A sidechain (also seen as side chain or side-chain) input is a special 'key input' found on audio compressor units and noise gate units. It's an additional input, separate to the main input found on the device.
The signal that's connected to the key input changes the behavior of the main output of the compressor or the gate. The device will have an option that allows you to select whether the unit should 'listen' to this key input, and the way the sound is affected depends on the unit and how the settings are adjusted.
The result is that the main signal being processed by the compressor or the gate takes on the characteristics of the side chained signal imposed on it.
There are lots of technical and creative ways that a sidechain can be used in the studio. I definitely think that the technique has had a massive impact on the sound of today's electronic dance music, as we'll look at below.
A side-chain compressor can be used in many different ways. One of the most common is called 'ducking' - where the main signal that passes through a compressor drops in volume level, whenever a signal enters the key input.
You'll most often hear this on the radio, when the DJ talks over the music that's playing. The volume of the music drops whenever the DJ talks, allowing the voice to be heard more clearly as it doesn't have to compete with the volume of the music. The music track is the main signal passing through the compressor unit and the DJ's microphone is connected to the key input.
Another use for it can be heard everywhere in electronic dance music, in what's known as 'pumping'. For example, if you have a kick drum playing as well as a synth bass sound, you can change the way that the two sounds interact with each other, which can result in a far more interesting feel and catchy rhythm for your production.
Here's how you can do it:
A common use for this type of gate is to have the main audio signal take on the rhythmical elements of the key input. For example:
Using the sidechain technique can take a little getting used to, but I definitely feel that using it can give you really interesting and creative sounds in your productions. If you get to know your compressors and gates well, you can easily apply this technique to help expand your range of production tools in the studio.