The Many Splendors of Sidechaining

by Phillip Nichols, iZotope contributor

December 1, 2017

sidechain_compression-1200x500

Compressing the bass when the kick hits

Over the last handful of years, I’ve noticed the term “sidechaining” being used like it’s the password to get by that big bouncer at the city’s hottest club. Even people who admit to not knowing what it is keep saying it. “I think I’ll have to sidechain the kick. It just needs it, you know? Then, maybe I’ll sidechain the bass because I can’t have it flatlining.” Meanwhile, I’m furrowing my brow and being slightly too judgy.

Sidechaining can be a great solution for some routine mixing problems, or it can shake the foundation of an otherwise solid sound. If you’d prefer to be in control of your sidechain activities and avoid self-sabotage, do read on.

The Concept

First, let’s review what happens without special sidechaining in a typical “compressor plug-in on the bass” scenario. The bass signal is what causes the plug-in to compress. If the bass level goes above the compressor’s threshold, the plug-in compresses. If the bass level falls below the compressor’s threshold, the plug-in stops compressing. The same truth applies to compression on any other instrument. Whatever signal goes through the compressor is the signal that triggers it.

The core idea behind sidechaining is to use one signal to trigger a processor on a different signal. It’s as if one signal is the key to unlock and activate an effect on another. As a result, some devices (software and hardware) use the term “key input” while others prefer the label “sidechain input.” Understand that a device’s key input (sidechain) is not mixed with its processed audio output. Some processors have a cool “key listen” mode, which allows you to monitor the sidechain signal, but once you turn off that mode, the sidechain will no longer be audible.

How would some fancy sidechaining change the typical bass compression scenario? Let’s see! If a kick drum is used as the sidechain/key source for the bass compressor plug-in, then every time the kick hits, the plug-in will compress the bass.

Wacky as that may seem, it can be quite useful when trying to get the kick and bass to cooperate. If the fundamental concept of sidechaining makes sense, the next step is to make sure you can make it happen.

The Sidechain Setup

Let’s stick with the kick and bass scenario for the first example and break down an average sidechain setup into bite-size morsels. The following steps don’t necessarily need to be done in the order given, but they are in a nice, logical layout. Open a session and follow along!

1.  Insert a compressor plug-in on the bass track. Since not all compressors have sidechain functionality, you might have to look through a few until you find one that has a key input or sidechain section.

2.  Dial in a decent starting point for your compression parameters—ratio, attack, release, and threshold; those are up to you.

Image 1-Basic Compressor Setup

Basic Compressor Setup

3.  Locate the key/sidechain input selector in the plug-in. Set it to an unused bus, which will serve as the signal path used to get the kick to the sidechain.

Image 2-Key Input

Key Input

4.  If your plug-in’s sidechain has an on/off switch, turn it on! The correct choice will typically be titled something like “External” or “Key” (see images below). Many processors automatically enable the sidechain after the key input is selected, saving you the extra step.

Image 3-Internal Sidechain

Internal Sidechain

Image 4-External Sidechain

External Sidechain

5.  On the kick track, assign a send to the previously unused bus from Step 3. I recommend setting the send to “pre-fader” so that the sidechain configuration will work even if the bass is soloed or the kick is muted.

6.  Turn up the send level. Start at around the same volume as the kick fader unless it’s clipping.

Image 5-Finished Setup

Finished Setup

7 . Play the track and observe that the kick is triggering the compressor on the bass.

Image 6-Working Setup

Working Setup

Success; things seem to be good to go! But what if the kick has a lot of bleed from the snare? The snare bleed could be loud enough to trigger the sidechain compression. That’s bad news. It’s for precisely that sort of situation that many sidechain sections offer a filter that only affects the key input.

If used properly, a filter can remove unwanted frequencies and minimize undesired triggering of the processor. In the image below, notice that the sidechain section has a filter with selectable filter type and frequency. By using a low-pass filter at 250 Hz, frequencies above 250 Hz (in the sidechain signal) will be attenuated before the compression is triggered. This leaves you with a more isolated “trigger kick” and improves the accuracy of the sidechaining.  

Image 7-SC Filter

SC Filter

Some plug-ins, such as Neutron 2’s Compressor, allow you to sidechain from a specific, definable frequency band (see image below).

Image 8-Neutron Compressor

Neutron 2 Advanced Compressor

Once you’ve got the routing and standard configuration taken care of, you can delve into some routine uses for this nifty process.

Common Applications

Typically, sidechains will only be found on dynamics processors such as compressors, gates, and dynamic EQs (though you may find exceptions). So, focus on threshold-based audio effects. To get started, here are some frequently-encountered situations that can benefit from sidechain processing.

  • Bass Drowning Out the Kick: Using the configuration detailed in the previous section, the bass will be compressed each time the kick hits. As a result, the kick will be more audible than the bass during those hits. This can be a great way to get the kick to punch through a bit more, especially if your kick and bass share the same frequency range.

  • Drum Overheads with Overwhelming Snare: Toss a compressor on the overheads, then send the snare (close mic) to its sidechain. Every time the close-miked snare is played, the snare-heavy overheads will be attenuated. If there is significant kick bleed in the close snare mic signal, use a high-pass filter on the sidechain to minimize it!

  • Bright Guitars Covering Up the Lead Vocal: Insert a dynamic EQ or multiband compressor on the offending guitars. Set it to subtly turn down or compress the frequency range where the vocal is present and bright. You know what’s next; send the vocal to the sidechain! Whenever the vocal is heard, the guitars will get just a little bit “mellower.”

  • Lots of Bleed in the Kick Out: Put a gate on the “Kick Out” track. Assuming that the “Kick In” track doesn’t have excessive bleed, send it to the gate’s sidechain. Now, the kick track without bleed controls the gate on the bleed-heavy kick. The bleed in the “Kick Out” won’t open the gate!

  • Fiending for Some ‘80s Snare Reverb: Set up a reverb plug-in on a spare channel. Insert a gate directly after the reverb plug-in. Send the snare track to the reverb channel and to the sidechain of the gate. Ho ho ho, dear readers. What happens as a result is not for seekers of the subtle. Each time the snare plays, the gate will open and let the reverb be heard, and the gate will close after each crack of the snare. So, the existing of snare signal triggers the reverb, and the lack of snare signal stops the reverb (abruptly or gently, depending on the gate settings). Wild and nostalgic!

Conclusion

As long as you have sidechain-capable processors and an unfaltering faith in the power of signal well-sent, the fruits of the sidechain tree are yours for the taking. Pull up some tracks and enjoy! You can also use sidechaining for some pretty creative mix effects, but that’s another story to be told some other time.