After the introduction of the microphone and electronic amplification in the late 1920s, the mastering process became electro-mechanical, and electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).
However, until the introduction of tape recording, master recordings were almost always cut direct-to-disc. Artists performed live in a specially designed studio and as the performance was underway, the signal was routed from the microphones via a mixing desk in the studio control room to the mastering lathe, where the disc was cut in real time.
Only a small minority of recordings were mastered using previously recorded material sourced from other discs.
Running times were constrained by the diameter of the disc and the density with which grooves could be inscribed on the surface without cutting into each other. Dynamic range was also limited by the fact that if the signal level coming from the master tape was too high, the highly sensitive cutting head might jump off the surface of the disc during the cutting process.
From the 1950s until the advent of digital recording in the late 1970s, the mastering process typically went through several stages. Once the studio recording on multi-track tape was complete, a final mix was prepared and dubbed down to the master tape, usually either a single-track mono or two-track stereo tape.
Prior to the cutting of the master disc, the master tape was often subjected to further electronic treatment by a specialist mastering engineer. After the advent of tape it was found that, especially for pop recordings, master recordings could be optimized by making fine adjustments to the balance and equalization prior to the cutting of the master disc.
Mastering became a highly skilled craft and it was widely recognized that good mastering could make or break a commercial pop recording. As a result, during the peak years of the pop music boom from the 1950s to the 1980s, the best mastering engineers were in high demand.
In large recording companies such as EMI, the mastering process was usually controlled by specialist staff technicians who were conservative in their work practices. These big companies were often reluctant to make changes to their recording and production processes—for example, EMI was very slow in taking up innovations in multi-track recording and they did not install 8-track recorders in their Abbey Road Studios until the late 1960s, more than a decade after the first commercial 8-track recorders were installed by American independent studios. As a result, by the time The Beatles were making their groundbreaking recordings in the mid-1960s, they often found themselves at odds with EMI's mastering engineers, who were unwilling to meet the group's demands to push the mastering process because it was feared that if levels were set too high it would cause the needle to jump out of the groove when the record was played by listeners.
In the 1990s, electro-mechanical processes were largely superseded by digital technology, with digital recordings transferred to digital masters by an optical etching process that employs laser technology. The digital audio workstation (DAW) became common in many mastering facilities, allowing the off-line manipulation of recorded audio via a graphical user interface (GUI). Although many digital processing tools are common during mastering, it is also very common to use analog media and processing equipment for the mastering stage.
Just as in other areas of audio, the benefits and drawbacks of digital technology compared to analog technology is still a matter of debate. However, in the field of audio mastering, the debate is usually over the use of digital versus analog signal processing rather than the use of digital technology for storage of audio.
Although in reality there isn't such a thing as an "optimum mix level for mastering", the example on this picture to the right only suggests what mix levels are ideal for the studio engineer to render and for the mastering engineer to process. It's very important to allow enough headroom for the mastering engineer's work. Many mastering engineers working with digital equipment would agree that a minimum of 3 to 6 dB of available headroom is critical to perform good mastering. Ideal peak levels should not exceed -3dBFSD and the average sum of the left and right channels should be at around -10 to -18 dBFSD (As shown on the picture to the right).
There are mastering engineers who feel that digital technology, as of 2007, has not progressed enough in quality to supersede analog technology entirely. Many top mastering studios, including Bernie Grundman Mastering (which has mastered 37 Grammy-nominated albums), and Gateway Mastering, still embrace analog signal processing (such as analog equalization) within the mastering process. Additionally, the latest advances in analog mastering technology include 120V signal rails for previously unavailable headroom of 150dB as well as frequency response ranging from 3Hz to 300kHz. In order to duplicate this frequency response in digital domain, a sampling rate of at least 600kHz would be required, by the Nyquist–Shannon sampling theorem. However, it is pertinent that the extremes in this frequency range (3 Hz - 300kHz), are effectively inaudible, existing outside the range of most professional microphones.
The specific medium varies, depending on the intended release format of the final product. For digital audio releases, there is more than one possible master medium, chosen based on replication factory requirements or record label security concerns.
A mastering engineer may be required to take other steps, such as the creation of a PMCD (Pre-Mastered Compact Disc), where this cohesive material needs to be transferred to a master disc for mass replication. A good architecture of the PMCD is crucial for a successful transfer to a glass master that will generate stampers for reproduction.
The process of audio mastering varies depending on the specific needs of the audio to be processed. Steps of the process typically include but are not limited to the following:
Examples of possible actions taken during mastering:
The guidelines above are mainly descriptive of the mastering process and not considered specific instructions applicable in a given situation. Mastering engineers need to examine the types of input media, the expectations of the source producer or recipient, the limitations of the end medium and process the subject accordingly. General rules of thumb can rarely be applied.