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Sidechain compression is a form of compression that uses the signal running through a compressor’s sidechain circuit to trigger compression. You can use sidechain compression to prevent frequency masking and add creative pumping effects to your mixes. In this guide, you'll learn how to apply sidechain compression using FabFilter's Pro-C 2.

Sidechain Compression Explained

A compressor normally responds to the signal level of the track you’ve applied it to. For example, if you apply a compressor to a vocal track, it will attenuate the level of the vocal when it gets too loud.

When you engage a compressor’s sidechain circuit and select another track in your DAW as the sidechain input signal, you tell the compressor to respond to the signal level of the other track, as opposed to the signal level of the track the compressor has been applied to.

If you apply a compressor to your bass track and set your kick track as the compressor’s sidechain input signal, the compressor will apply gain reduction to the bass track every time the kick plays.

Don’t overthink this concept; it’s as simple as it sounds. When a compressor has its sidechain circuit engaged, the sidechain input signal trigger’s the compressor.

To be clear, compression is not applied to the sidechain input signal. Just like a compressor being used normally, a compressor responding to an external sidechain input signal will still compress the track you applied the compressor to.

How to Set Up FabFilter's Pro-C 2 to Apply Sidechain Compression

The way you go about routing audio into the FabFilter Pro C-2's external sidechain input will vary slightly depending on the DAW you’re using. In Ableton, it’s a rather straightforward 3-step process that's quite similar to how you would apply sidechain compression using stock Ableton effects. If you're using Cubase, Logic Pro, or Pro Tools, you can use this guide to set up an external input; once you've done this, skip ahead to Step 2.

1. Set Up an External Input

Apply a compressor like the FabFilter Pro-C 2 to the track that you want to apply gain reduction to. If you’re trying to prevent your bass from masking your kick, apply the compressor to your bass track.

‍Figure 1: FabFilter's Pro-C 2 applies to a track in Ableton.

Select your kick track from the External Source (Sidechain) dropdown menu.

Figure 2: A track labeled "KICK" selected as the compressor's External Source.

2. Open the FabFilter Pro-C 2's Side Chain Section and Select "Ext"

Open the FabFilter Pro-C 2 and click the “SIDE CHAIN” button at the bottom of the user interface. This will open the compressor’s Side Chain section.

Figure 3: The "SIDE CHAIN" disclosure button is highlighted in green.

Click “Ext” to tell the Pro-C 2 to respond to the External Source you set up. Now, when you engage playback, the compressor will apply gain reduction to your bass track based on the signal level of your kick track. One of the great things about the Pro-C 2 is that it’s extremely visual, so you can see gain reduction being applied.

Figure 4: The "Ext" button within the sidechain section of FabFilter's Pro-C 2.

3. Dial In Your Compression Settings

There are a few settings you should tweak to get the most out of your sidechain compressor. To start, disengage Auto Gain so that you’re able to make accurate A/B comparisons when you bypass the compressor.

Attack Time

You’re going to want to use a fast attack time (1-10 ms) to attenuate your bass as soon as your kick plays; the faster you set the attack time, the faster the bass will duck out of the way.

Release Time

The release time you should use depends on the effect you’re in pursuit of. For a transparent form of compression, use a fast release time; this is useful if you’re just trying to get the “click” of a kick to punch through your mix. To create creative pumping effects that draw attention to themselves, use a slow release time.