The Basics of Side Chaining

Nihil sub sole novum (There is nothing new under the sun).

The ability to configure VST plugins with a side chain is appearing in more products every day. While side chain capability may be advertised as a “new feature”, it’s a technology that dates to the earliest days of broadcasting.

Like so many things in recording, side chaining was developed to solve a specific problem and then quickly adapted to other more creative applications. In this article we will explore the history and function of side chaining and then walk through the corrective and creative applications.

The fundamental concept of a side chain is to allow one audio signal to control another. Or more precisely, allow one audio signal to control a device through which another audio signal is passing.

One of the earliest applications of side chaining was in radio. The goal was to reduce the volume of the music whenever the announcer spoke. This application is often called “Ducking”.

First, the engineer would split the announcer’s signal into two parallel feeds. One feed went directly to the broadcast signal. The other feed was (literally) plugged into the side of a compressor. (Compressors of course provide a type of automatic volume control; more on that in another article.)

Once configured, when the announcer spoke, the side-chain signal would activate the compressor which in turn reduced the music’s volume. When the announcer stopped speaking, the compressor would return the program material to normal volume.

Figure 1: Yellow arrow represents the side-chain signal

Probably the most common (creative) application of side chaining today is the ubiquitous “pumping pad” heard on almost every dance, house or trance song. The most common way to achieve this is by making a separate track of just the kick drum then routing that track to the side-chain input of a compressor on the pad track. Then adjusting the compressor’s ratio and level controls for the desired amount of pumping.

Figure 2: This configuration will cause the synth pad to pump with each kick drum.

The steps to set up a side chain will vary from DAW to DAW. In Cubase, (my go-to DAW) the procedure starts with enabling the side-chain option on the compressor.

Figure 3: In Cubase, clicking on the "Activate Side-Chain" icon, shown here in orange, creates the side chain routing.

Once the side-chain option is activated, a new routing option for the side-chain input appears in the track inspector. From here you simply change the output routing for the kick drum from “stereo out” to “Sidechains”.

Figure 4: Output routing of the "key" or "trigger" track switched from “Stereo Out” to “Side-chains” for the Synth Pad compressor.

Once you get the routing for your DAW sorted out, you can begin to experiment with other options.

One popular way to use side chains creatively is to swap out the compressor for a noise gate. A noise gate is like a compressor in that it is an automatic level control. But when a noise gate operates it shuts the signal flow off entirely. Hence the name. Noise gates are commonly employed on drum mics to eliminate the noise, rattle and phase issues associated with a live (but unused) microphone.

But what if you pair a noise gate with a side chain? Now you can derive all kinds of creative options.

One trick David Bowie was rumored to use was to set up two vocal mics. One close mic (in the typical fashion) and another one further away (a room mic in other words). The room mic had a noise gate side chained to his primary mic and set to open only at extremely high volume. The result caused the loud passages to sound “louder” without having to raise their level in the mix.

Another trick is to use side-chained gates to introduce additional signal into your drum sounds.

1. Start by creating an audio track of static (pink noise or white noise)

2. Add a noise to this same track

3. Create a “key” or “trigger” track by isolating and duplicating your snare drum

4. Route the duplicate snare track to the noise gate’s side-chain input

Now, each time the snare drum hits, the noise gate opens momentarily and allows a burst of white noise through. You can process and mix this in to add depth, color or good old fashion funk to your snare sound!

You can use the same trick on your kick drum. But instead of “keying-in” white noise, try filling a track with a 60Hz tone. Now each time the kick drum hits, you’ll get a burst of sub-bass to play with.

Want to really get weird? Make a duplicate of the main vocal or maybe a guitar track, run it through a bit-crusher to really whack it out and key that from your kick or snare! Now every time the kick hits, you get burst of vocal or guitar (all smashed up) to blend in or play with in the background. Adds a lot of options for remixing.

If we switch back to the original compressor scenario, you can use the same basic set up to apply a mild compression (ducking) on the drum overheads to prevent phase interference from the snare. Simply use the snare track as the key to both overhead compressors.

You can think of side chaining as a distant relative to the modulation matrix in most contemporary synths: using one signal to control another. And just like the modulation matrix, once you get the basic workflow under your hands, the creative options come tumbling out hot and heavy!

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