There are as many ways to master a song as there are to arrange a song, perform a song, record a song, or mix a song.
There are a few strategic purposes to mastering a song that go beyond the tactical level of things like "adding punch" or "smoothing". Any number of these purposes can come into play for any given song, depending on the situation:
CUT AND POLISH
The mastering engineer (ME) may talk to the performer, producer and/or tracking/mixing engineer to get an idea of just what those that came before him in the chain have in mind as far as what they have had in mind in creating the production up to that point as far as mood, style and effect. The ME then listens to the mixes given him and decides just what needs to be done to make what's there sound "better". He/she then applies whatever processing they feel the rough mixdown needs to make it shine as best as possible and as close to the desired effect as possible.
Think of this in much the same way as a jeweler whose given a rough diamond and asked to make a such-and-such style wedding ring out of it. They first have to examine the diamond and consider what kind of look or cut the client is looking for to decide just where to cut the diamond and how, where the facets should go, what impurities to hide and what ones to showcase, and then once cut, poilish it to a perfect shine.
Just as the jeweler doesn't automatically take any old diamond and give it a rose cut, and even if he does, doesn't just arbitrarly make the cut anwhere but rather must first figure out the crystalline orientation of the stone, the ME doesn't automatically assume that they are going to "add punch", and even if they do, they don't just arbitrarily set the limiter to 'X' and boost away, they analyze the song and decide what it needs by its inherent nature.
THE SETTING
Just as many diamonds, once prepared, are then placed into a setting, most songs an ME gets are not stand-alone productions but are just one of many songs to go onto a CD. In these cases, the ME usually also has to consider just how the song he is mastering is going to fit into and match the rest of the CD, just as the jeweler has to match the right stone to the right setting.
Here they must consider how the RMS (average) volume of the song is going to fit with the volume of the other songs. Not every song can or should be maxed to -3dBRMS, it depends upon the genre, style and arrangement of the song and mix. Also decisions have to be made about the flow of the CD; how the song will match up with the previous and next song sonically and temporaly. Not only must they sound right tone- and volume- wise in sequence, but crossfades may have to be made, timing betwen tracks tempoed out, etc.
THE ENGRAVING
Finally, once the ring is made, there is the engraving to be done. For the ME, this is the burning of the premaster disc (often referred to as the "master", though this can technically be a misnomer in many cases). This involves ensuring solid Redbook specs and solid error rates, and it also involves proper PQ editing, and optional inclusion of CD text and registration codes when requested, etc. It's not just a matter of burning a CD-R with Nero; they do it to much tighter specification than your average home burner and cross a lot of Ts and dot a lot of Is that many of us don't even realize exist.
SUMMARY
Mastering is a process just like everything else in the production chain is. What is actually performed in that mastering process is just as dependant upon the content of the mix and the desires of the clients as everything else is. And just like every other process in the chain, no two mastering sessions are identical and there are not canned "recipes" for a proper mastering job.
This is the big difference between real mastering engineers and home recordinsts that offer bargain basement "mastering". The bargain basement shops are making money off of the uneducated ear (a.k.a. 99% of their clients) by running all music through canned procedures using off-the-shelf plugins (can you say "maximize"? I knew you could!) and then A/Bing the difference. The client will hear "louder" and then go "Oooh!"
The real mastering engineers not only know that this is an ancient parlor trick once used to sell 5-band EQs to unsuspecting hi-fi customers and to sell speakers by putting their Bic lighter in front of a tuned bass port and then hitting the "Loudness" button on the receiver to blow the lighter out with a little bass, but knows that it takes much more than parlor tricks to polish a diamond.
G.
