When using a sidechain, you can either enable an internal sidechain - which determines what frequencies trigger the processing, or an external side chain - which is an outside audio source that triggers the processing. Both have distinct pragmatic and creative purposes, and are typically used on a compressor.
When mixing or mastering, an internal side-chain is typically an internal equalizer, with which you can determine which frequencies trigger the processor. For example, I can use a compressor’s internal side-chain to make the high frequencies not trigger the compressor, by introducing a low-pass filter.
Let’s take a listen to it.
An external side chain is an extrinsic, or outside audio source that affects the processor’s behavior; for example, I could use a kick as an external side-chain on a compressor, and whenever the kick hit, the compressor would respond. This feature has multiple creative and practical applications when mixing and mastering.
Let’s listen to a drum track as the external side chain, affecting a compressor on the rest of the track.
One popular way to use an internal side chain in a mix or master is to use a high-pass filter, in turn, ensuring that the low-frequencies don’t trigger the compressor. This is especially helpful on a mix bus where the kick may be the most common trigger.
By cutting it out, you only cause compression when the mids and highs are loud enough to trigger it, resulting in the retention of your dynamics.
Let’s take a listen.
If the compressor you’re using allows for it, you can switch your signal from left and right to mid and side - then you can increase the input of the mid chain to cause more compression to the mids. The side will be louder by comparison, causing dynamic stereo expansion.
This effect can be somewhat finicky, so keep an eye on your attenuation and to dial in the threshold, input and other factors just right.
Let’s take a listen.
Bass ducking is a very popular use for external side-chain; with it you attenuate the bass whenever the kick hits to create more room in the low-end. This helps clear some space in the low-end since the 2 instruments are both powerful and occupy roughly the same space.
Simply put, the compressor goes on the bass track, the external side chain is set to the kick track.
Let’s take a listen.
Vocal ducking is popular in live performances or when a DJ needs to break in to say something, but if used very subtly, it can work well in rap and pop tracks. Using the vocal as the external side chain, you can attenuate the full mix.
If you do try this, be sure that the attenuation isn’t more than 0.5dB, since it can become noticeable very quickly.
Let’s take a listen.
TrackSpacer is a really unique plugin that utilizes an external sidechain like a compressor but then introduces frequency-specific dynamic EQ to react to the incoming signal. This way, if the track’s frequencies and side-chained signal’s frequencies overlap too much, that range will be attenuated.
The low and high cut help you isolate the range, and use the amount dial to control how much attenuation occurs.
Let’s take a listen.
Automating your external side-chain can be a little difficult, since the send occurs pre-fader, or can’t be controlled by adjusting the fader, mute, or other functions. With that in mind, it’s best to automate the settings within the plugin that you’re using, typically the threshold is the easiest option.
Let’s automate the threshold of the compressor and in turn the amount of compression caused by the external side chain.
Sometimes, you want your processor to respond in a consistent way - in other words, you don’t want it to respond dynamically, but instead, introduce identical processing consistently. To accomplish this, set up a signal generator, mute it, and set it as the external side chain to your compressor.
Then adjust the threshold until you get the exact amount of attenuation that you want.
Let’s take a listen.
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