What is Mix Bus Compression?

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Written By Tanya

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Because of their complexity and confusion, there are few topics or techniques that amateur mastering engineers can avoid. Stereo bus compression and auxiliary buses compression are two of these topics.

Our goal today is to eliminate this mystery and allow everyone to enjoy this advanced mixing technique. Once you know what you aren’t supposed to do, it’s not as difficult!

Your compressor settings will allow you to get out of danger and your songs will have a greater sense of coherence.

Let’s also be very clear:

This method is not preferred by all professional mixers. Others prefer to leave all the work to the mastering engineer.

However, this is not the level of compression where adjustments can be made after the mix has been finalized. Mixing into a compressor is part of the process. It should be done lightly so that there are plenty of dynamics and headroom to allow the mastering engineer to fine tune.

Let’s now get to work, get this information out there, then get back to mixing…

What is Mix Bus Compression?

Mix bus compression refers to the mixing of master output through a compressor, but the term is also used to describe sub-mixing sets on an auxiliary bus, such as drums.

It’s used to create a sense o “glue and punch” in the mix, thereby creating cohesion among the tracks.

You can achieve this with either plugin compressors, or outboard, hard compressors. With modern plugins, neither one is disadvantageous. The choice of compressors is up to you.

It is important to remember that we are not printing the mixture and then compressing it. This should be left to the mastering phase of the process.

We apply compression to the mix bus before we do anything. This allows us to make sound choices and not disrupt our carefully balanced balance later. The setup is as simple as this: all tracks output to a mix bus where you apply the compressor, which then leads to the master out.

This effect provides the glue: Think about what happens when you compress an entire waveform consisting of one sound. Consider compressing a waveform made up of multiple combined sounds. They will all be affected together. This is their core difference.

Which Bus Compressor Mix Should We Use?

There are four types of compressors available, digital or analog.

  • VCA (Voltage Controlled Amplifier).
  • FET (Field-Effect Transistor)
  • Variable-Mu Tubes
  • Optical Compressors

The circuitry used to reduce gain is what makes them different. Each one may have access to different time-based controls, including attack, release and the ability to set the compression threshold or ratio.

Although there is much debate about which type of compression is best, I believe that it doesn’t matter if you use a transistor, vacuum tube or valve to accomplish the task. It doesn’t matter what your preference is for transparency or coloration, nor how “musical”, either option.

The compression effect is what we refer to as musical. You can even influence the compression effect in large part by choosing the settings.

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You can gain a better understanding of each one by using a plugin that allows for you to switch between the emulated circuit types with the same settings. This will allow you to listen to the changes. This ability is available in most default stock plugins, but there are at least four that you can bypass to make the comparison.

It is important to mention that the API, SSL and Neve plugins are just as good as their hardware counterparts, and even though they are more expensive, they are still considerably cheaper than comparable outboard options.

Tip: This is not negotiable. Two dual-mono compressors must be capable of stereo link so they can both respond equally to peak levels in each channel. You will constantly offset your center line of imaging.

Don’t make your listeners sea-sick. This route will allow you to disconnect the stereo link and track mono instruments, which will stretch your budget.

You could use any of our best compressions. But if we had to pick one, it would be the Empirical Labs Ditressor.

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The Distressor is timeless and trendy. Although the one shown above is mono, you can get a stereo pair for half the price of two mono’s.

Side-chain detectors are not required (as discussed below), as these beasts have high-pass filters and mid-range detector options.

Which Stereo Bus Compressor Settings are Best?

Although most people already know the answer, I am answering this question.

There are no “set-and-forget” settings. Each song will require a different set of parameters.

There are some features that you’ll appreciate that will make your life easier and eliminate the need to compress too many buses. Let’s take a look at these and then, reluctantly, I’ll give newcomers a place to start.

AUTO-RELEASE

We are no longer referring to individual instruments (and therefore predictable), so it is advantageous to accept the assistance of computers and look ahead algorithms.

A few compressors have an auto-release feature that allows you to set the time it should hold on to compression before releasing and returning to full amplitude.

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All tracks will be received by the mix bus, so there won’t be any set dynamics. It will be very variable. The auto-release can be used to help you attain the “musicality” that we have spoken of.

