Pro Tools: How Side-chaining Works

Screen 1: A common production technique is to high‑pass filter a compressor’s internal side‑chain, so that kicks and basses don’t trigger excessive compression. Some compressor plug‑ins have these filters built in, but if not, you can set up your own filtering in the Pro Tools mixer.

Pro Tools offers a number of ways to implement the power of side‑chaining in your mixes.

In audio production circles, side‑chaining is one of those terms which is often used in a rather narrow and not entirely accurate way. You’ve probably encountered the term as shorthand for one particular use: compressing a source using an external signal as a trigger, to create a ‘ducking’ or ‘pumping’ effect. While valid and very effective, that technique represents only one of the ways side‑chains are used. For simplicity I’ll refer to compression, but these principles apply to any dynamic process: compression, gating, expansion or limiting.

In Pro Tools, as in other DAWs, dynamics processors rely on side‑chains. Whether you’re using Smack or Pro Compressor, you’re working with a detector circuit that operates with reference to a side‑chain. The better you understand that, the more control you’ll have.

What Is A Side‑chain?

All conventional dynamics plug‑ins control the level of audio passing through them by monitoring the level of a control signal and comparing it to a reference level: the level at which an action should begin. This reference level is called the threshold, and what happens in response to the control signal (the side‑chain signal) crossing the reference level (the threshold) depends on the type of process (compression, expansion and so on), and the settings of the other controls (principally ratio, attack and release times).

The side‑chain is the path carrying the control signal that feeds the detector in a dynamics processor. It’s what the plug‑in is “listening to” in order to decide when to apply gain reduction, and how much. By default, the side‑chain signal is derived from the input signal to which compression is being applied. This is known as an internal side‑chain.

All compressors, even simple ones like Dyn3 Compressor/Limiter, have a side‑chain path. What varies is how accessible and adjustable that side‑chain path is. Some plug‑ins, like the 1176‑style BF76, have unconventional control sets with no threshold control or access to the side‑chain path for filtering, but the side‑chain path is still there. More advanced Pro Tools plug‑ins like Channel Strip and Pro Compressor expose additional controls for shaping the side‑chain signal, allowing you to fine‑tune how the detector responds.

By using a different audio signal as the side‑chain, the compressor can ‘duck’ audio out of the way of another signal.

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