Mastering, a form ofaudio post production, is the process of preparing and transferring recorded audio from a source containing thefinal mix to adata storage device (themaster), the source from which all copies will be produced (via methods such as pressing, duplication orreplication). In recent years,digital masters have become usual, although analog masters—such as audio tapes—are still being used by the manufacturing industry, particularly by a few engineers who specialize in analog mastering.[1]
Mastering requires critical listening; however, software tools exist to facilitate the process. Results depend upon the intent of the engineer, their skills, the accuracy of the speaker monitors, and the listening environment.Mastering engineers often applyequalization anddynamic range compression in order to optimize sound translation on all playback systems.[2] It is standard practice to make a copy of a master recording—known as a safety copy—in case the master is lost, damaged or stolen.
In the earliest days of the recording industry, all phases of the recording and mastering were entirely mechanical processes. Performers sang or played into a largeacoustic horn and the master recording was created by the transfer of acoustic energy from thediaphragm of the recording horn to themastering lathe, typically located in an adjoining room. The cutting head, driven by the energy 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 fromwax; this gave rise to the colloquial termwaxing, referring to the cutting of a record.[4]
After the introduction of themicrophone andelectronic 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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).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 cutdirect-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 ofmagnetic tape. Magnetic tape was invented for recording sound byFritz Pfleumer in 1928 in Germany, based on the invention ofmagnetic wire recording byValdemar Poulsen in 1898. Not until the end ofWorld 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 rpmLP record—meant that the audio quality,dynamic range,[a] and running time[b] of master discs were still limited compared to later media such as thecompact 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 finalmix was prepared and dubbed down to the master tape, usually either a single-trackmono or two-trackstereo 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 asEMI, 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 inmulti-track recording[c] and did not install 8-track recorders in theirAbbey Road Studios until the late 1960s, more than a decade after the first commercial 8-track recorders were installed by American independent studios.[5]
In the 1990s, electro-mechanical processes were largely superseded by digital technology, withdigital recordings stored onhard disk drives or digital tape and mastered toCD. Thedigital audio workstation (DAW) became common in many mastering facilities, allowing the off-line manipulation of recorded audio via agraphical 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 ofdigital 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 enoughheadroom 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 aloudness war in commercial recordings.[8]
The source material, ideally at the originalresolution, is processed usingequalization,compression,limiting and other processes. Additional operations, such asediting, specifying the gaps between tracks, adjusting level, fading in and out,noise reduction and other signal restoration and enhancement processes can also be applied as part of the mastering stage.[8] The source material is put in the proper order, commonly referred to as assembly (or 'track') sequencing. These operations prepare the music for either digital or analog, e.g. vinyl, replication.
If the material is destined for vinyl release, additional processing, such as dynamic range reduction or frequency-dependent stereo–to–mono fold-down and equalization may be applied to compensate for the limitations of that medium. For compact disc release,start of track,end of track, andindexes are defined for playback navigation along withInternational Standard Recording Code (ISRC) and other information necessary to replicate aCD. Vinyl LP andcassettes have their own pre-duplication requirements for a finished master. Subsequently, it is rendered either to a physical medium, such as a CD-R or DVD-R, or to computer files, such as aDisc Description Protocol (DDP) file set or anISO image. Regardless of what delivery method is chosen, the replicator factory will transfer the audio to aglass 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. 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.
Steps of the process typically include the following:
Examples of possible actions taken during mastering:[8]
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Amastering engineer is a person skilled in the practice of takingaudio (typically musical content) that has been previously mixed in either theanalogue ordigital domain as mono, stereo, or multichannel formats and preparing it for use indistribution, whether by physical media such as a CD, vinyl record, or as some method of streaming audio.
The mastering engineer is responsible for a final edit of a product and preparation for manufacturing copies. Although there are no official requirements to work as an audio mastering engineer, practitioners often have comprehensive domain knowledge of audio engineering, and in many cases, may hold an audio or acoustic engineeringdegree. Mostaudio engineers master music or speech audio material. The best mastering engineers might possessarrangement and production skills, allowing them to troubleshootmix issues and improve the final sound. Generally, good mastering skills are based on experience, resulting from many years of practice.
Generally, mastering engineers use a combination of specialized audio-signal processors, low-distortion-high-bandwidth loudspeakers (and corresponding amplifiers with which to drive them), within a dedicated, acoustically-optimized playback environment. The equipment and processors used within the field of mastering are almost entirely dedicated to the purpose; engineered to a high standard, often possessing low signal-to-noise ratios [at nominal operating levels] and in many cases, the incorporation of parameter-recall, such as indented potentiometers, or in some more-sophisticated designs, via a digital-controller. Some advocates for digital software claim that plug-ins are capable of processing audio in a mastering context, though without the same degree of signal degradation as those introduced from processors within the analog domain. The quality of the results varies according to the algorithms used within these processors, which in some cases, can introduce distortions entirely exclusive to the digital domain.
Real-time analyzers, phaseoscilloscopes, and also peak, RMS, VU and K meters are frequently used within the audio analysis stage of the process as a means of rendering a visual representation of the audio, or signal, being analyzed.
Most mastering engineer accolades are given for their ability to make a mix consistent with respect to subjective factors based on the perception of listeners, regardless of their playback systems and the environment. This is a difficult task due to the varieties of systems now available and the effect it has on the apparent qualitative attributes of therecording. For instance, a recording that sounds great on onespeaker/amplifier combination playing CD audio, may sound drastically different on a computer-based system playing back a low-bitrateMP3. Some engineers maintain that the main mastering engineer's task is to improve upon playback systems translations while the position of others is to make a sonic impact.[9]
Prolonged periods of listening to improperly mastered recordings usually leads to hearing fatigue that ultimately takes the pleasure out of the listening experience.[10]
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