What is mastering and why do I need it?
This subject gets really deep, really quick so I'll attempt to explain all this in very broad strokes.
Simply put, mastering is the process of preparing and organizing your music for mass distribution. Mastering is both an artistic and a technical process so in order to understand it, you need to examine it from both perspectives.
In the big leagues, audio mastering is accomplished in what's called a "mastering studio". Mastering studios are designed, equipped and staffed specifically to handle all the chores associated with mastering audio for wide-spread commercial release. The mastering studio is the bridge between the recording studio where you recorded your project and the duplication company that cranks out the millions of copies of your album that end up on record store shelves.
The typical scenario goes something like this: you go into a recording studio to record songs for your upcoming album. Each song consists of many individual tracks of audio that have to be mixed together. After all the tracking and editing is finished, the songs are mixed down to stereo and then burned onto a CD-R disc. You can play that CD-R disc in any compact disc player and it sounds pretty good (hopefully) but it's not necessarily the thing you'd go to press with. The songs might not be in the correct order, or you may have recorded more songs than needed for the final product. The music might sound great in one place and not so great somewhere else. The point is, there's still more work to be done. The next step is the mastering studio. The mastering studio engineer takes the CD-R disc (or it could be an analog reel tape or WAV files) that you got from the recording studio and runs the audio through special mastering processors to improve the sound quality. Then they determine the track order and the amount of quiet time (if any) between the tracks. Once that's done, they incorporate other elements into the final product such as ISRC and PQ codes, CD text, index marker locations, hidden tracks, PC features (some audio discs have special hidden PC features which are activated when you insert the disc into your computer). Once all that final prep work is done, they burn another CD-R which is referred to as the "pre-master". The pre-master is sent to a duplication company, along with all the album artwork, where the final product is produced. Vinyl is way more involved! When it comes to vinyl we're talking lab coats and microscopes and oil and lathes and EQ curves and... well... it's a mess.
From an artistic viewpoint, there are usually a number of things that can be done to improve the sound quality of a recording. When I say "improve", what I really mean is that the audio is being modified in order to make sure it translates well in the outside world. People listen to music on all different types of stereo equipment and in many different environments (a car, a home theater, an iPod, a desktop PC, a dance club, a boombox, etc). It's the function of the mastering studio and the mastering engineer to make sure that the final product sounds good everywhere.
In order to accomplish this, mastering studios are equipped with ridiculously expensive, exotic, audiophile-grade playback equipment, in rooms designed from the ground up to reveal all the nuances of a recorded performance. If you've never experienced the thrill of listening to music on a playback system costing $100,000 (or more), in a room which has been finely-tuned for critical listening, then you don't know what you're missing. Sonic details that might have otherwise been clouded or masked are brought to life. The mastering engineer is trained to listen for these sonic details and to make whatever corrections or changes are needed to improve the quality of the audio.
The other main artistic consideration to mastering is song selection and track order. A band will sometimes record more songs than they need for the final product. Once you start piecing everything together, you get a clearer picture of how the final product is going to sound. Track order is an important part of the album mastering process. You basically want to get the songs to flow into and out of each other. As with the rest of the process, selecting which song is first, second, third, etc. is a very subjective call. I've seen heated discussions on the subject. The alpha male usually wins out :-) I read an interview in Recording magazine a while back with David Singleton who is the mastering engineer for Robert Fripp and King Crimson. David & Robert would mix an album and put the songs in a particular order. Then they would burn a CD and take it home and listen to it over the course of many days or weeks. As they were listening to the CD, they would note the point where their minds would start to wander off. They would then note that particular song and replace it with something else. This exercise would continue over and over until they could listen to the entire CD without losing attention. So, in effect, each song was in the perfect order to keep their attention focused on the music.
I guess it's worth mentioning that the internet is fast becoming the main method for music distribution. Compact discs sales are plummeting. The album "concept" might be dying along with it. These days, iTunes/MP3 singles are king. The idea that songs are meant to be played in a specific order doesn't hold much weight anymore thanks to portable listening devices like the iPod. So perhaps, the mastering concept will eventually change also. I do a lot of singles mastering which means I deal mostly with just a few songs or songs in no particular order these days. Irregardless of how many songs are involved, my main goal is make sure each individual song competes with other songs on the market. Since the iPod "concept" is so popular, you basically want your music stand up to other songs - especially if they are in rotation on an iPod. You don't want your songs to be lower in volume or muddy compared to whatever else might be out there (unless that's a deliberate decision made by the artist).
The technical process of mastering involves such things as:
* Normalization (or peak limiting). The volume level of most commercially released music these days has been "pumped up" using mastering compressors and brick-wall limiters. So you will probably want to make sure your music has also been "pumped up" to commercial standards if you intend to compete with what's already out there. You also want the overall volume level of the album to be consistent from song to song.
* ISRC codes (International Standard Recording Codes) - the ISRC is an international standard for song identification codes used mainly to track airplay (including internet radio) or digital downloads. Each song on an album gets a unique code. These codes used to be free but now there is a modest cost to obtain them. Most of the better CD replicators out there can provide the codes so if you're going to be ordering duplicates from them anyway, might as well just order the codes at the same time.
