Audio: Buses Vs. Subgroups, Aux Sends/Returns, VCAs & Groups


Auxes, buses, sends, returns, VCAs, groups, subgroups, and many more terms make up the wide lexicon of audio technology. When learning about music and audio production, the glossary of terms can be overwhelming, and what's worse is that many terms are used interchangeably. Let's consider these related terms and their definitions to help you keep up with the learning process.

What are the differences between buses, groups, auxiliary sends/returns and VCAs in audio technology? Here are the key differences, in point form:

  • Bus: any kind of audio signal path that allows the combination or “summing” of different signals to be routed/processed together.
  • Subgroup: a grouping of instruments or tracks that are mixed or inserted into the “main” mix as a composite signal.
  • Auxiliary track: a type of bus for creating auxiliary mixes or common parallel processing.
  • Auxiliary send: the aux bus that feeds the auxiliary effects and processors.
  • Auxiliary return: the mixer channel fed by the auxiliary effects and processors' output.
  • VCA (voltage-controlled amplifier): a voltage control source that acts as a master control over the volume levels of the channels in the VCA group. VCAs do not pass audio.
  • Group: the linking of certain user-defined functions, notably level, between tracks (other options include editing, automation, alternative/playlist select, panning, solo, mute, and record arm).

These simple definitions will give you a solid idea of what these terms mean. However, because some words are often used interchangeably and may be used differently between DAWs and hardware mixing consoles, we should dive deeper into their full definitions. That's exactly what we'll do in this article.


A Primer On Signal Flow

To understand buses, subgroups, auxiliary tracks, sends, returns, VCAs and groups, we should understand signal flow.

As the name would suggest, signal flow refers to the flow of an audio signal, whether we're going live, recording or mixing.

Where does the audio signal originate, and where does it end? Where does the signal go between its start and end points? What inputs and outputs does the signal goes through, and how is the signal carried between each output and input? There are the questions of signal flow.

In analog systems, we actually have electric current flowing through electronic devices. In digital audio, the same idea applies, except it's based in a digital representation of audio.

Let's consider the signal flow of an electric guitar being recorded into a mixing console as a related example:

  • The guitar string vibrates (sound, not audio).
  • The pickup(s) convert the vibrating magnetic string into an electric current (audio signal).
  • The audio signal passes through any volume, EQ, etc., potentiometers on the electric guitar.
  • The audio leaves the guitar output and travels through the patch cable to the amplifier input.
  • Upon arriving at the amplifier input, the signal is amplified through tubes, transistors, transformers, digital modelling, etc.
  • From the amplifier output, the signal travels to the cabinet speaker input.
  • The speaker produces sound to recreate the audio (sound, not audio).
  • A microphone picks up the sound from the cabinet.
  • The microphone outputs the signal through an XLR cable to a mic input at the mixer.
  • The mixer applies gain at the mix input, and the signal can be processed by any processes on the designated channel (EQ, compression).
  • The signal is sent through the mixer (the topic of this article), eventually to the mix bus and the output of the mixer.
  • The signal is sent to the input of a power amplifier, amplified and outputted through speaker cables to the inputs of a pair of studio monitors, which produce sound representing the audio.

Note that sound is technically not part of signal flow. I just wanted to give a complex example to give you an idea.

Related article: What Is The Difference Between Sound And Audio?

And that's only touching the surface. Plenty of signal flow and routing can be done inside the mixer (or the digital audio workstation, for that matter). Signals can be combined, split, recombined and resplit in a multitude of ways, which is what we'll be discussing shortly.

In this article, we'll examine several of the most popular routing options for signal flow within mixers, notably buses, subgroups, auxiliary tracks (including sends and returns), VCAs and groups.

To set ourselves up for success, let's quickly go over the start and end points of a typical mixer.

The audio signals in a mixer generally come in on a channel. A channel is designed to accept microphones, line-level devices such as amplifiers, preamps, signal processors (EQs, compressors, etc.), and/or direct instrument-level signals.

