Study Hall

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Caution: Channels Merging – Thoughts & Approaches When Combining Like (Or Similar) Inputs

Just because we're able to add a microphone (or two) to every source onstage, does it mean we should?

When I first started mixing club bands back in the early 1980s, I thought I was pretty cool behind my 16-channel analog mixing board. Most bar bands had about the same number of channels on their stage, although a few larger bands had 24- or even 32-channel consoles. Wow!

We simply put a single microphone or DI on every input on stage and away we went. With today’s digital consoles, even a moderately priced package can support 48 to 64 inputs from the stage, which allows us to use multiple mics and DIs on some instruments to give us an A/B option, or a combined sound from two different mics or a mic/DI pairing.

But just because we can, does it mean we should?

Too Much?

A lot of less experienced engineers (and even some of us more seasoned folks) fall prey to thinking we need to use more than one channel for a given input because someone we admire or read about in an audio magazine has a “killer way” of manipulating multiple “like” channels. This is often done using multiple mics, a mic/DI combo, or “double assigned” channels that both receive the same single input.

As I’ll explain here, cool techniques and tricks can be used to get better results than can be achieved using only one channel, but before I describe some of these methods, I recommend first asking yourself, “Do I really need more than one input for this instrument?”

In certain scenarios (a “throw-and-go” festival, for example, where you’re mixing a band you’ve never heard before) it’s usually smarter to focus on a simpler methodology and whittle down the inputs to one per instrument. Of course, there are situations where it really can help to have more than one input, such as snare top and bottom, but as we continue this discussion, we’ll see that many problems can arise when extra input channels are added.

It’s also important to understand the impact of combining “like” inputs, because usually some element is slightly different between the two, which can create a problem. An example: deploying two different mics on two different speaker cones of a guitar player’s 4 x 12 amp. (A common combo is a Shure SM57 dynamic mic on one speaker and a Shure KSM32 condenser mic on the other.)

While we might get lucky and simply bring up both mics in the PA (same level, same EQ and a little panning) and discover total magic, most often there’s an anomaly in the overall tone. Maybe one mic has more low-end than the other, so the high-pass filter (HPF) is set higher to get rid of some of the “woof” – but then the two mics don’t quite combine correctly from a tonal standpoint.

Frequently, mix engineers just barrel ahead with both mics hot, simply because they’ve seen this done so often that it must be the way to do it. But the result can be a huge loss in tone and volume, so we may reach for the “polarity” reverse button (ø) on the console in the hope that it might improve the situation.

Side note: I say “polarity” reverse even though most consoles are labeled “phase” reverse. What actually happens when this button is engaged is that a “polarity flip” happens. “Phase” is a little more complicated. For a more in-depth discussion on this topic be sure to check out Jonah Altrove’s “Phase vs Polarity” on ProSoundWeb.

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It all begs asking the question again: “Do I really need a second input for this instrument?” It may be that using only one guitar mic and EQ’ing it a bit is a much better solution.

Make It A Double?

Another popular technique among experienced engineers is “double-assigning” an input into two channels on the console. An example: when there’s only a single kick drum mic but you’d like to have two kick channels on the console that are EQ’d differently to emulate having an inside mic (with more attack and “snap”), and an outside mic (with more low-end and “fullness”). Unfortunately, this approach is often handled incorrectly.

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