Mastering is a form of audio post-production that processes and prepares a final mix into a digital master. This digital master can then be used to produce commercial copies on any audio format required.
There are many processes that may need to be addressed during mastering. These can include: equalization and tonal balancing, dynamic compression/expansion, peak level limiting, noise reduction, stereo width adjustment/enhancement, de-essing and de-clicking, editing, phase correction, fading in/out.
mastering it is important
to carefully balance the music so that it will translate accurately on
any number of playback systems it may encounter. Audio must be optimized
for the most expensive
boutique hifi system, and the cheapest mini radio. It is important to
control the bass and low end content so that the music sounds great when
played in clubs and public venues.
Every mastering job is different and may require any of the processes mentioned above to correct or enhance certain aspects of a recording or mix. Some of the music I receive will benefit from some analogue dynamic compression, and tonal balancing (EQ). During mastering I will adjust for a correct musical balance if necessary, often making small dips and cuts to certain prominent frequencies and areas in the mix. I can achieve this very accurately in my acoustically treated and calibrated studio. Full range monitoring and high quality digital to analogue (DA) conversion allows me to hear minor details that may need to be delicately balanced to enhance the listening experience. Stereo width may need to be adjusted (widened or center focused), and editing and fading in/out of the beginning and end of the track may be required for smooth and clean transitions. Finally, I may use more dynamic compression or limiting to achieve commercial loudness.
There is some discussion these days about modern commercial music being pushed to the extremes of loudness using certain mastering techniques. Once the maximum amplitude for a digital audio signal is reached (0 dBfs), then perceived loudness can be increased further by using audio compression and brick wall limiting, to reduce the dynamic range of the music. Extreme dynamic range compression and limiting will add distortion artifacts to the audio signal, and the music will suffer as a result, sounding fatiguing and uncomfortable when listened to for extended periods.
In my own personal experience, some songs can be pushed louder than others before dynamic range reduction starts to have a detrimental effect on the sound quality of the music. I master a lot of bass heavy techno and electronic music which gets played by DJ's in clubs and live venues, so I usually push these tracks less to achieve a better quality low end. Compression and limiting are powerful tools when mastering, and applying them sensibly will allow a correctly balanced (dynamic) master to sound much better on a club system at high volumes, than an already pushed to the extreme loud master.
I am always sensible when I set final loudness, and I can achieve a final master that provides a healthy amount of volume whilst retaining mix transients, musical dynamics and bass content.
Of course if you require a louder than average master, (and some musical styles often benefit from being 'in your face' loud) then I can often get a couple more dB's of loudness for most tracks. I can do this in a way that minimizes the distortion artifacts that occur when dynamic compression and hard limiting are applied, in order to achieve a commercially competitive 'hot' digital master.