Before the invention of CDs and digital technology, mastering was done using mechanical processes that involved the master recording being cut direct to disc as the artist performed in the studio.
As technology advanced, particularly in the 1940s with the advent of the magnetic tape, the mastering process became separate from the recording process. Since then, mastering has become a more skilled craft, which has been known to make or break recordings.
Since the 1990s, digital technology has taken over the mastering process through the use of the digital audio workstation. This has helped the process become much more streamlined, requiring fewer stages than electro-mechanical mastering.
There are three main types of mastering used once audio recordings are ready to be mastered onto their final medium for CD duplication.
Glass mastering uses glass as a substrate to hold the master image of the CD while it is being created and processed. This requires a clean room free of contaminants such as dust, smoke, hair, etc., because these could cause errors rendering the master useless. The round plates of glass used must also be scratch-free as even minor scratches will affect the quality of the CDs produced from the master.
Photoresist mastering makes use of a light-sensitive material that forms the pits and lands on the master blank through the use of a blue or ultraviolet laser.
Dye-polymer mastering (also known as Non-photoresist mastering or NPR) also uses a laser to record onto the dye-polymer, creating pits in specific areas. This creates more accurate pit depths than photoresist mastering.
The process of mastering a CD can be time-intensive, and the steps required from recording to recording can vary depending on the quality of the original audio being used.
Processes used during this stage can include, but are not limited to, compression, equalization, limiting and noise reduction, as well as editing, fading, leveling and pre-gapping.
At any given moment during the mastering process, the masterer could be sequencing individual tracks into their final order, processing audio for the best sound quality, transferring audio onto the DAW, implementing noise reduction for background hisses, adjusting volume, editing noticeable flaws, fading tracks out as they near the end or applying peak sound limits for all tracks as a whole, among various other tasks.
The benefits of CD mastering are most evident when you listen to an audio recording both before and after it has been mastered. Audio that has not been mastered can have an amateur sound to it, which could be caused by a variety of reasons. There could be distracting hissing in the background. The loudness of the sound could vary throughout the track, being louder at some parts than at others. There could be gaps of silence in between tracks. None of these things, however, should be noticeable if a recording has been properly mastered.
Using a third-party mastering engineer who is not involved in the recording process can also be beneficial as he may notice additional problems or opportunities in the recording that have gone without notice by the recording team.
"A good-sounding album needs a good relationship between the bass and high end," says Tom Scheponik, a senior mastering engineer at DiscMakers.
"There has to be a clean bass response. Muddiness in the low end can make the sound of the song lose its focus."
Dave Hevalow, also a senior mastering engineer at DiscMakers, adds, "CD mastering is your last opportunity to make a positive impact on how your music will sound ... . Quality CD mastering is essential."