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Mix Bus 101: Why, When, and How to Group Tracks into a Bus

by Daniel Dixon, iZotope Contributor August 21, 2019
What's a mix bus? Learn here!

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We've got your guide to everything mix bus-related. Learn how they work, the difference between a group, aux bus, or master bus, when you should route your tracks to a group bus, and how to do it the right way.

What is a mix bus?

Every single mixing board, whether physical or digital in your DAW, performs exactly the same task—it takes multiple signals from different channels, combines them together, and routes that blend of tracks to an output of your choice. A mix bus (also spelled mixbus) is where all those combined tracks are routed and merged together so you can take collective action on them. 

What is a master bus?

The most popular mix bus is the basic main stereo mix bus (also called the "master bus"). You'll find it on every single mixing console (usually as the last channel on the right side). To put it in simple terms, the mixing board takes all of the different instrument channels and combines them into a single pair of Left and Right channels. Those channels are then output to your Left and Right speakers or headphones.

The main stereo mix bus acts as your master control center, where you can make final adjustments before the audio leaves your board (or your DAW) and hits the speakers. At the very least, the main stereo mix bus will allow you to control the volume of all the combined tracks (useful if you want to create a fade-out or make everything louder/softer). Most mixers will also let you add different insert effects (like iZotope's mix assistive plug-in Neutron 3) to the main stereo mix bus, to make your songs sound incredible.

The main stereo mix bus isn't the only mix bus you can create in your audio flow, however. There are two other types of mix buses that can significantly help in your mixing process: subgroups and aux tracks.

What is a subgroup versus a bus?

Here's one of the simplest ways to explain subgroups and busses: Imagine you're a teacher in charge of 60 kids. For a field trip, you hand each child a T-shirt with a specific color, which puts them in color groups (green, red, and blue). The principal gave you a list, which specifies the T-shirt color that every student is supposed to receive. 

From here, there are options. You can read down the list, check the T-shirt color, grab that color from the appropriate bin, and hand it to the student. So, "Jimmy, blue," get a blue shirt, find Jimmy, and give him his T-shirt. "Alissa, green," get a green shirt, find Alissa, and give her her T-shirt, and so on with 60 students. Needless to say, that's a lot of work and would take a lot of time.

Thankfully there's an easier way. You could group the kids by T-shirt color and have them board separate buses. Meaning, you could read off all the names of the kids that need green T-shirts and have them all get on the same bus (we'll call it the green bus). Then have all the "blue" kids board the second bus and the "red" kids the third. Then, all you would have to do is get on each bus with the appropriate bin of colored T-shirts and hand them out to all the kids on board.

A subgroup mix bus is just that: a group of tracks controlled as a single group. Imagine you want to add distortion to 10 background vocal tracks. You could do this track-by-track, or route all the background vocal channels to a bus. 

In doing so, you mix down all 10 tracks into one stereo channel (let's name it “BG Vocals”). This bus would, in turn, output to the “Master Bus.” Add your distortion plug-in to the “BG Vocals” bus, and bam.

Take a look at the diagram below and notice how I created a subgroup mix bus for drums (often times called a Drum bus). Each of the individual drums channels (orange-colored tracks) has been routed to output to Bus 10. In turn, the Drum bus channel has been set to receive input on Bus 10. All those individual drum channels mix down to a single stereo track for easier mixing.

Creating a drum bus

Pro Tip: the term “Bus” can refer to any type of mix bus, but most often is used to describe a subgroup mix bus. In Logic Pro X, Subgroups are called “Stacks.” ProTools calls any kind of bus “Aux,” regardless of its function.

 

Aux channels and the importance of routing

Auxiliary channels (generally referred to as “aux channels”) are similar to group buses, as they can receive input from multiple sources. Just like group buses, aux channels usually output to the master bus. Where they differ, however, is in their routing. Aux channels send a copy of the signal, rather than sending the complete signal like group buses do. Sending a copy allows you to maintain the original signal untouched, alongside your processed track, giving you independent control between the two signals.

Take this example: you’ve got a guitar track that you want to add reverb to. You could insert the reverb plug-in directly on the guitar track. It gets the job done. However, this method poses a few problems. First, if you want to change the amount of reverb versus dry signal, you have to open the plug-in each time and readjust it inside the plug-in. Additionally, this method is also problematic if you want to apply the same reverb effect to multiple instruments. In theory, you could set up a sub-group bus to apply the effect to multiple tracks together. But it doesn’t solve the problem of using different amounts of reverb on different tracks.

This is where send-based aux buses come in handy. Rather than inserting the reverb effect directly on the instrument track, create a new bus (we’ll call it “Reverb”) and insert the reverb effect on this bus. Then route the “Send” of your guitar track to the “Reverb” bus. This will send a copy of the guitar output to the reverb bus, still sending the original signal wherever it was going (in my case the Master Bus). That way you end up with two signals: one original, dry guitar signal and a copy of the guitar signal affected by the reverb plug-in. This means you’re able to have separate faders giving you independent control over the dry (original) and wet (reverberated) signals.

An example of routing from AUX channels

If you want to apply the same reverb effect to multiple tracks, just send copies of their signals to the already existing reverb bus! What’s even better is that almost all DAWs and boards allow you to decide how much of the original signal you want to send to a particular bus. Meaning, you can get more reverb on the guitar and less on the hi-hat by adjusting how much you send of each signal.

For a more detailed review of sends/returns and the difference between pre-fader and post-fader sends, check out the Get Sent section of Phillip Nichols’ great Understanding Audio Signal Flow in a DAW article.

Conclusion

Whether you just want to organize your tracks in sub-groups or set up complex effects audio flows with auxiliary buses, mix buses can be a huge timesaver and simplify your work. And, as always, less work for you means more time staying creative! So get the best of both worlds: stay organized and creative.

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