This was written a while back so excuse the legacy look, but it is still current info.
This is one of the more creative features of the compression, not just the standard Logic compressor plugin, but of many of the good old analogue compressors. Instead of the compressor doing it’s stuff on the signal going into the regular input (let’s call this the main signal), what happens is that a completely different signal can be fed into the "sidechain" and instead of the main signal dictating how the compressor behaves, the sidechain signal does this so the compression applied to the main signal is no longer based on what that is doing dynamically but it is based on what is happening on a completely different track.
Back in the old days this was a very useful feature for radio DJs. If you wanted to talk while a track was playing, it made sense to lower the volume of the track. However this could happen automatically if the track was going through a compressor, and the DJ’s voice was the side chain. There would be no compression applied to the track while playing until the DJ started to talk. The compressor reacted only to the voice so that it would would automatically"duck" the track as the DJ spoke. Instead of having to manually lower the fader while speaking, the DJ could just leave it all to the compressor, leaving his/her hands free to get on with more important stuff. Very cool in those days. Sidechain compression is still used for the very same thing, but can also be used very creatively, especially in a mix.
How to do it with the compressor in Logic
You can work out the best way to use this in your own mixes, so I shall only demonstrate a very basic way to make it work using the standard Logic 7 instruments. The pictures here show you what to do but you can download the Logic project using the button at the top of this article to investigate further. Here we have an ES P instrument plugin on Instrument 1 playing a chord for 4 bars (yellow notes in the matrix). On Instrument 2 there is a simple Ultrabeat 2-step kit which comes in on bars 3 and 4 (red notes - this will be the sidechain):
I have inserted a Logic Compressor on the ES P track and made some quite radical settings so that we can hear the effect easily (you may want this to more subtly!). You can hear the drum sidechain acting on the pad after a couple of seconds:
In this example there is a very low threshold and high ratio, therefore a lot of compression, and the attack and release times are short to make the effect obvious. The sidechain is set to bus 3, i.e. it will not be affected by the ES P chord, but will "listen to" whatever is on bus 3 and act accordingly.
The Ultrabeat is sending to bus 3 so that it operates the compressor. The output is set to "no output", so this signal does not go into the mix. I have done this so you can hear the effect more easily - if you want to also hear the sidechain signal you just need to set the ouput of this Instrument to your mix bus.
©2003 Pete Thomas