The more channels a mixer has, the more audio signal can be connected and routed through it. This is true of recorded signals, live signals, auxiliary “return” channels, subgroup mix channels, and more.

In terms of semantics, channels are often called “tracks” in mixing.

The output of each channel is typically automatically routed to the main stereo output of the mixer (often referred to as the mix bus). The bus, which sums signals together, combines all the channels together to output them properly in the main outputs.

Each channel typically also has a few auxiliary sends, which effectively split the audio to send it to both the channel output and to a separate auxiliary track channel. The auxiliary track/channel, like the other channels, typically has its output routed to the mix bus/main stereo output.

Some mixers, and nearly all digital audio workstations, will have several bus options for the channel outputs, which we can use to group similar channels together into submixes for more streamlined mixing. It's generally easier to process many tracks' audio at once through a subgroup than processing each track/channel individually.

A VCA (voltage-controlled amplifier) isn't a channel and doesn't actually affect the routing within a mixer. Rather, it can be used to control the gain of multiple channels at once on a single fader. No audio passes through the VCA as it's not a channel but simply a fader that uses a control voltage (or a digital representation of a control voltage) to alter the gain of multiple tracks at once.

The term “group” in modern digital audio workstations has little to do with routing. It generally refers to linking parameters between multiple channels for easier mixing and editing. For example, we can group all the guitar tracks together and set up the group so that moving one fader moves all the faders, editing the audio at a certain playhead position edits all the audio at that playhead position, sending one track to a certain auxiliary track sends all the tracks to that auxiliary track, and so on.

Otherwise, the term “group” is synonymous with “subgroup”.

The terminology can be different depending on the mixer or the DAW. Although I'll do my best to generalize throughout this article, your specific equipment and software may put slightly different labels on what we're about to discuss.

So with that long primer, let's get into buses, subgroups, aux tracks/sends/returns, VCAs and groups to better understand their roles in mixing, recording and audio production more broadly.


What Is A Bus?

A bus, in audio, is a signal path that can carry audio from multiple sources from one place to another. Buses, themselves, are not channels. Rather they are signal paths than can connect channels. A track/channel output or its auxiliary send can be set to a specific bus (Bus 1, for example), and a subgroup or auxiliary track's input can be that same bus.

Most mixers and DAWs automatically bus everything to the mix bus, which goes to the output of the mixer or DAW. However, we can use buses to route signals elsewhere in the mixer, like to subgroups and auxiliary tracks, before routing those channels ultimately to the mix bus.

Mixers with more buses have more routing capabilities than those with fewer.

Buses are not audio tracks or MIDI tracks that contain audio clips or MIDI information. Rather they route audio within the mixer, console or DAW.

In analog devices, a bus refers to a physical point where multiple signals are summed together. In digital devices, this is done virtually through digital means.

Busses are general-purpose and are, therefore, often vague in their description.

Much confusion about buses comes from how we discuss them in practice. For example, we'll often call our subgroups by their name followed by the word bus, i.e., drum bus, guitar bus, vocal bus, etc. When doing so, we often reference the subgroup channel on the mixer or DAW rather than the audio conduit that carries the audio signals themselves.

So if we want to be technically correct, it's better to call these “buses” subgroups.

For more info on using bussing in your mixes, check out my video below:

To learn more about buses, check out my article What Are Audio Buses? (Mixing, Recording, Live Sound).


What Is A Subgroup?

Subgroups are groups of [typically] similar tracks/channels summed together. These tracks are bussed together on the same bus, and the subgroup is the channel with that bus as the input.

Subgroups are not audio tracks, meaning we can't record audio to them. Rather, they're set up to receive the outputs of other tracks in the mix via a common bus.

Subgroups are sometimes referred to, often confusingly, as groups, bus groups, submixes, submasters, or even as “buses”, depending on the gear or software.

Subgroups are fantastic for simplifying a mix, especially when we're dealing with limited resources (outboard gear and/or CPU). We can streamline the mixing process by having a dedicated channel and fader for a group of tracks.

There's a philosophy known as top-down mixing that starts processing the mix bus before moving onto the subgroups and finally to the individual track (if any more processing is even warranted by then).

I have a video on top-down mixing worth checking out if you're interested in learning more:

As an aside, submixes can be defined slightly differently than subgroups. While subgroups are generally designed into the mixer itself, submixes generally utilize another, smaller mixer to mix a group of tracks into a composite signal (mono or stereo) and send that signal to the main mixer. The difference is subtle, and in modern DAWs and digital mixers, the two terms are often interchangeable.

Not only can we process all the similar tracks together in the subgroup channel, but we can simplify our auxiliary sends by sending from the subgroup rather than from each track.

So if we need specific control over individual tracks, we can do so at the track level. However, it's much more convenient to work with subgroups in most cases!

For more information on subgroups, check out my article What Are Subgroups? (Audio Mixing, Recording, Live Sound).


What Are Auxiliary Tracks, Sends And Returns?

An auxiliary channel or “aux track” is designed as a flexible routing option/bus destination to take “sends” or outputs from the individual tracks and subgroups of the mixer.

The subgroup is one use of an auxiliary track. Another common use is the “aux send”.

Auxiliary tracks are not audio tracks (they do not record audio) but rather take in audio from other tracks within the mixer. When not used for subgroups, aux tracks will not be fed by the outputs of the tracks/subgroups. Instead, each track/channel will have some number of “sends” to effectively feed the channel's audio to the specified auxiliary track without affecting its output.

As a matter of semantics, some would argue that auxiliary tracks are different than subgroups. A subgroup is effectively set up on an “auxiliary track”, though not all auxiliary tracks are set up as subgroups.

With the flexibility of modern digital audio workstations, the terms and definitions often overlap. In analog hardware mixers, which are much more limited, these distinctions hold more weight.

For example, a mixer could have four auxiliary sends. These sends are effectively buses that can combine signals from any of the tracks. Each send will route the signal from the track to its appropriate auxiliary channel. We can then do as we wish with each of the four auxiliary channels, all without affecting the output of the channels themselves (which can be routed to subgroups or straight to the mix bus).

Auxiliary tracks are the aux tracks/channels, complete with their own fader, inserts, sends, etc. They are incredibly flexible in their usage and are an important part of any mixer.

Let's break down aux sends versus aux returns.

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.

For more information on inserts, check out my article Audio: What Are Inserts? (Mixing, Recording & More).

The aux return is an auxiliary track that is outputted back into the mix (it's “returned” to the mix), often directly to the mix bus. These returns are used for many applications and are super flexible.

Let's consider a few ways auxiliary tracks are used in mixing.

Auxiliary tracks are often used for sending different mixes to foldback or in-ear monitors so that different musicians can have more or less of certain tracks in their monitor feed. We can effectively have different mixes by dialling in the send pots rather than the faders (which ultimately send to the mix bus).

Note that when we're creating auxiliary mixes like this, the outputs of the auxiliary channels need to be separate from the main outputs. We output from the mixer/console or the audio interface connected to our computer/DAW to send the aux mixes where they need to go.

When it comes to mixing the main mix, we can use auxiliary return channels for return effects and parallel processing.

Multiple tracks or subgroups can be sent to the same aux return, and that aux track can be processed independently of the original channels. Remember that each channel will have its own send control to adjust the level being sent to the aux bus.

Effects return channels, as they're often called, are popular for time-based effects like reverb and delay. Sending individual channels to a dedicated reverb or delay effects return channels gives us tremendous control over the space in the mix.

I discuss how to use effects sends/returns and the benefits of doing so in the following YouTube video:

We can glue tracks together with common time-based processing without having to insert these effects on each channel. We also have independent fader control over the time-based effect, meaning we can mix it perfectly, independent of the original tracks, and automate it to our liking. It also means we can process the effects return (with EQ, compression, saturation, etc.) to help the time-based effect sit perfectly in the mix. These are controls that are extremely difficult to achieve when inserting effects directly on audio tracks.

Effects return channels are also used for parallel processing, which is effectively the same as setting up independent time-based effects, routing-wise.

