This is a quick rundown on how to achieve side-chain compression. I understand this quite a frequently asked question; this article is a detailed run-down on how side-chain compression works and applications in which it can be used.


What is side-chain compression?
Side-chain compression is a secondary function performed by many compressors. It aims to lower the volume of one signal based on another signals input. This input is commonly known as a ‘key input’.


Why would I want to use side-chain compression?
You would use side-chain compression if:

you wanted to avoid ‘clashing’ of frequencies or;

you wanted to rhythmically separate two elements of your piece or;

you just wanted to do it as a purely creative process.

How do I side-chain compress something?
To side-chain compress something. You need two signals. One signal is going to be the signal that is being compressed and the other signal is going to be the key-input for your compressor. In this example I am going to use the classic example of a kick and bass.


Step 1: Create an audio and MIDI channel (by default Ableton Live opens with an audio and MIDI channel ready to go).





Step 2: Create a kick pattern on the audio channel (this can be a MIDI channel if you like, however I like to arrange my drums in audio).





Here I have created a two bar kick loop where the kick plays on every 1/4 beat.


Step 3: Create a MIDI clip with 1 long note playing in it (preferably a bass note for this example)








Here I have created a bass patch in Operator and while I was at it I decided to create a drum loop also.


Step 4: With this long bass note playing over the kick drum, there is a bit of frequency clashing. The kick gets a little bit lost in the mix and the bass seems to have no rhythm to it. This is where side-chain compression comes in handy. To begin applying side-chain compression to the bass, add a compressor to the end of the effects chain after your synthesiser. In this example I am dropping the compressor after Operator and EQ8





Step 5: As it is now, the compressor is set-up to compress based on the signal running in to it. We want to tell it to compress based on another signal that is coming in to it (the kick drums signal). To do this; click on the little arrow between the device on/off switch and the device name ‘Compressor’ to open the side-chain panel up.





Your compressor should now look like this.


Step 6: To tell the compressor to compress based on the kick drums signal, enable the ‘Sidechain’ button and underneath it in the drop down menu define which channels signal you would like to use as the key-input. In this example we want the ‘Kick’ channel to be the key-input.





That’s it!, your compressor is now set-up to react based on the kick channels signal.


The following describes how all the other more parameters will now behave (from left to right):


Second drop down menu: Selects where in the signal chain the key-input is taken from.


Gain: Defines how loud the compressor will receive the kick channels signal (don’t worry, you won’t hear it unless you enable the little greyed out headphone icon)


Dry/Wet: Will create a balance of compression between Operators signal and the kick channels signal.


Headphone icon: Allows one to hear the key-input signal.


EQ: Enables the side-chain EQ. After enabling this you will have the option of six different filter types and frequency and resonance controls. This will define which frequencies of the key-input will trigger the compressor. For instance, if you only want the compressor to react to high frequencies of the kick drum enable the high pass filter and adjust the frequency control.


Envelopes (Peak/RMS/Opto):
Peak: Will react quickly to short transients, much like a peak program meter (ppm) reacts to signal.

RMS: Is less sensitive to transients, much like VU metering.

Opto: Enables slower release behaviour, rather then the signal dropping directly to -infdB as it may in Peak mode.

Attack: Will define how long after the key-input signal is received that Operators signal will be lowered in level.


Release: Will define how long the signal takes to return to unity level after the attack phase.


Threshold: Defines where compression will begin.


Ratio: Defines the ratio of input to output. For instance a ratio of -inf:1 means that you can push as many decibels as you want in to the compressor and you will still only get one out (this is essentially turning your compressor in to a limiter). A ratio of 4:1 means for every 4dB over the threshold the signal will only increase by 1dB. Therefore with a ratio of 4:1 if you put 8dB in to the compressor you will only get a 2dB increase.


Knee: Blurs the line between the threshold and ratio.


Model: Chooses the compressor model. Prior to Live 7, there were two compressor models. ‘Compressor 1′ and ‘Compressor 2′. This chooses either of them plus a new one knows as FB (feedback) which is supposed to emulate old-school analog hardware compression.


Note: When choosing a model in side-chain mode, FF1 seems to be the only one that does not yield a ‘click’ or ‘pop’ whenever the key-input is played.


Lookahead: Sets how fast the compressor acts. Seems we are working in digital world 0ms is an option and in theory this means that the compressor sees in to the future and judges it’s reaction before any signal is played in to it.


Output/Makeup: This is the master output level of the compressor. It is commonly used to match input and output levels of a signal. Louder signals always seem better to our ears, so this make-up function can be used to cut (or boost) the level of the compressors output for fair A/B comparisons.


In this example I will also create a short hit of white noise produced by Analog, add a reverb to it and also compress this signal to the kick. Now the reverb tail will be side-chained. This gives the effect of the mix breathing.


Hear the differences here:

Here is the Music Player. You need to installl flash player to show this cool thing!

With Side-Chain Compression: Notice it breathes and drives a lot more? It adds an extra rhythm to the bass and the noise. It also leaves room for the kick frequencies to develop a bit more and gets rid of the ‘muddiness’ that was incurred without the side-chain compression.


Without Side-Chain Compression: It sounds muddy, it doesn’t drive as much and it sounds a little bit boring.


Download the project file here: Mr. Bill – Side-Chain Compression Project


If you enjoy this information and you benefit from it please consider donating!


Cheers,
Bill.