What is a 2-bus in Mixing and what is it used for?

What is a 2-bus in Mixing and what is it used for?

Those of us who have been around for a while have heard the expression 2-bus before, but for the new guys and gals, the term 2-bus doesn’t make much sense. So, here to shed a little light on this fundamentally important topic is a rundown of the what, where, how and why of the 2-bus.

What is a 2-bus in mixing? The 2-bus, also known as the Master Bus (plus a few other names), is the stereo channel where all individual mix tracks finish up. So, on a mix, a mono vocal track, stereo drum overheads, DI Bass, live Bass, and everything else, are all routed to the 2-bus. The 2-bus is the link to the real world of two-channel, home hi-fi, earbuds, car audio and other listening environments.

The term 2-bus is commonplace online and in books, but it is also widely known using other terminology.

A 2-bus by any other name:

  • Master Bus
  • Stereo Mix Bus
  • Master Track
  • Master Channel
  • Mix Bus
  • Main Stereo Bus
  • Stereo Bus
  • Two Track Mix Bus

The 2-bus processing originated from the two channels that are output from an analogue mixing desk. The stereo output often went through a compressor and EQ stereo processing unit to ‘glue’ the track together. The final mix processing step provides essential fine-tuning before being passed on to the Mastering Engineer.

In more recent times, the 2-bus concept has found itself at home in the digital world. Although Digital Audio Workstations (DAWs) allow an unlimited number of buses to be created within a project, workflow still tends to mimic the analogue world. In reality, this is very logical, as music today is consumed mostly in stereo.

The term 'Bus' in modern digital mixing is borrowed from the analogue world. A bus is the electrical industry's term for wires or conductors that carry voltage along similar paths.

The beauty of the 2-bus is that it focuses on final mix processing, allowing all the tracks to be treated simultaneously. When mixed skillfully, the final stereo track sounds ‘together’ and ‘like a real record’. The tracks are no longer a set of separately recorded instruments, but a coherent song. The coherence is often referred to as ‘glueing’. We will talk about that more later.

Read on to learn more about the 2-bus. In this article we will look at:
  • Mixing with the 2-bus
  • How the 2-bus feeds Stereo Processors
  • How tracks are glued together
  • What is a typical 2-bus Mixing Chain
  • 2-bus Limiting

Mixing with the 2-bus

The 2-bus is probably best explained using a diagram. See Figure 1 below.

The 2-bus runs top to bottom in the diagram. The 2-bus is shown with a left channel and right channel picking up all the recorded instruments and delivering them to the stereo processing. To simplify the explanation, we are assuming panning is L-R-C.

In the diagram, the left drum overhead is routed to the left channel of the 2-bus. The right drum overhead to the right channel. The kick, and snare, are routed to both the left and right channels (essentially putting them in the middle of the stereo field).

Figure 1: Explanation of how to mix with the 2-bus

The other instruments Bass DI, close-mic Bass, Guitar A, Guitar B and Main Vocals, are also all routed to both the left and right channel of the 2-bus. Backing Vocals A are routed to the left. Backing Vocals B are routed to the right.

That's it! That’s the concept of the 2-bus. The 2-bus is picking up all the individual recorded instruments and delivering them to the stereo processing units.

Following delivery by the 2-bus, stereo processing is applied to all the instruments. A stereo file is bounced that can be passed to a Mastering Engineer who will finish off the processing and prepare the final audio ready for the ‘real-world’.

Note: Figure 1 show the 2-bus runing top to botom, but just remember that on a typical DAW setup the 2-bus runs left to right.

Glueing It Altogether

Can you see on from Figure 1 that the stereo processing will be applied to all of the individual tracks? The drums, bass guitar, guitar, and vocals added together and processed. First, compression is added, then harmonic distortion. Next EQ is applied, and finally, the whole mix is given some stereo width. Hopefully, you can recognise the power of 2-bus processing.

This is an essential step in mixing as the processing:

  1. Takes quiet parts of the mix and makes them louder using compression.
  2. Takes louder parts of the mix and makes them quieter using compression.
  3. Adds grit and definition to the finished audio using harmonic distortion.
  4. Achieves tonal balance of the complete mix by adding and subtracting EQ.
  5. Enhances the stereo width of the completed song.

Mixing directly into the 2-bus provides a shortcut (I don't like that word) for amateur mixers. It takes a mix from zero to eighty with a 'template' setup. Of course, things need to be tweaked each time for individual songs but this a great way to turn mono recordings into a great sounding first-round stereo mix. 'Glueing', in mixing terms, is another way to describe 'combining' or 'blending'.

All of the processes, in their small way, add to the total cohesion of the finished audio through the 2-bus. We’ll talk more about these important processes a little more, in the next section, when we look at typical 2-bus mixing chains.

What is a typical 2-bus Mixing Chain?

In another article, What Is The Best Mix Bus Chain for Mixing Music?, I discuss 2-bus processing in some depth, but for this discussion, I’ll just give the highlights.

The 2-bus will typically include the following four processes:

1. Mix Bus Compression

Excellent results can be obtained by compressing the 2-bus by about 3dB for ‘Glueing’ effects or about 7dB for ‘Pumping’ effects. Use a highpass sidechain filter to get more control over the low end.

Well-know 2-bus compressors include:
  • SSL-G Series
  • Cytomic 'The Glue'
  • API 2500
  • Fairchild 670
  • Dangerous Music 16x2 and 2-BUS+
  • UA 2-1176

2. Harmonic Distortion

Digitally recorded audio often lacks a little ‘je ne sais quoi’, and Harmonic Distortion adds a significant amount of clarity and character to the whole sound. Just don’t over-cook it!

3. Mid-Side and Stereo EQ

EQ processing on the 2-bus is done in STEREO. 2-bus EQ refines the tone of the music and can help the width and spaciousness of a mix. EQ is applied sparingly and with broad EQ curves on the 2-bus.

4. Stereo Width

I like to add about 20% to each side of my mix. There are a few ways to accomplish widening and plenty of decent plugins to help. At the time of writing, Ozone’s ‘Imager’ is free to download and use in your projects.

The order in which the four processes are sequenced changes from engineer to engineer, however if you are new to the 2-bus, just stick to this order for now. With more experience you can tweak the order and the settings.

2-Bus Limiting

Should you mix into a limiter?

If you’re asking this question, then the advice should be ‘No, do not mix into a limiter’. A preferable method when starting out is to:

  1. Make sure you are recording and mixing with enough headroom (use the K-20 system).
  2. Use the 2-bus compressor to add a bit of glue (about 3dB should do it).
  3. Check that your final mix does not peak anywhere.
  4. Leave the limiting to the mastering phase. If you are going to master your own tracks, carry out the mastering in a different DAW session. If not, leave the limiting to the mastering engineer.

Whatever you do DO NOT mix into a limiter when mixing and then remove it when you bounce the track down and pass it on to the mastering engineer - you will undoubtedly not get the results you desire.

About Me

Hello! I’m Tim Williams.

Back in 2009, I bought myself a copy of Pro Tools and recorded some home made music. It was challenging to start with, as I had no idea what I was doing. I made many mistakes on my journey - some fun, some expensive, and many time-consuming! I find running a Home Music Studio a fascinating and rewarding hobby and still enjoy it every day. This website is where I’d like to share everything that I’ve learned.

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