Grouping, Busing, and Parallel Processing

Let your tracks live in a parallel universe

By Phil O'Keefe

One of the really cool features of most DAW software is a customizable virtual mixer, where you can create layouts and templates for recall at a later date. But in terms of customization, I’m also a huge fan of being able to add auxiliary channels to the mixer for functions such as effects sends and returns.

Usually with an affordable hardware mixer you are limited to a just few aux buses, so those tend to get used up pretty quickly for reverb, headphone monitoring sends, etc. But in software, adding an almost unlimited number of sends is usually just a few mouse clicks away. It certainly beats having to sell your hardware board and buy a new model just to get some more aux sends! Having lots of available aux sends opens up the possibility of using some of them for “bus compression” or parallel processing. By parallel processing I don’t mean dual CPUs in your computer, but parallel audio paths for the direct signal and the processed signal, with each on a separate fader.

ENTERING THE PARALLEL UNIVERSE

Suppose you have a set of drum tracks, or several background vocal tracks, or multiple acoustic guitar tracks, and you’d like to compress all of them a bit — either to smooth them out, or add a bit of punch. You could insert an individual compressor inline on each track by using your software mixer’s channel insert points, and sometimes that may indeed be your best option. It will certainly allow you to use different attack and delay times — or a completely different compressor — on each track if desired. But it takes more CPU power to run all those compressor plug-ins simultaneously, unless you call on some external hardware assistance (e.g., SSL Duende, FocusRite Liquid Mix).

Besides, sometimes you just want a bit of compression across all the drum tracks, or background vocal tracks, using the same type of compressor. This is where routing aux sends to a compressor inserted on an aux return channel can come in handy. The concept is similar to using an aux bus and aux return as a reverb send and return, instead of inserting a separate reverb plug-ins for each track.

SETTING IT UP

1. Add a stereo aux return to your software mixer layout. In Pro Tools, do this by hitting Ctrl-Shift-N, and selecting “1, Stereo, Aux Return” in the ensuing dialog box.

2. Upon creating your new stereo aux return, assign the input of that aux return channel to an aux send. In Fig. 1, the input for the aux return channel is set to aux (“bus”) 1-2.

3. Add an aux send (again, using aux/bus 1-2) to each track you want to send to the processor. If you have “Send levels follow groups“ enabled in your Pro Tools preferences (under the “automation“ tab), you can then raise or lower all the aux sends on any grouped set of tracks by simply raising any one of the aux sends — and all of the aux sends assigned to the same bus will follow suit for every channel of the group. Of course, you can disable the group and adjust the send levels for any tracks individually as well.

4. Adjust each send’s panning individually. With drums, I normally use the same panning for the aux sends as for the stereo mix.

5. After setting the levels and panning, tap the signals off of the original tracks, and bus them to your aux return channel; this is where any inserted effects or processor plug-ins will process the signal before sending it to the stereo mix bus via the aux return channel fader. This allows you to bring the compressed signal up on a separate fader, and blend it in with the original, unprocessed tracks.

Figure 1: Drum tracks 1-6 are all sending some of their signal to Bus 1-2. This goes to a Compressor/Limiter, the output of which returns into the virtual mixer via track 7.

This technique makes it easy to fine-tune the amount of compression you want, and because it’s working in addition to the original source tracks, you get a more balanced and natural final result, with less “squash.” Just raise the aux sends until you’re hitting the compressor as hard as you want, then slowly raise the aux return fader until you hear the desired mix of unprocessed and processed signals.

Of course, you can insert other types of plug-ins either before or after the compressor on the aux return channel. Placing an EQ plug-in before the compressor will tend to change the way the compressor responds, while placing it immediately after the compressor will tend to shape the overall sound and timbre of the compressor’s output. Neither approach is “right or wrong” and both can be useful, so experiment with both approaches and decide which one sounds best.

Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.