Resources for the Recording Musician
September 22, 2005

Side Chain

Lately I have been playing with compressors and gates. My church just picked up a Presonus ACP88 and I noticed it has side chains for both the compressor and the gate. I have read and seen examples on how to use them both. My question is how much do the “pros” use side chains in recording and mix downs?

Side chaining is used in many ways by lots of pros... some of them do some very creative things:

One creative example is keying the bass from the kick drum (kick drum signal feeds the side chain input) on a compressor to make them pump a bit, or to simply pull the bass level down a bit when the kick drum hits to keep the low end under control.

I've also heard of people keying a synth pad or guitar part from the hi-hat track through a gate, so that you get a pulsating part that has the same rhythm as the hi-hat.  In fact, I was just reading about that in a recent magazine discussing the classic track "Le Freak" by Chic (I think that was it, can't remember for sure) where they did that to clean up the sloppiness of the guitar part... kind of an old method of audio quantizing.

I personally haven't played around with those techniques myself because I've never had the need, but I use another method of side chaining once in a while via a De-Esser.  I've had vocalists that just couldn't control their sibilance or were too bright, and a De-Esser helps quite a bit.  A De-Esser is just a compressor with a side chain input where a filtered (EQ) version of the source is fed to the side chain input so that the compressor reacts more to specific frequencies, in this case, the hi frequencies where the "S" sounds are located.  De-Essers are used quite a bit by pros... Another trick with a De-Esser is to strap it across the overhead mics for a drum kit to help keep the cymbals under control if they are too loud or too bright.

Another very common use is with gates when trying to reduce bleed on drum tracks (such as hi-hat bleed into the snare mic, or just to keep the tom tracks turned off whenever the toms aren't being played).  The good gates have side chain filters on them to let you zero in on the tone that you want to use to open the gate and hopefully reject as much of the other stuff you don't want as possible.

Yet another example is for mix bus compression, or mastering.  If you've got a bass heavy song, such as a rap or RnB song with deep and loud kick drums or bass synth, you can sometimes get an undesirable pumping effect using a single band compressor which reacts to the low frequencies much more.  One way around that is to use a High Pass Filter on the side chain to remove some of the low end so the compressor isn't pulling down the whole mix as much every time the kick or bass hit heavy.

Many of these side chain features are built into hardware, and software, compressors & gates.  De-Essser plugins, or hardware, for example, already have all the side chain functionality built in, usually including adjustable EQ/filters to hone in on the problem frequencies.  Most plug-in gates also have built in side-chains with their own EQ sections for finding just the right frequencies to trigger the gate without getting false triggers.  Similarly, many modern hardware compressors have built-in circuits to tailor the response to the low end for mixing duties, to help avoid pumping.  For example, the Drawmer 1968 compressor I own has a "Big" switch that filters out some low end for the side chain.  The API 2500 buss compressor I own has three response curve settings for its side chain circuit.

So, those are just a few examples of how side chains are used by the pros... and, yes, they are using them.

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