In a nutshell; it is the process of getting your mixed track prepared for release on it's intended format (CD, Online Download, Vinyl) using a mixture of technical and creative engineering techniques.
The creative side of mastering is allowing tracks to sound the best that they can individually, whilst at the same time, flowing nicely with the other tracks on the release. Changes made to the audio are of a subtle nature; simply refining and enhancing a few overall details, retaining the feel and energy created at the mixing stage.
This means using just small adjustments of signal processing (normally EQ, compression, limiting) to help balance and to add shine to a mix. More significant changes should have been made at the mix stage as it is far easier to adjust individual elements of a mix (vocals need to be louder, a common issue).
Once the tracks have been tweaked and are all sitting well together sonically; the next part of mastering is the limiting stage. This involves bringing the track's gain up to the maximum acceptable volume; what is acceptable is a highly contentious issue on it's own. I have written about it in more detail below in the 'HOW LOUD?! SORRY I CAN'T HEAR YOU!' section but the long and short of it is that louder isn't always better!
After limiting; track spacing, gaps and overlaps are decided. This dictates how the record will ebb and flow as a whole. The final stage is to convert tracks to the correct file type(s) and to do any required labelling and coding of the tracks.
'How loud?! Sorry, I can't hear you!'
Ever since the 1990's, you may or may not have been aware of something called the 'Loudness War' within the music industry. Put simply; with technological advances allowing for far more aggressive audio limiters (known as brickwall limiters); it has become a competition to see who can make the loudest record; with the loudest tracks standing out the most on radio playlists.
Why is this important you ask? What's wrong with being louder? Well in exchange for maximum volume, a track will lose it's dynamics, sometimes it's feel and often becomes fatiguing to listen to. Put on your favourite records from the 60s, 70s and 80s, notice how much quieter they are? Now turn up your stereo; notice how much more life they seem to have compared with 'loud' mainstream tracks from today, when listened to at comparable volumes?
It's a hard one to call as untrained ears (the majority of listeners for most records) won't necessarily notice the loss of dynamics and will simply hear a louder master as better. Therefore when handing the mixes over to the mastering engineer; the artist must decide themselves how far they are willing to compromise on sound in return for loudness.
But no matter what you decide, be it loud or dynamic and regardless of my own preferences; as the mastering engineer I'm here to help you achieve the best possible outcome with minimum compromising on the quality of audio.
Want to know more? Google 'Loudness War' and take a look at this site: Turn Me Up
My approach to mastering...
I approach a new track with the outlook that the band or artist has signed off on the mix because they love the sound. That means for me that I am not looking to make drastic changes but simply to make small changes so that the track will sound the best that it can on as many playback devices as possible, giving the best representation of the music.
In terms of volume; I look to find a good compromise between volume and dynamics that I think suits the mix and genre. I usually veer on the quiet, more dynamic side. Quieter tracks also tend to convert better to compressed file formats (MP3, AAC, etc) and whilst it would be great if the world was geared towards higher quality audio files; most people these days consume their music from compressed files (something to bear in mind with your loudness decision).