Using the Sidechain in Logic’s Compressor
This is one of the more creative features of 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 instruments. The pictures here show you what to do but you can also download the project to investigate further.
Here we have an Alchemy instrument plugin on Instrument 1 playing a pad chord for 4 bars (blue notes in the piano roll). On Instrument 2 there is a simple Ultrabeat kit playing a series of short notes which comes in on bars 3 and 4 (green notes – this will be the sidechain):
I have inserted a Logic Compressor on the Alchemy 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:
There is a low threshold so that there is a lot of compression, and the attack and release times are short to make the effect obvious. The sidechain is set to bus 1, i.e. it will not be affected by the Alchemy chord, but will “listen to” whatever is on bus 1 and react only to that input.
The Ultrabeat is sending to bus 1 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 output of this Instrument to your mix bus.
To learn more about this effect, play with the compressor. I have used quite extreme settings in regard to threshold, attack and release. You can also use the track send for the bus rather than the output.