Parallel Compression

Parallel Compression (PC from here on) is a method to add compression without loosing transients or audio fidelity in general. It is traditionally used to make drums more stable (to stabilize them) without loosing the attack but could be theoretically used to alter/add compression on anything in your track. For dance music producers, PC is a commonly used method for sound design, especially on drums. PC is also known as New York Compression, Motown Compression or Upwards Compression. The story goes that this method was discovered by a Motown studio engineer in New York, hence the variations of the Name.

How It's Done
A common problem when you apply compression is that it while it might make your audio louder and more powerful, it will also lead to a loose of fidelity. Your drums can loose attack for example. PC is a simple but effective method to combat those problems. The basic idea of PC is keep the fidelity of the uncompressed material but getting the power of the compression by mixing the uncompressed audio with a compressed copy of the audio. Doing this is pretty simple. Duplicate the audio and apply the compressor only to the second channel. Now you can mix the original and the compressed material to taste. This can be very useful for use on a drum bus, as you will keep the attack of the original drums, but can you can add as much additional power from the compressed drums as you like. You could also equalize the compressed drum bus and compress only certain parts of the frequency range.

Links
Videos which explain this technique in detail:

http://www.youtube.com/watch?v=iAT1P0UBF58&hl=de Good explanation, uses Fruity Loops

http://www.youtube.com/watch?v=i-NxcOgIGB8 - Good explanation, uses Pro Tools

http://vimeo.com/4677832 - Tios on PC by Mike Chav, an american studio engineer, gives some interesting thoughts how to use PC on other stuff than drums.