SIDE-CHAIN DETECTION

While a sidechain option is standard on plugins, it’s not a feature that all hardware compressors have. It allows you to force the compressor’s response to a different track while still acting on the original.

Consider a modern rap song that has a sub-bass kick as both the kick and bass. This will create a pumping sensation when the kick fires off.

Side-chain detection allows you to run a copy through an equalizer on the bus to EQ low frequencies out, or even a duplicate of the mix with no kick. Now you can bypass pumping and apply sub-mix compression without ignoring the kick.

MULTI-BAND COMPRESSION

Parametric equalization has made this concept more familiar to most of us. The compressor can do the same thing. The frequency spectrum can be divided into bands of two or three (up to eight usually). This means that you could compress the 2k-5k range of frequencies by yourself.

I believe that mastering engineers should handle this. If you are having problems with multi-band work, it is best to focus on the individual tracks and not on the mix bus.

AUTOTHER CONSIDERATIONS

Keep in mind the main purpose of compression

  1. To achieve consistency within the variances in amplitudes
  2. You can personalize each musical event according to your needs

The second purpose is often overlooked by amateurs. They will usually attack as quickly as possible, and then release.

Sometimes we just want the initial transient hits to be there, so we choose a slower release. We might want to achieve a Pat Metheny-like sound on the guitar, so we choose a slower release. Maybe we are dealing with rap vocals, and need fast settings for both.

These options should be considered when setting up stereo bus compression. Pay attention to the individual instruments and how they are affecting the overall mix.

Mix with your ears and not your eyes!

If you don’t pay attention, it’s easy for the music to become dull. This is directly related to the song’s tempo and genre. A faster tempo will require a quicker attack and release.

Your release should not take longer than the time it takes for your next kick or big guitar strum to come through. If you want your snares popping, you will need a slower approach.

A DECENT STARTING POINT

This is not a place to squash anything, but glue it all together. A low ratio with medium attack and release is a good place to start. Slowly lower your threshold until you can see anywhere from one to three decibels compression.

  • Ratio 1.3 to 1 up to 2
  • Attack: 50 milliseconds
  • Release 150 milliseconds
  • Threshold 1 to 2. dB gain reduction

This is a guideline, not a template. Each song is unique, so each one will need to be adjusted accordingly.

Pro-Tip The make-up gain is used to match subjective levels of the mix, not for headroom and staying within the converters’ sweet spot. This will let you bypass the compressor and allow you to easily compare the effect. After you are done, you can dial in the make-up gain again.

Let me add a few words of caution to make sure you don’t destroy your mix.

Do not mix with Master Bus Compression Settings

Mix bus compression is often fraught with problems. They set too high a ratio or too low a threshold. This is how it should be thought about…

Most likely, you are already compressing each track separately. Others might push them through an additional bus compressor. Finally, they hit the stereo bus compressor. There might be even a limiter!

Problem is that faders can become sloppy. They stop doing the things you expect.

Let’s take, for example, if you want to increase the volume on a synthesizer by itself, you will push the fader up. However, your compressor is still post fader so you can also hit the mix bus. It doesn’t matter how loud you crank it up, it won’t increase in volume. The sound gets worse.

The mix bus compressor is compressing the rest of the mix harder, so you don’t get a louder synth. Instead, you get an objectively quieter synth that is subjectively louder because of the root-mean square amplitude. This means you are getting less volume from everything else.

It was difficult to read and type that, and even more confusing to mix that way. We recommend that you compress your stereo bus very gently!

You should also mention that you shouldn’t need or use limiters if the intention is to send the mixes off to mastering engineers. The mix shouldn’t have too much make-up.

It is not necessary to try to be perfect, but just semi-perfect. You don’t have to pay for a rescue mission, but headroom or dynamics to use.

A Bus Compressor with Care is a good choice

There isn’t much else that can be said. You can’t put into words what you don’t hear.

You can start with the settings below and adjust them according to your song’s tempo, genre and dynamic material. Then, as you mix it into it, you will hear the cohesion that you have been looking for but not yet achieved.

Mix bus compression can take some time to master, but once you do, your listeners will be able to notice. Until next time, happy mixing!

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