For a lot of people coming into sound recording and mixing nowadays (myself included) our first introduction to mixing and effects has been with the graphical mixers and digital plug-ins available through DAWs like Pro Tools, Logic, Reason, and GarageBand. While Auxiliary channels and buses within a DAW graphically represent what an engineer does on a mixing console and with outboard gear, it is often hard to conceptualize what auxiliary channels and buses do without physically seeing the objects and pathways that connect and represent them.
What are Auxes and Buses?
Let’s first define what a bus and an auxiliary track are. Unlike an aux track, a bus is a connection of many different signals, seen sometimes as a channel strip’s send. The send controls however much of the signal you want the bus to send or “transport” to a specified location, such as an auxiliary track.
An auxiliary track, unlike a bus, is an actual track available for manipulation and instantiation on your mixer. An auxiliary track’s role is to take whatever amount of the signal that is sent to it via a bus and process it with effects like reverb, compression, delay, etc. The auxiliary track “hosts” these effects, and an auxiliary channel with reverb is essentially your reverb – whether it is an outboard reverb unit (hardware) or a plug-in reverb (software). Another way to think of it is that Auxiliary channels are actual tracks, a destination for your buses. All buses do not have to go to auxiliary tracks, but all auxiliary channels need buses as inputs.
Why it’s important to use auxiliary tracks and bussing: the more complex your songs get, the more tracks you are hypothetically using. Let’s say you have 16 different tracks just for your drums – mixing drums with that many tracks can be difficult, and one of the things you can do to make it easier on yourself is with the creation of submixes. A submix is essentially many tracks grouped together on buses and sent to auxiliary tracks representing each of those groups. Submixes help organize your mix and help preserve your mix. To keep with the example of 16 drum tracks, say you’ve mixed these 16 tracks roughly how you want them relative to one another. But say when you add bass, guitar and vocals to the mix you find that the drums are a bit too loud. Instead of sliding each one of the faders down to accommodate this necessary reduction in volume, sliding down the auxiliary tracks (your submix) is much more convenient and will preserve your previous mixing.
In addition to the benefits of submixing, hosting effects on your auxiliary tracks allows you to apply effects such as reverb and delay to a lot of things without instantiating said effect on each individual track. Within the digital landscape, it is very easy, and conceptually easier to just put your reverb effect (as an example) onto the particular track you want to apply reverb to. The problem with this (potentially) is that when you want to effect a number of tracks with similar reverb, it becomes confusing and CPU intensive to instantiate a reverb plug-in on each individual track. Instead it is easier to apply effects like reverb to a number of tracks via an auxiliary track and using aux sends (buses), helping minimize wasted CPU power.
Buses and auxes are also critical for more complex musical processes. The concept of “side chaining” and “key gating” occur through the use of buses, routing and sending the signal from one effect to another, creating interesting and stylistically helpful musical techniques. In a lot of electronic music that we hear today, much of the bass lines feature some kind of ducking (side chaining the bass line to the kick drum). These more advanced musical techniques are impossible without applying buses and auxiliary tracks in more creative ways.
So when you are mixing, and when you are coming up with new creative things to do in your music, look to the use of auxiliary tracks and buses to not only make your mixing life much easier and organized, but to also explore the vast array of interesting effects you can create by making use of them.