the be-all, end-all signal. This final stop in the life of the mixer’s signal represents the best musical output of singers and instrumentalists, and the diligent work of the sound engineer.

In addition to stereo XLR balanced jacks, most mixers have unbalanced 1/4-inch or RCA jacks as well. These are useful for routing the full stereo mix to a recorder, video system, radio transmitter or other stereo destination. Some mixers offer a separate output level control for the unbalanced outputs, which can come in handy.

Virtually all mixers offer insert points at the main stereo output. This is where you’d patch in any effect or processor desired on the whole stereo mix. Because it has the potential for ransacking your whole sound, any stereo mix processor must be set up with great care.

Mono Output
Mono outputs are growing more common, even on low-cost live sound mixers. On these low-cost mixers, this represents the true final stop for the signal—the mono output is simply a combination of the left and right outputs (see Figure 2a). On bettermixers, the mono output carries its own discrete signal. Just like the stereo bus, you can assign channels or subgroups directly to the mono output (see Figure 2b).

The best live sound mixers derive their mono signal from a true LCR (left-center-right) panning system. With LCR panning, the pan knob controls the channel’s placement across three outputs (left, center and right) instead of just left and right (see Figure 2c). This gives the best imaging and placement in an LCR three-cluster speaker system.

Final Output
Our journey, which started at the input stage of the mixer, has now ended at the mixer’s outputs. Along the way, we’ve seen numerous different points where the sound engineer—that’s you—has the ability to influence the sound for better or worse.