Auxiliary tracks are invaluable tools in mixing, recording, live sound, and any other job that utilizes an audio mixer. If you're at all confused about aux tracks, sends and returns, or you'd like more information to help you make the most of your aux tracks, you've come to the right place!
What is an auxiliary track? An aux track is a track/channel with a specified bus input that takes in audio from other channels or from external sources. Multiple tracks can have their signals routed to an aux channel via pre or post-fader sends or via their outputs. Each aux track output can then be routed elsewhere.
What are auxiliary sends and returns? An aux send is a bus path that can send audio from a channel independently of that channel's output, either pre or post-fader. This bus feeds an auxiliary track. If that auxiliary track's output is routed back into the mix (rather than to separate mixer outputs), it is considered a “return” channel.
That's about as concise as I can define these mixer tools. If you didn't catch that or want to learn more of the intricacies, I'll be elaborating on auxiliary tracks, sends and returns in this article.
A Short Primer On Signal Flow
Auxiliary tracks, sends, and returns are invaluable options for advanced routing in our mixers. To fully understand routing, we should first develop an understanding of signal flow.
As the name would suggest, signal flow is the study of how an audio signal “flows” through its signal path. Where does the signal originate, where is it ultimately going, how is it routed along the way, and what processes does it pass through?
In typical mixer situations, the audio tracks will be fed by either live audio (live mics, instruments, playback devices, etc.) or recorded audio (the audio embedded in physical or virtual media). The outputs of these tracks will normally be routed, by default, to the mixer's main outputs via the mix bus (or whatever synonym the mixer uses).
Audio signals in these tracks can be routed through inserts, via auxiliary sends (the topic of this article), and through their outputs.
Inserts are slots on the channel where processors (EQ, compressors, etc.) can be inserted in-line. If we inserted an EQ in insert 1 and a compressor in insert 2, the audio would come into the channel, flow out of the channel to the EQ (first processor), back to the channels, back out of the channel to the compressor (second processor), and then back into the channel for further routing.
For more information on inserts, check out my article Audio: What Are Inserts? (Mixing, Recording & More).
Depending on the mixer, we could also have built-in processors (notably EQ and sometimes compression) that the signal will pass through along its signal path.
The auxiliary sends typically come after the inserts and can effectively split the signal to route it to other channels (aux tracks) without altering the output of the channel.
And finally, we have the output, which routes the channel to where it needs to go, typically either directly to the main outputs via the mix bus or to a subgroup via another bus.
For more info on using bussing in your mixes, check out my YouTube video here.
To learn more about buses, check out my article What Are Audio Buses? (Mixing, Recording, Live Sound).
With that short primer out of the way, let's dive deeper into auxiliary tracks, specifically.
What Are Auxiliary Tracks?
Auxiliary tracks are versatile in their usage in mixers, making it difficult to give a succinct definition.
Auxiliary is defined as providing supplementary or additional help and support, which is fitting for these aux tracks.
Auxiliary tracks can be set up as subgroups, alternative mixes, or for parallel processing or effects returns.
With the flexibility of modern digital audio workstations, the versatility of aux tracks has increased, and indeed, the terms and definitions of the various routing options have become somewhat blurred. The terms auxiliary track and subgroup may be completely separate in analog hardware mixers, which are much more limited. I should also mention that the labels of these routing options often change depending on the brand/manufacturer of the mixer/DAW.
Versatility aside, auxiliary tracks are tracks with specific bus inputs. The signals on the specific bus could be from different sources.
In the case of subgroups, which can be argued as being distinct from aux tracks, the bus is generally fed by the outputs of similar tracks. For example, all the drum tracks could be outputted to the drum bus; all the guitars could be outputted to the guitar bus. In this example, we could have a subgroup or “aux group” for the drums and the guitars.
For more information on subgroups, check out my article What Are Subgroups? (Audio Mixing, Recording, Live Sound).
The more common use of the term “aux track” is when the aux track is fed by the auxiliary sends of the mixer channel. If available, each channel will have some number of “sends”, which are buses that will feed their respective auxiliary channels.
Aux tracks can also be fed by external sources via aux inputs if need be. Again, they're versatile.
These sends are independent levels controls that will send audio signal from the channel separately from the channel output. The amount of level sent from the channel to the aux track can be adjusted (typically by a knob control). It can be sent pre-fader (independent of the channel fader position) or post-fader (dependent on the channel fader position). More on that later.
The aux track will have inserts, its own sends (in advanced mixers), and its own fader and panning controls. We can process the audio passing through the aux track as we see fit, which is extremely powerful in mixing, recording and live sound.
The auxiliary track itself can then be routed where it needs to go. We can output the aux track back into the main mix or to separate mixer outputs (other than the main mix). We can output it to the mix bus, to a subgroup, and can even send it to other auxiliary tracks in more advanced mixer setups.
So to recap, the auxiliary track is a dedicated mixer channel with its own bus input. It doesn't record audio but can pass the audio through for additional routing. Aux tracks can be routed outside the main mix or kept within the mix.
If you'd like an in-depth discussion on routing, check out my detailed video below:
Aux Track Inputs (Sends)
The aux send is the bus that allows us to mix varying levels of each channel and send them to the auxiliary track. These sends are typically potentiometers (real or virtual) so that we can send different amounts of each channel (or nothing from any channel) to the aux track in question.
