What exactly *is* compression?
A compressor, to get all technical on your ass, is a non-linear amplifier. The easiest way to conceive of a compressor is as an automatic volume control. When a signal exceeds a certain level (called the threshold), its gain is reduced, effectively turning down the volume at superhuman speeds. You might also think of this as a kind of "soft overdrive" or waveshaping. The amount the signal is reduced by is called the ratio of the compressor. The attack and release time settings determine how aggressively this reduction kicks in and out, respectively. Many compressors have adjustable or soft knees as well, which let you control how gradually the volume reduction happens. Soft knees tend to sound more transparent, but hard knees give more aggressive volume control.
Cool, so what do I do with that?
Well, the answer is, "it depends, what's wrong with your signal?" Compressors can either be used for leveling (making slow volume adjustments) or limiting (lowering the volume of loud events in a signal), and they can be applied anywhere from individual tracks to masters. Knowing what you need is the first step. If a track is too "spiky" or "jumpy," you probably want limiting. If something is more subtly uneven and just needs some extra thickness, you want a leveler.
Setting up a limiter
I find the easiest way to set up a limiting compressor is to start by maxing everything out: shortest attack, longest release, highest ratio, threshold at 0 dB. This makes the effect of the compressor painfully obvious. From there, adjust the threshold so that only loud spikes are cut off; parts of the signal below that should sound normal. Next, shorten your release time as much as possible without inducing any uneven pumping (volume changes that don't match the rhythm of the music). After that, increase your attack time so that an appropriate amount of punch and detail comes through before volume reduction. Finally, adjust your ratio to get the desired amount of gain reduction; typical limiting ratios are 4:1 and above. More than 3 dB of gain reduction tends to sound unnatural, but sometimes unnatural might be what you want.
Setting up a leveler
Leveling compressors are less dramatic to set up, but harder to tweak, as they have a much more subtle effect. I start by setting the threshold just below the lowest signal level, with a 50 ms attack and 1 s release. From there, set your ratio between 1:1 and 2:1 for the desired effect and adjust your attack and release as needed.
Am I doing this right?
Compressors can be overwhelming at times, so here are some typical settings to see if you're on the right track:
- Vocals: 3:1 - 4:1 ratio, .5 ms attack, 80 ms release, -24 dB threshold
- Drums: 5:1 ratio, 40 ms attack, 200 ms release, -18 dB threshold
- Mix Buss: 1.5:1 - 2:1 ratio, 50 ms attack, 180 ms release, -6 to -12 dB threshold
- Leveler: 1.1 ratio, 100 ms attack, 1500 ms release, -35 dB threshold.
