Modular Quick Tip: Sidechain Compression

Sidechain compression is a method whereby an external source is used to trigger a compressors, rather than the signal that’s being fed into its signal path.

It’s common place in electronic music and has been popularised by various french house producers. Hear the unaffected sample from the start and listen to how the kick drum attenuates in from 2:09.

I wanted to work out a way of executing this technique using eurorack modular sans any compression. I am sure there are various other ways to do this, but this method made the most sense for me. Here’s the video:

If you want to understand better about what I’m doing, read on:

You Will Need

A VCA: this stands for voltage controlled amplifier and is just that, it can amplify sounds according to an incoming control voltage.
An inverter: this inverts a signal, i.e flipping it around from positive to negative or vice versa.
Something to sidechain compress, such as an oscillator.
Some sort of kick drum sound, either coming from a drum module or as audio routed from your DAW.
…and an envelope follower: this listens to an incoming audio signal and outputs a control voltage that emulates its shape.

I’m going to use just what I have to hand, which is a Doepfer A-132-3 Dual VCA, a Pittsburgh Modular Toolbox (for voltage inversion), a Tiptop Audio BD808 and a Doepfer A-119 Envelope Follower.

Some other modules used in this patch are an MFB Dual LFO to trigger the kick drum, an old intellijel Dixie as my sound source and a Pittsburgh Modular Mixer to sum the kick and oscillator together for monitoring.

The Patch

Let’s start by triggering our kick drum. I’m using our LFO’s square wave output routed to our BD808’s Gate In.

The kick is routed to the Asym. In on the A-119. This has two audio outputs which is useful in itself, but it gives us an envelope out too. Route one of the audio outs to the mixer and another to the inverter, which is then routed to the CV input on a VCA.

Now let’s sort our synth out – route the output of the Dixie (or whatever oscillator/sound source you are using) to the VCA. I’m using the sawtooth output as it has the brightest, fullest sound and it will be easy to hear the pseudo-compression.

On the VCA turn the Gain (or bias) up. This is counter intuitive to how we might route a normal synth patch, but stay with me. Now, turning the CV up, every time the kick hits, an envelope shape will come out of the A-119, be inverted by the toolbox and then attenuate the VCA.

This is fine in isolation but if you want to make the patch more playable some steps might need to be taken.

If you’re using a keyboard to “play” the oscillator, route the output of the VCA to another VCA (see why I used the dual one now?) and route the keyboard’s gate output to the CV on that, thus effectively turning our VCA in a simple AND gate – i.e when the signal is present AND a key is depressed, the VCA will output a signal.

If you want to use a DAWs kick, that can be routed to the Symm. In on the A-119 and the same steps can be taken. Beware that this method might need some degree of attenuation as this input is expecting an instrument level sound, not a line level one.

Lastly to make this more versatile you could use a white noise module and an ADSR instead of the dummy kick – this means using the ADSR you can shape the size of the trigger, rather than it being fixed to your kick’s length. More on this another time.

Post Script

I’ve just been made aware of these two excellent videos, one from Make Noise’s official YouTube channel on using MATHS as an envelope follower, and the second a video specifically on using MATHS to achieve sidechain compression. Here go: