Sidechain compression: the sidechain compression technique explained

Sidechain compression can be a really useful technique in mixing. That’s because it opens up the option to compress a track in your mix in relation to what is happening on a different track. For instance, you can have a compressor apply compression to the bass guitar every time the kick drum is played. Or you can have a compressor attenuate the level of a guitar every time the vocalist sings.

Understanding compressors and sidechains

To understand sidechain compression, it helps to first understand how a compressor works. When you insert a compressor onto a track, the track’s signal enters the compressor through the input. The signal then passes through the compressor and leaves the compressor via the output. Compressors contain a gain controller. The gain controller is the device that turns down the level of the signal that is passing through the compressor whenever compression is required.

Upon entering the compressor, a copy of the signal is fed to the compressor’s sidechain circuit. The sidechain circuit analyses this copy of the signal to determine when the signal requires compression. It makes this determination by measuring its copy of the signal against the compressor’s threshold setting. When the sidechain circuit calculates that compression is required, it instructs the gain controller to turn down the level of the signal that is flowing through the compressor. The gain controller then turns the signal down in accordance with the ratio setting. So, put very simply, your compressor’s sidechain tells your compressor when to turn the signal down.

By default, the sidechain is fed with a copy of the native signal i.e the signal from the track that the compressor is inserted on. But many compressors offer you the option to feed the compressor’s sidechain with a different signal. So now, the sidechain analyses a different signal and instructs the compressor to apply compression based on that which is occurring on a different track. We call this sidechain compression.

Why use sidechain compression?

To understand why this technique is useful, it helps to look at an example of its application. Mixing engineers often use sidechain compression to turn down a bass guitar whenever a kick drum is played. Mixing engineers do this to help prevent the bass guitar masking the kick drum in the mix. To achieve this, you insert a compressor on your bass guitar track and you feed its sidechain using the signal from the kick drum track. The compressor applies gain reduction to the bass guitar track. But the sidechain of that compressor is being fed with the signal from the kick drum track. So the compressor applies compression to the bass guitar track in accordance with the kick drum track’s signal.

Setting up sidechain compression

The process of setting up sidechain compression is quite straightforward. To set up side chain compression, you begin by inserting a compressor on the track that you want to apply compression to. Then, on the track that you want to use to feed that compressor’s sidechain, you assign the signal to an available bus. If your compressor supports external sidechaining then it should have some kind of input selector. Set this input to the bus that you have selected. Your compressor will likely have a button to switch the sidechain’s input from the native signal to the signal assigned to the input selector. Your compressor’s sidechain will now be fed by your chosen signal.

Common applications of sidechain compression

As previously mentioned, turning down a bass guitar when a kick drum hits is a common use for this technique. You can also use sidechain compression to turn down guitars, keys or other instruments when the vocalist sings. In dance music, a common application is to set up sidechain compression with the kick drum feeding the side chain of a bass, synth or even the whole mix. In this instance, the attack and release are then set so that a pumping effect is created.

What ever the application, sidechain compression offers you the ability to take more advanced control over the dynamics in your mix.

Do you use sidechain compression in your mixes? If so, what do you use it for?