In this episode, we'll concentrate on dynamics processing during mixdown, and we'll see how to use compression to give consistency to your tracks. This usage, in my opinion, is the most important one, since it allows you to balance instruments in relation to each other, which is one of the main goals of mixing.
The goal you are after here is to make the sound of the processed instrument more consistent in terms of level throughout the entire song. This type of compression will help reduce volume fluctuations on the recording that resulted from the musician accidentally moving a little closer or further from the mic. It can also provide you with a smoother and better balanced track that will sit better in the mix and be less likely to disappear during soft passages. But be careful. While this kind of processing might result in a loudness gain, that's by no means the main goal here. Likewise, you shouldn't try to sculpt sound in any way at this stage. On the contrary, you are going for a musical and natural compression that respects the musician's performance, as well as the instrument's timbre.
Step by step
In order to achieve today's objective, proceed as follows: Set the threshold so the compressor "captures" all dynamic movements of the instrument. In other words, even the slightest peak ought to trigger the compressor.
Setting the ratio is an extremely delicate matter. If it's set too high it will make the compression effect audible, in detriment of the feeling of liveliness of the signal. And, if it is too low, it will not produce the desired effect. There's no magic recipe. Try different ratios until you find a setting that's high enough to get the job done without altering the character of the signal.
Now, regarding the attack and release (aka "time constants"), there are two cases to consider. First, for percussive instruments (drums, percussion, etc.) and for those for which the rhythmic sensation is crucial (bass, rhythm guitar, etc.), make sure to set the attack time long enough to preserve the transients. Setting it too short will dull the attack of the string or drum or percussion instrument. (Editor's note: Attack settings above about 12 ms will typically preserve the transients.) The release time, for its part, ought to be set according to the rhythm of the song, to stick to its groove. For all other tracks, set the attack as short as possible so the compressor has every jolt under control. But be careful, because a too short attack time can produce distortion in the low end. As for the release, favor a moderate setting so that the end of the compression blends with the natural decay of the sound.
The secret to getting a transparent result is similar to using a "soft knee" setting. In fact, a soft knee setting makes the compression kick in more gradually, and is therefore almost invisible to the ear.
Finally, it is very common to filter out the lows of the processed signal so that the excess of energy in the low end doesn't trigger the compressor needlessly nor in a way that is too obvious.