One of the questions I see a lot is how to use a compressor in mastering. Of course, nothing beats a lot of time spent using a particular compressor on many different tracks to get a feel for it, but there are some techniques that can help to speed up the process. This is not the only way, but it's one that works for me, after a lot of trial and error, and if you are mastering for punch/clarity/"glue" etc., as opposed to just squash. I will say upfront that it is almost completely paraphrased from Michael Paul Stavrou's excellent book, 'Mixing WIth Your Mind', which despite some now outdated sections on digital audio, is one of the best recording books I have ever read.
So, here's the technique: First choose a basic stereo compressor and an unmastered, highish tempo, steadily rhythmic track (e.g. something Disco, House or Techno etc).
1) Set the main controls like this: Attack as short as it will go. Release as short as it will go. Ratio as high as it will go. Threshold/Input Gain depends on the compressor type, but basically be smashing it as hard as you can. It sounds really bad, right?
2) Now start tweaking the Attack control and listen to the attack of the transients on the kicks, snares, bass line, arpeggios or whatever. You should be able to find a nice spot where it really lets the transients 'thwap' through whilst clamping down on everything else. Use your ears. Where does it sound best? Less snap and more smash = shorter Attack, more snap and less smash = longer Attack. I can usually find a sweet spot very quickly. I usually err on the side of a little too short rather than a little too long. Don't touch the Attack again, you are done there. It still sounds really rubbish though, doesn't it?
3) Move on to the Release control. We are now at the real magick. What you are looking for here is an indefinable certain something that grooves. Again, use your ears. Might be best left at minimum, might sound good thinking about the tempo of the track, so it clamps down and swings back just before the next kick (although it might not...), it might want even longer than that for a volume wave/RMS/loudness levelling like function that just "feels" right and rides the groove. It's the ineffable control, but the most important for adding feel. So use your ears, not your eyes, not your screen, not your meters, not the compressor markings, just your ears. When you are happy, don't touch the Release again, leave it, you are done there.
4) If the comp has a HPF in the sidechain, I usually play with it here. On full band tracks I usually find somewhere between about 60-300Hz will work to stop those kicks and bass lines pulling things down too much, but I'll also usually go back and tweak this again later. I almost always have the HPF in the sidechain for mastering compression, very rarely off altogether. It really adds to compression transparency. 'Suck, Thwap, Suck, Thwack', it still sounds bad, even though it's now "bouncing" along nicely with the music.
5) Move on to the Ratio control. Bring the Ratio control down as far as you possibly can, all the way if possible, until that groove you have established becomes almost subliminal. Mastering ratios are often very low, like 1.1:1 or 1.5:1, very rarely more than 2.0:1. You've now got that groove sounding really nice!
6) Threshold or Input Gain should be set so that the amount of Gain Reduction is never clearly audible. Try not to do more than about 1.5-3dB of Gain Reduction on the highest peaks. You have added a subliminal certain something to the snap and groove that the artist will hopefully appreciate. If they had wanted an obvious 'Suck & Thwap', they would have done it themselves, or asked you to do it. If they did ask you, that's great, go ahead and slam it a little more by lowering the threshold, pushing the input gain harder, or raising the Ratio back up.
7) It should hopefully now sound much better, have a certain 'je ne sais quoi', glue, warmth, togetherness blah blah blah when you A/B it with the original track. You are done!
Notes: Play with the compressors' extra controls, as they all work and sound slightly different. My Chandler Germanium compressors are great because they also contain two input and two output transformers you can gain stage for more or less saturation, and an output gain with feedback control to adjust the type of 'colour'. They also offer six different transfer curves/knee types, I usually use the two or three widest knee types as they are the most transparent/least "grabby". I also have a "Clean/Dirty" switch which changes the amount of harmonic distortion the gain reduction causes. Usually Clean, but sometimes Dirty can be just what the doctor ordered.
I hope that was somewhat helpful. Feel free to post any comments or alternative ways of working, I'd love to hear feedback!