It’s hard to talk about electronic music without mentioning compressors. Most people probably associate audio compression with the ‘pumping’ characteristics made popular by acts like Daft Punk, but that’s not really their intended function in most cases. So I figure it’s high time we run through what a compressor is.
A compressor is a device that reduces the dynamic range of an audio signal. Or to put it plainly, it’s used to make the loud bits quieter and the quieter bits louder. On guitars for example, they’re often used to increase the sustain of a note without getting the volume spike caused by hitting the strings harder. The compressor will limit the initial volume spike of the plucked string and boost the volume of the string ringing out - handy for solos and other melodic parts. Where things get more complex is understanding the controls available on compressors.
Here’s a few standard controls that you’ll commonly find on a compressor:
The threshold is the level at which compression begins to occur, i.e. if your threshold is set to -10dB no compression will occur until your signal’s volume rises above -10dB.
Attack time is the time it takes for compression to kick in when the signal hits your threshold. This is a matter of microseconds, but you’d be surprised how much it can change the sound. For example, having a slower attack time on your drums such as 75ms will let a super brief transient through before compression sucks the volume down. That may not sound ideal, but letting that brief signal through can create the impression that your drums are really smacking hard.
Compression ratio is the level of attenuation your signal receives once it reaches the threshold. If a signal goes 2dB beyond your threshold and the ratio is set to 2:1, the signal will be attenuated to 1dB above the threshold. If it’s 8dB above, it would be reduced to 4dB above, or 4:1 reduced to 2dB above, etc. Many compressors have an ∞:1 setting, which is basically the compressor doubling as a limiter - it ensures the signal does not exceed the threshold.
Release time is, as you might expect, the time it takes to release the compression effect once the signal amplitude drops below the threshold. This is generally slower than attack time as a fast release tends to create an un-musical pumping effect. A longer slope back to the uncompressed sound means it’s more difficult for the listener to detect. On the flipside, an exaggerated longer release time can be used as its own rhythmic effect.
Once you’ve applied your compression, you’ll likely find the overall volume of your signal has suffered, since all you’ve done is reduce the amplitude of its loudest parts. That’s where output gain comes into play. Boosting this a few decibels will give you back your volume, but now with a less dynamic signal - the loudest parts will be quieter and the quiet parts louder.
I’ve obviously simplified things here, but if you can wrap your head around those concepts, then you’re well on your way to grasping compression. As with all audio lessons, hearing is understanding, so experiment and experiment some more. In the next issue, I’ll talk about the different types of compressors and their implementation.