* CD text, PQ codes and other embedded mumbo jumbo. Important? Yes. Critical? No.
* Analog-digital conversion. This is very important if your music was mixed down to analog tape (still very common in the big leagues). Quarter-inch reel tape is still used for mixdown. Yes, analog still competes favorably in this digital age we live in.
* Phase cancellation - Mastering studios have been known to kick a mix back to the recording studio from whence it came in order to fix phase cancellation problems (among other things). Phase cancellation is a technical problem that potentially occurs when you record a single sound source with more than one microphone. Sounds might be reaching the microphones at different times and when the two signals are mixed together, they start to cancel each other out. The effect is like you're listening to the sound from inside a tin can - kinda hollow sounding. This can't be fixed in mastering. Has to be corrected during mixdown.
Coffee is running out and your brain is starting to hurt, ain't it?
How do I do it?
The answer to this question depends on whether I'm doing the tracking here or if I'm working with stereo mixes done from somewhere else. I'll be honest, I don't have a single magic recipe for mastering. I approach each project as unique and pick-and-choose the tools I need based on the type of music being recorded (or the needs of the artist in general). Specifically, I use software plug-ins for mastering.
Samplitude (my main recording application), comes with a host of plug-ins designed specifically for audio mastering. Generally, I always have a mastering compressor inserted into the master bus of the program mixer so that the artist can hear an approximation of how the overall mix will sound with a bit of mastering "muscle". I don't spend a lot of time tweaking the mastering settings during tracking. I'll pull up a preset that seems appropriate and just go from there. Later on, after tracking and mixing is done, I'll spend a little bit fine-tuning the mastering settings.
If I'm doing the recording (meaning I have all the individual tracks at my fingertips), I usually don't go to the mastering plug-ins if the overall mix needs more or less of something. For example, if the mix needs a little more low end I will go to the bass and/or kick drum tracks and work directly from them. If something needs to be brightened up, I'll go directly to that particular track and change the EQ settings on that track. That's a much different scenario compared to working with a stereo mix from another studio. With a stereo mix, I don't have the ability to get at individual elements as easily (it's just a left and right two channel stereo mix-down). So if I need to change something within the two channel stereo mix, I'll need to use the mastering plug-ins to try and surgically "get at" those frequencies. Occasionally, the client brings me "stems" which are various "groupings" of tracks. Stems are sort of like submixes of elements of the overall final mix. Or I get multiple mixes done with smallish changes (like vocals mixed up or down at different volume levels so we can pick the best sounding mix).
The mastering processors are combinations of tools - usually consisting of a multi-band compressor limiter, EQ with graphic spectral analysis adjustments, master compressor/limiter, stereo widening, reverb or delay effects, etc. How much or how little I choose to use depends on the source material. Rock music (in general) gets a "loud" treatment - mainly to compete with all the other loud mastering jobs out on the market. Do a search for loudness wars and you'll find plenty of information regarding how and why music sounds loud and squashed these days. Classical and folk music tends to have very gentle mastering settings.
Once all the songs are mixed and mastered, then we then group all the songs together and establish the song order and the amount of quiet space between each song. I use Sony's CD Architect program for this purpose. After all the songs have been properly positioned and the track indexing is complete, the final step is to burn a (pre)master CD-R to Red Book specs. Voila! We're done. Now you can take that (pre)master CD to any duplication service and have thousands of CD's printed up. The (pre)master should be treated with respect. Don't use the (pre)master CD for anything other than making dup's. Put it away in a safe place when you're done.
A few extra comments and observations:
It's generally not a good idea to record, mix and master all in the same day. Ear fatigue is a very real problem. I prefer to give my ears at least 12 - 24 hours to decompress before mixing and mastering a project. It's amazing how different things can sound when you've been away from the project for a while.
I like to spot-check the volume levels of each song so that there isn't a big difference in volume from one song to the next. I find this works best by quickly jumping from song to song and noting the loudness of each song. I want the first song and the last song and everything in between to be more or less the same volume level. I do this work with Sony CD Architect which is the program I use for CD authoring. If there is a noticeable difference in volume for a particular song, I will go back and remaster the quieter song so it's more in line with it's neighbors. You don't want people reaching for their volume controls all the time.
I always tell the client to spend a few days with the final master and listen to it on as many stereos and in as many different environments as possible. If any changes are needed, the client should make detailed notes by track number and return to the studio for another pass. The goal is to get the master to sound good everywhere. If your CD sounds good in one place and bad in another, consider the location and type of equipment being used before you freak out. Some rooms/stereos aren't very musical sounding and you may think you're hearing problems with the mix when, in fact, you're hearing other unrelated problems.
Give yourself a week or two, if possible, to live with the music before you start duplicating. Wait a day or two before doing any critical listening. A fresh perspective always seems to bring out things you missed the first time. I've gone back and listened to mixes/masters I did months or years ago and usually always hear something I think could have been done better. So, the point is, the more time you give yourself for mastering, the better the final product will sound when you go to press.