Parallel processing means that the aux bus output is routed to the mix bus as well, allowing the channels being sent to the aux bus to also be outputted to the same mix bus. This way, we are able to mix the processed audio from the aux with the tracks that feed the aux. In other words, we can process and mix the signals in parallel.

Common parallel processing techniques include parallel compression, saturation and aural exciting. In the same way that we set up the time-based effects sends, we can send multiple tracks or subgroups to an independent channel and process that channel with compression, saturation, saturation/high-pass filter, or whatever else will enhance the mix.

When mixing in parallel, we maintain our “dry” original channels and mix in completely “wet” processed channels (the aux returns). We can be aggressive with the parallel processing to add weight, colour and interest to the original tracks.

We can also insert multiple processes, including EQ, into the parallel channels to have them fit perfectly in the mix.

I have a video dedicated to parallel processing. Check it out here:

To learn more about auxiliary tracks, check out my article Mixing/Recording: What Are Auxiliary Tracks, Sends & Returns?

Let's now move on to VCAs.


What Is A VCA?

VCA stands for voltage-controlled amplifier, an electronic amplifier that varies its gain depending on a control voltage.

In mixing, a VCA or VCA group is an independent fader that we can use to control the signal levels of multiple channels simultaneously without altering the channels' faders and without having to route the channels to their own subgroup.

VCAs do not pass any audio signals and do not give us any channel controls (inserts, panning, sends, etc.). They are a voltage-controlled source that controls the volume level of the channels in the VCA group.

Therefore, the channels of the VCA group will still be routed to their respective outputs. Additionally, the relative balance between the channels in the VCA group will remain intact regardless of the VCA fader position.

VCA groups are yet another way to simplify the mixing process. We can choose to automate the VCA for even more control over the mix.

VCA groups will have a fader and will sometimes offer mute and solo options, depending on the mixer/DAW.


What About Groups?

As we've already established, things can get a bit confusing in terms of semantics. The term “group” could be synonymous with “subgroup”. However, in modern digital audio workstations, it generally means something different.

In terms of signal flow, modern DAW/digital mixer groups do not have anything to do with routing. Rather, they link specific parameters and controls between the tracks/subgroups of the group.

For example, if we have several tracks grouped together, we can set a multitude of options so that altering one track will produce the same relative results in the other track. Linked control options include:

  • Editing: cuts, crossfades, nudges/moves, clip gain adjustments, etc., made on one grouped audio track will be made on all other grouped audio tracks.
  • Input: changing the input of one track will change the inputs of all other grouped tracks to match.
  • Record arm: arming one grouped track to record will arm all other grouped tracks to record.
  • Inserts: inserting a processor on one grouped track will insert that same processor in the same insert slot on all other grouped tracks.
  • Auxiliary send: setting up an aux send on one grouped track will set up that same send on all other grouped tracks.
  • Send levels: adjusting the send level on one grouped track will cause relative adjustments on all other grouped tracks.
  • Panning: adjusting the pan position on one grouped track will cause a relative pan position adjustment on all other grouped tracks.
  • Fader position: adjusting the fader position on one grouped track will cause a relative fader repositioning on all other grouped tracks.
  • Output: changing the output of one grouped track will change the output of all other grouped tracks to match.
  • Solo: soloing one grouped track will solo all other grouped tracks automatically.
  • Mute: muting one grouped track will mute all other grouped tracks automatically.

Changing one parameter of one channel in a group changes that parameter in all channels of the group (whatever parameters you choose to link). Groups are great for editing and getting bulk mixing moves done quickly.

For more information on DAW groups, check out my article Audio: What Are Groups? (Mixing & Editing In DAWs).


This article has been approved in accordance with the My New Microphone Editorial Policy.

Arthur

Arthur is the owner of Fox Media Tech and author of My New Microphone. He's an audio engineer by trade and works on contract in his home country of Canada. When not blogging on MNM, he's likely hiking outdoors and blogging at Hikers' Movement (hikersmovement.com) or composing music for media. Check out his Pond5 and AudioJungle accounts.

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