What is mastering?

CD mastering has a reputation as a mysterious art known only to a musical high priesthood. In fact, the right mastering can make a good recording sound excellent, and possibly turn a great one into a legend. Most discs can be made competitive for radio and in-store play.
To use an analogy - Every album has a "voice" in which the message of the artist is delivered. A strong performance and good recording technique will set the basic tone for this voice. Mastering can then profoundly affect it's impact and resonance. How? A wide range of techniques can bring out an album's best sound. Depth, punch, sense of air and detail can all be enhanced. Experience and a feel for the music determine the best path.
The process: Most people are familiar with the idea of recording music in a live concert or multy track recording studio. You create individual tracks for voice, music, instruments to be mixed down to a 2 track stereo mix. This is not a mastered mix. It is the next step that is taken to get that great radio sound. The process of creating the final master is called mastering. It has three steps:

1. Sweetening: When engineers first began cutting master discs used to produce vinyl records, they designed signal processors such as compressors, limiters, and equalizers (EQ) to prevent overloading the cutter head. They noticed that changing the settings could also have beneficial effects on the music, especially in Pop styles. Equipment and techniques were developed to further "sweeten" the sound. Since then, this has been considered the heart of the process, where clarity, smoothness, impact and "punch" are enhanced, depending on the needs of the music. The goal is to increase the emotional intensity. If the performance, arrangement and recording quality are good to start with, then the final master sounds even better than the mix tapes, and even casual listeners can notice the difference. This is what albums need in the pop markets: big, radio-ready sound and a competitive edge.
2. Assembly editing: The tapes from the location recording or mixdown sessions are transferred to a digital editor. The tunes are sequenced into the order you specify and correct spacing is made between cuts. Beginnings and ends of cuts are faded to black (silence) or room tone (the natural background noise of the performing space), or the cuts are crossfaded as you wish. Pops, clicks and strange noises can often be fixed at the mastering stage, depending on their source and severity.
3. Output: The finished music tracks are transferred to the media needed for mass production, usually CDR (Recordable CD). This master disc can be played on any CD player, so the client can audition it and give final approval. Occasionally cassette replicators prefer 4mm DAT tapes.

You need a great mix to get a great master:

Most people who send albums to LAHitman Studios have created a competitive product. There are no major technical problems, and the overall level, balance and EQ are in the ballpark. The producer just wants a big, radio-ready sound. At the other extreme, there are mixes that were seriously flawed. Sometimes it’s better to do a remix than depend on the mastering to fix the problem. .
This brings out a key point: mastering can only improve a good product. It cannot fix bad mixes, mushy tracks, poor arrangements or sloppy playing. If the kick drum sounds great and the bass guitar is terrible, there will be problems getting both to sound good. If you are at the mixing stage, I welcome you to send an early reference mix for free evaluation. I may hear a problem that you can fix before mastering. 

Note - To CD replicators, mastering means creating the glass master disc that is used to make stampers (which are then used to press the CDs). Almost every CD plant prefers to make it's own glass masters, for quality control reasons. The glass master should be a perfect mirror image of the CDR master disc produced by the music mastering facility. producing a CD.

Why not just transfer my mixes to CDR?

If your mixing engineer is one of the rare few who can mix and master at the same time, then a straight transfer might do fine. But there are at least three reasons why professionals send tapes out for mastering:

1. Usually a lot can be done to improve the mixes. The market is demanding. If you want the disc to compete in the radio markets, in-store play, and the homes of consumers accustomed to excellent music, it has to be right sonically. It's like very expensive cars - without a good paint job, few people will appreciate how great the car is. Also, since the mixes were recorded at different times of day over a week or more, you end up with differences in level and tone. Mastering creates a seamless whole out of a collection of individual tracks.
2. The mastering engineer has fresh, experienced ears. By the time the mixes are done, everyone involved is fried. It's tough to keep perspective after you've heard a mix 50 times. A new outlook, a new set of listening skills - attuned to the complete presentation rather than the detail of the mixes - can make a huge difference.

Mastering engineers must be fluent in both the artistic and technical areas of music-making. Good communications skills are also critical, since a lot of terms are used to describe sound. For example: "It needs to sound big, resonant, fat, warm, ambient, taut, sweet, present, smooth and live. It needs air, sparkle, depth, brilliance, impact, punch, focus, clarity and definition." The goal is to understand and accept the producer's guidance and then add or remove only what the music requires, not a bit more. It sounds simple... (Was it Michelangelo who referred to carving statues as "getting rid of the unnecessary stone"?) Often the changes are subtle, but the response from the producer or label exec should always be "That's it! Sounds like a record!"

3. The mastering facility has ultra-clean processors that are built to handle stereo signals. This may be obvious - it is one thing to run a guitar through a limiter and equalizer, and another thing to run your whole mix through it. A finished mix is a complex balance that can be made worse as easily as it can be improved. Its worth using the best equipment available.