Friday, March 8, 2013

Compression, The Short Answer

There may not be any subject as talked about in audio as compression, yet many people have a hard time wrapping their minds around it. In a sea of discussions on the nuances it can be hard to find a basic definition to get over that first speed bump to understanding. Here's a quick couple of paragraphs that I wrote on a forum a while back. It is in no way a complete description but it's about what I'd say to someone trying to understand compression for the first time.

A compressor is a circuit that automatically lowers the volume of the signal going through it. You set a threshold and everything below that threshold goes straight though untouched. When the input is higher than the threshold the circuit turns itself down. You control how much by setting the ratio. 1:1 does nothing, 2:1 means that the input has to increase by 2 dB to see an increase at the output of 1 dB, 4:1 takes a 4 dB increase to get 1 more dB at the output. Ratios of 8:1 or 10:1 are generally considered limiting because their effect is pretty severe. Brick wall limiting will not allow an output higher than the threshold setting.

By setting the attack and release you can control how quickly the circuit acts. A fast attack might make drums sound mushy but relaxing it a little with let the first loud crack come through before it starts acting on the resonant sound. Release times can be changed to keep the sound from "pumping" or to make it pump on purpose. Longer releases are often used on vocals because the circuit lets go of the signal in a way that is less audible.

When all is said and done you start with a signal that has a given dynamic range, the distance to the highest peak, and get one with less dynamic range at the output. Essentially you lower all the highest peaks in the program. Then you can turn up the gain at the end to make up for the level you cut out. It has the effect of bringing up the quieter sounds in the signal.

1 comment:

  1. It's often funny how everybody who does any kind of audio manipulation knows (at least in some way) what a compressor is and or does, but everybody outside that bubble doesn't have the slightest idea that such a thing even exists :)

    Seems like most people get the concept behind it pretty quickly when you explain it to them, though. Because it just makes sense.

    Now, using a compressor in a useful way is a whole entire matter. When I started out I spent years just turning knobs. I heard that compression makes things sound better, but I was totally clueless as to how and why. I was listening, but I wasn't sure what exactly was happening. So more often than not, the "auto" setting on the old 266 was my best friend. I just trusted it. Years later I found the best approach to setting compressors in Michael Stavrou's book "Mixing with your mind" (which is a great book full of unusual approaches). Helped me immensely, and I basically use a similar technique to this day.

    I first set the attack to the fastest possible setting (all the way counter-clockwise, the release to a really quick setting and the ratio way up. Then I lower the compressors threshold so much, that the signal gets compressed ALL the time. Then I slowly turn the attack knob clockwise until there's enough "attack" (crack, snap, click) coming through. Twiddle with it a bit. I then "turn up" the release until it sounds right. Sometimes I land on longer release times for that bouncy feel, sometimes I choose shorter release times for that gaspy breathy feel. Whenever I'm happy with the overall "compressor sound", I set the ratio to a less agressive setting, and raise the threshold so the compressor's doing something like 3-8 db reduction on the loudest peaks.

    More often than not this approach gives me something useful. I might alter my technique when I do more exotic things like parallel compression (where the original signal takes care of the transients) or when using any type of sidechain compression (or if I use frequency dependent/key compression).

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