Many of the folks I speak with have no idea what "mastering" is. Here's the scoop:
Mastering is the final production step a record goes through before a master is sent to the duplication plant.
Mastering involves these four mandatory (in my opinion) steps which are done to the mixed two-track (stereo) material:
1. Cleaning up beginnings and endings of songs with edits and/or fades.
2. Establishing the spacing between tracks that "feels" good.
3. Balancing the relative levels between songs, so song A doesn't feel loud or soft compared to song B or C... Generally this is based on the lead vocal, and is done by ear.
4. Generating the code information denoting track start and end locations. These are called PQ codes because those are the sub-sectors of the CD where the data is written. The duplication plant needs this info to do their job correctly.
Mastering can ALSO involve: Additional 2 track processing like compression, limiting, eq and a host of other processes can be done at a mastering house. Removing extraneous noises, clicks, pops and the like are also things that are frequently done when mastering. If you can keep extraneous noises out of the mixes, so much the better. Sonic micro-mangement can also be done; raising and lowering or eq-ing sections of music, or even individual notes to get the best from a piece. CD-Text title and artist names can be enbedded on the cd, but this info is only used by a few players (mostly in cars-- though I do have one rackmount player here which displays cd-text). Most info that people see is from "Gracenote" also known as CDDB (cd data base), which is an online database of cd track and artist info. When you insert a cd into your computer iTunes (and most other computer media players) compare the number of tracks and their associated times to the Gracenote database and bring up the titles that match. Submitting to Gracenote is free and relatively easy. ISRC codes can also be added at the mastering stage. For more information please visit usisrc.org.
In general, a good mastering job can really help a record feel like a "record," and no mastering, or poor mastering, can sometimes make a cd feel more like a collection of songs than a cohesive whole. If you want to learn more about the technical stuff (dither,noise shaping, etc.), I'd suggest heading over to Bob Katz's mastering web site. He's got plenty of info there for those who are curious.
CDs have a maximum level that they can reproduce and as time has gone by some clever folks have found ways to keep nearly all the music on a disc within a very small dynamic range very close to the maximum level. Having a "loud" cd is not always the best thing for a few reasons. First, it's been well reseached that we humans respond to dynamic changes on an emotional level; which is to say that when a dynamic change happens we feel it and move with it. The super loud CDs have little or no dynamic variation. Second, a loud cd gets old fast. The ebb and flow of a record has a lot to do with volume. A CD at full bore for 45 minutes is pretty monotonous. Third, the standard compressor for radio (the Orban OptiMod) turns down signals that are too close to the maximum level to keep from over-modulating. The system is designed to only turn down the peaks of the music and otherwise bring things up to an average level, however, the Optimod sees these maxed out CDs as ALL peak. So it all gets turned down. There goes the idea of making it louder for the radio. Though it's tempting to squash, please leave some dynamic range in your music! Go back and listen to Miles Davis' Kind of Blue if you have forgotten how integral dynamics are to a great sounding record that stands the test of time. Or compare the un-remastered Dire Straits Brothers in Arms CD with most any major label CD bought in the last year...which is more pleasant to listen to? More VERY good info is at http://www.turnmeup.org/ Regardless of how loud you want your cd to be, let your mastering engineer put the final squish on your record. Don't limit your mixes to death or we may have to turn entire tunes down to make them fit with other songs. If there's more dynamic range to play with, we can push the tunes up only as far as they need to go to fit with the other material on the disc, which makes for a more cohesive end product.