pM,
Below is my standard blurb - the basics of compression in the fewest number of words possible.
--Ethan
A compressor or limiter is an automatic volume control that reduces the volume when the input gets too loud. Originally they were used to prevent AM radio transmitters from distorting if the announcer got too close to the mike. Then some creative folks discovered that a compressor can sound cool as an effect on voices and musical instruments.
The primary controls on a compressor are:
Threshold - also called ceiling - This sets the point at which the automatic volume reduction kicks in. Below that volume the compressor does nothing. When the input gets above that level, the compressor reduces the volume automatically to keep the signal from getting much louder.
Attack time - how quickly the volume is reduced when the input exceeds the threshold. If it's too slow, then a short burst of loud music can get through and possibly cause distortion. So when using a compressor as a tool to prevent overload you generally want a very fast attack time. But when used on an electric bass to get a little more punch, 20-50 milliseconds is often good because that lets a little burst of the attack get through before the volume is reduced. So each note has a little extra "definition" but without the full length of the note being too loud.
Release time - how quickly the volume comes back up once the input is no longer above the threshold. If it's too fast, you'll hear the volume as it goes up and down. That sound is called "pumping" or "breathing." Sometimes this sound is desirable for adding presense to vocals, drums, and other instruments, but often it is not wanted. The best setting depends on whether you're using the compressor as a tool to prevent overloading, or as an effect to create a cool sound or add more sustain to an instrument. If you don't want to hear the compressor work, set the release time fairly long (one second or more).
Compression ratio - 1:1 does nothing. 2:1 means if the input rises to 2 dB. above the threshold, the compressor will reduce the level by 1 dB. so now it's 1 dB. above. 10:1 means you have to get 10 dB. above the threshold for the output to go up by 1 dB.
Makeup Gain - since a compressor can only reduce the volume when the incoming signal is too high, the Makeup Gain control lets you bring the compressed audio back up to an acceptable level.
Some compressors also have a Knee setting, which has affects only signals that are right around the threshold level. With a "hard knee" setting signals below the threshold are not compressed at all, and as soon as they exceed the threshold the gain suddenly starts being reduced by exactly the amount that the ratio dictates. A "soft knee" setting works a bit differently. As the signal level approaches the threshold it starts to be reduced in level, and the reduction gradually increases until the level crosses the threshold. The compression does not reach the full value of the ratio until a bit above the threshold.
Besides serving as an automatic volume control, a compressor can also make notes sustain longer. To make a note sustain requires raising the volume of a note as it fades out. That is, making the trailing part of a note louder to counter its natural fadeout is what makes it seem to sustain more.
To do this with a compressor you'll set the threshold low enough that the volume is reduced most of the time. Then as the note fades the compressor reduces the volume less, which is the same thing as raising the volume. For example, when you play a note on an electric bass the compressor immediately reduces the volume by, say, 10 dB. because the start of the note exceeds the threshold by 10 dB. You don't hear the volume be reduced because it happens so quickly. But as the note fades over time, the compressor raises the volume which gives the effect of adding sustain.
That said, I pretty much stopped using compressors a few years ago. Now I use volume envelopes in Sonar as needed to raise soft syllables or lower too-loud stuff. Programming volume changes manually rarely takes longer than finding the right compressor settings, and of course you can change the volume envelopes any time in the future. The big advantage of avoiding a compressor is to not add pumping and breathing sounds. These days the only things I compress - and always after recording, non-destructively - are
acoustic guitar and electric bass if they need a little more sustain as an effect.
