Mastering, a form of audio post-production, is the process of preparing and transferring recorded audio from a source containing the final mix to a data storage device (the master); the source from which all copies will be produced (via methods such as pressing, duplication or replication). The format of choice these days is digital masters although analog masters, such as audio tapes, are still being used by the manufacturing industry and a few engineers who have specialized themselves in analog mastering.
In the earliest days of the recording industry, all phases of the recording and mastering process were entirely achieved by mechanical processes. Performers sang and/or played into a large acoustic horn and the master recording was created by the direct transfer of acoustic energy from the diaphragm of the recording horn to the mastering lathe, which was typically located in an adjoining room. The cutting head, driven by the energy transferred from the horn, inscribed a modulated groove into the surface of a rotating cylinder or disc. These masters were usually made from either a soft metal alloy or from wax; this gave rise to the colloquial term waxing, referring to the cutting of a record.
After the introduction of the microphone and electronic amplification in the mid-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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).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.
The recording industry was revolutionized by the introduction of magnetic tape in the late 1940s, which enabled master discs to be cut separately in time and space from the actual recording process. Although tape and other technical advances dramatically improved audio quality of commercial recordings in the post-war years, the basic constraints of the electro-mechanical mastering process remained, and the inherent physical limitations of the main commercial recording media—the 78 rpm disc and the later 7-inch 45 rpm single and the 33-1/3 rpm LP record—meant that the audio quality, dynamic range, and running time of master discs were still relatively limited compared to later media such as the compact disc.
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 made so that the resulting record would sound better. This was done by making fine adjustments to the amplitude of sound at different frequency bands (equalization) prior to the cutting of the master disc.
Record mastering became a highly prized and 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 as well as Marty Munsch, 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 3 Hz to 300 kHz. In order to duplicate this frequency response in digital domain, a sampling rate of at least 600 kHz would be required, by the Nyquist–Shannon sampling theorem. However, it is pertinent that the extremes in these frequency ranges (3 Hz to 19 Hz and 21 kHz to 300 kHz) are effectively inaudible and only existing outside the range of both the human ear and most professional microphones.
The music mastering studio is very different from a normal audio recording studio. In fact, all the equipment and gear found in most recording and mixing studios can actually hinder the acoustics of a room to accurately monitor sound. Thus, the correct room acoustics and arrangement of the equipment inside a mastering studio is an important factor since the mastering engineer (ME) needs to hear in detail each mix. This room design should be non-environmental or with a minimum room interference. By working with an experienced mastering engineer, the recording artist is also open to more creative opinions and technical advice.
The source material, ideally at the original resolution, is processed using equalization, compression, limiting, noise reduction and other processes. More tasks, such as editing, pre-gapping, leveling, fading in and out, noise reduction and other signal restoration and enhancement processes can be applied as part of the mastering stage. Once all the material has been “mastered” and is aurally satisfactory, the premastering process begins. This step prepares the music for either optical disc or vinyl replication. The source material is put in the proper order, commonly referred to as assembly or (track) sequencing.
If the material is destined for vinyl release, additional processing, such as dynamic range reduction, frequency dependent stereo–to–mono fold-down and equalization, may be applied to compensate for the limitations of that medium. Finally, for compact disc release, Start of Track, End of Track, and Indexes are defined for disc navigation. Subsequently, it is rendered to either a physical medium, such as a CD-DA–formatted CD-R, a CD-ROM–formatted CD-R or DVD-R or, to a DDP file set, the standard method of secure delivery for CD and DVD replication masters. 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 media, chosen based on replication factory requirements or record label security concerns. Regardless of what delivery method is chosen, the replicator will transfer the audio to a glass master that will generate metal stampers for replication.
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.