This is a post by mastering engineer Barry Gardner who operates SAS Mastering
http://www.masteringmastering.co.uk/onlinemasteringservices.html
What is side chain processing?
Side chain compression is often seen as a complicated production technique but assuming you understand the basics of compression there should not be any troubles understanding how it works. Most audio compressors work by controlling the input signal using the same signals dynamics, so for example when there is a loud section in a vocal recording the peak in the vocal gets reduced in volume. This has the effect of evening out the level of the singing. It is a very common and useful technique for many types of instrument and most modern music mixes will have a reasonable amount of compression occurring.
Side chain compression requires that any given compressor has what is known as a side chain input, key input or external key. This is a hardware or software facility which allows the compressors detector circuit to be controlled by an external signal other than the one that is being compressed. Side chain compression requires 2 signals, the signal being compressed and the ‘control’ signal. Side chain compression allows us to be more creative and produce both useful technical solutions and also creative compression solutions.
Corrective or creative?
One very common way of using side chain compression would be in a background music and voice over situation. At times the music may become over bearing and stop the words from being clearly understood. This process is a technical use of side chain compression and is known as ducking. (this is because the music is ‘ducked’ under the voice) So the music is compressed (reduced in volume) when the voice appears. For this type of result a fairly fast attack time is set, say 10 milliseconds and a fairly slow release time of around 1 second. When the voice appears the music drops quite quickly to improve intelligibility of speech and the music is allowed to slowly recover to it’s original volume when the speech stops.
A common creative use of compression is a production technique which allows the kick drum to have a little more of it’s own space in the bass line. Often a kick drum and bass line occupy the low frequencies and as such they can sometimes clash. This clashing can be reduced a little by side chaining the bass line with the kick drum pattern. When you insert your compressor across the bass line you need to ‘key, ’ control’ or ‘side chain’ the bass line with the kick drum tracks output. There is normally a sequencer (digital audio workstation) specific way of achieving this so refer to the user manual if it is not intuitively routed. What practically occurs is that when the kick drum plays the bass line is dropped slightly in level for the duration of the kick to allow the kick drum some extra prominence.
For this kind of effect I recommend a good starting attack and release times as being 1ms and 200ms respectively. These of course have to be fine tuned to your kick drum decay length and individual bass line so serve only as a guide. The goal you are looking for will depend on the exact kick and bass line in your track so these time constants will need carefully manipulation to hit the sonic ‘sweet spot’.
Summary
Side chain compression is a great way to add some creative effects to your music. Try experimenting with side chaining various elements with your mix sources and see if they enhance the musicality of your tracks. It can take a little while to master the procedure but is very rewarding when you have perfected the technique.