Mastering (audio)

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, 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.[3] 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.[4]

After the introduction of the microphone and electronic amplifier 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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded). Until the introduction of tape recording, master recordings were almost always cut direct-to-disc.[3] Only a small minority of recordings were mastered using previously recorded material sourced from other discs.

In the late 1940s, the recording industry was revolutionized by the introduction of magnetic tape. Magnetic tape was invented for recording sound by Fritz Pfleumer in 1928 in Germany, based on the invention of magnetic wire recording by Valdemar Poulsen in 1898. Not until the end of World War II could the technology be found outside Europe. The introduction of magnetic tape recording enabled master discs to be cut separately in time and space from the actual recording process.[3]

Although tape and other technical advances dramatically improved the 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 later the 7-inch 45 rpm single and 33-1/3 rpm LP record—meant that the audio quality, dynamic range,[lower-alpha 1] and running time[lower-alpha 2] of master discs were still limited compared to later media such as the compact disc.

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.

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[lower-alpha 3] and 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.[5]

Optimum Digital Levels with respect to the Full Digital Scale (dBFSD)

In the 1990s, electro-mechanical processes were largely superseded by digital technology, with digital recordings stored on hard disk drives or digital tape and mastered to CD. 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 are still a matter for 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.[2]

Digital systems have higher performance and allow mixing to be performed at lower maximum levels. When mixing to 24-bits with peaks between -3 and -10 dBFS on a mix, the mastering engineer has enough headroom to process and produce a final master.[6] Mastering engineers recommend leaving enough headroom on the mix to avoid distortion.[7] The reduction of dynamics by the mix or mastering engineer has resulted in a loudness war in commercial recordings.[8]