RATIO: This knob determines how much gain or volume reduction you are going to experience after the threshold has been passed and the compressor starts working. Most compressors have a variable ratio from 1:1 to infinity. 1:1 is no effect and infinity is a brick wall. If a setting of 4:1 is used then before the threshold is reached the compressor has no effect and the gain is 1:1, but after the threshold is passed the gain changes from 1:1 to 4:1. This has the effect of reducing the overall volume range of the sound that is being compressed. Although compressor ratios can go all the way to infinity, you will find that 4:1 is a lot of compression.
ATTACK: This knob sets how fast the compressor reacts to the sound after its volume has passed the threshold. My experience says you want immediate response so set the knob all the way CCW which should be its fastest reaction time.
OUTPUT: This knob sets how much volume you want the compressor to produce. Generally you want unity gain. That would be 0-dbm.
As you can see the threshold and ratio knobs are where the action is. Learning how to set them correctly is very important. Beside the knobs there are usually one or more meters to let you look at the input and output volumes and also the amount of overall compression that is being achieved. For live applications, it is very important that no more than -6db of reduction show on the compression reduction meter or feedback can occur when the music stops. You adjust the overall reduction by changing the threshold and/or ratio knobs. I usually set the ratio that I want and then use the threshold knob to get the amount of compression that I need, being very careful not to over compress the signal.
Author: William Mellon