The magic of two-bus compression
The definition of a ‘mastering’ compressor is a little fuzzy, but it’s generally a design that will react gracefully to complex program material, without being too aggressive and ‘grabby’ on the attack, or pumping too obviously in release. FabFilter Pro-C 2’s Mastering mode is a perfect example: it can add density and glue, but it’s so gentle and well behaved that it’s difficult to make it sound bad.
TDR’s free Kotelnikov compressor can be similarly transparent, though it’s also capable of adding subtle colour, especially if you shell out for the GE version.
Sometimes, though, these designs are too ‘safe’, so we move into another category, equally loosely defined as ‘bus compressors’. These tend to be more aggressive and colourful, and a bit more prone to pumping (which can sometimes help to make a mix bounce and groove better), and feature complex, program-dependent behaviour that helps them to sound good on full mixes.
This kind of compression is often used on the master output at the mix stage, but if the mix needs more punch or glue, or needs to bounce more, go ahead and apply it at the mastering stage as well.
Cytomic’s The Glue plugin is a good example of this style, modelled after the bus compressors built into large SSL consoles. Ableton Live users will be familiar with this one, as there’s a version included in that DAW.
Alternatively, Klanghelm’s MJUC plugin is excellent, and very different in character, based on valve Vari-Mu designs instead of VCA. Three very different compressor models are included, which makes the €24 price tag even more of a bargain, all in all.
You can do mid/side with any compressor by converting manually from L/R to M/S and back, but it’s obviously more convenient to have it built into the plugin. FabFilter’s Pro-C 2 makes it easy to compress mid or side, though you’ll need two instances to do both at once. Elysia’s Alpha mastering compressor is a bit of a beast, and presents you with full control over mid and side compression simultaneously.
Be aware that compression adds extra harmonics, which can potentially result in aliasing. All the plugins mentioned above provide high-quality linear phase oversampling, which will reduce aliasing to inaudible levels. This needs to be specifically enabled in some compressors such as Pro-C 2 and The Glue, but it’s always on in Elysia Alpha.