These sends can often be configured to send the signal pre- or post-fader. Pre-fader means that the send pot will be the only control over the signal level sent to the aux track (at unity, it will send whatever signal level is in the channel—generally after any internal processing/inserts). Post-fader means that the fader position of the track will also affect the send level, along with the send pot position.
In modern digital audio workstations, we'll often have a lot of flexibility with the buses we choose for our aux sends. It may even be the case that the aux tracks themselves have their own sends, making advanced routing easy.
In analog boards, each channel may only have a few aux sends to feed the few aux tracks.
Aux Track Outputs (Returns & “Non-Returns”)
Moving on to auxiliary track returns, let's begin with the situation where the aux track is not “returned”.
The term return, in this case, means that the auxiliary track output is returned back into the mix.
Therefore, if the auxiliary track is used for alternative mixes and is outputted to mixer outputs other than the main outputs, it's not a return.
Auxiliary track returns are when the aux track is outputted back into the main mix, either directly to the mix bus or to another subgroup or aux track that's ultimately fed to the main mix.
Typical Uses Of Aux Tracks
Now that I've covered what auxiliary tracks, sends, and returns are, let's get into the typical uses of auxiliary tracks and how to set them up. Notably, we'll discuss the following:
Using Aux Tracks For Alternative Mixes
Using auxiliary tracks for alternative mixes is incredibly useful when tracking or mixing live sound. If needed, we could have a completely independent mix of levels for each available aux track/output.
To set this up, we need to ensure the aux tracks are routed to their proper outputs (other than the main mix/output). From there, we can adjust the sends of each track to have an appropriate mix in each aux track.
For example, if the drummer wants less guitar and vocals in his mix, we can send less of the vocals and guitars to his specific aux output. Meanwhile, if the vocalist wants more vocals and less bass, we can increase the vocal sends and decrease the bass sends in her aux track.
We would then route the drummer's auxiliary outputs (or whatever outputs the aux tracks are sent to) to the drummer's headphones or monitors and the vocalist's aux mix to the vocalist's headphones or monitors.
Using Aux Tracks For Subgroups
I already touched on this one since it's not the most common use of the term “aux track”. However, in many cases (especially in digital audio workstations), an “auxiliary track” will be used to host our subgroups.
Setting up subgroups is fairly simple. We find the tracks we want to group together and output set their output to the same bus. Depending on the mixer layout, the bus options and subgroup channels could be pre-determined. Otherwise, we'll likely have to set up an aux track with the subgroup's bus as its input.
To reiterate the example from earlier, we could have a drum subgroup with all the drum tracks routed to a dedicated aux track. We could also have all the vocals routed to their own subgroup.
Using subgroups can get us quick wins and make mixing more streamlined overall. Processing the main mix, followed by the subgroups, and moving on to the individual tracks is part of the top-down mixing approach.
I have a video discussing top-down and bottom-up mixing in more detail. Check it out here:
Using Aux Tracks For Effects Returns
Auxiliary tracks are often set up as effects returns.
As was discussed previously, returns are when the auxiliary track's output is “returned” to the main mix (rather than being outputted through the aux output or another mixer output).
The term “effects return” means that the audio sent to the aux track is being processed by effects before getting returned to the mix.
The most common effects on effects are the time-based effects, i.e., delay and reverb.
Setting up an effects return is pretty simple. Send an appropriate mix of tracks/subgroups to an auxiliary track, have the auxiliary track output sent, ultimately, to the mix bus, and insert an effect on the aux track.
Effects returns have several benefits over inserting effects directly on the tracks.
First, we can use fewer effects when we're processing several tracks' sends at once. This allows us to do more with hardware effects and lowers CPU load in digital systems. It also helps to glue the tracks together by processing them with the same effect.
I talk about the concept of glue in more detail in this video:
Second, we have complete, independent control over the effect signal (wet) and the original tracks (dry). We can dial in the perfect amount of delay, reverb or another effect relative to the original tracks without affecting the original signals whatsoever.
This not only means that we can easily adjust and automate the fader and panning of the effect return (aux) channel. It also means that we can add additional processes like EQ, compression and saturation to alter the sound of the effect without affecting the original signals either.
Furthermore, it makes things a lot cleaner when it comes to mixing the individual tracks as we won't have inserted time-based effects (or other effects) tied to the original tracks.
I discuss how to use effects sends/returns and the benefits of doing so in the following YouTube video:
Using Aux Tracks For Parallel Processing
Parallel processing is set up, using auxiliary tracks, in the same way as an effects return. The difference is mostly in terminology. While effects returns typically have “effects” inserted (delay, reverb, etc.), parallel channels typically have “processes” inserted (compression, saturation, etc.).
So then, setting up parallel processing means sending an appropriate mix of tracks/subgroups to an auxiliary track, having the auxiliary track output sent, ultimately, to the mix bus, and inserting a processor on the aux track.
When using parallel processing, we can get fairly aggressive with the processing and mix the parallel aux track underneath the original tracks. Parallel compression and saturation are common ways to thicken up and add character to tracks and subgroups, including vocals, drums, bass and more.
I have a video dedicated to explaining and showcasing parallel processing. Check it out here: