Mastering (audio)

Magnetic tapewas commonly used to create master copies.

Mastering,a form ofaudio post production,is the process of preparing and transferring recorded audio from a source containing thefinal mixto adata storage device(themaster), the source from which all copies will be produced (via methods such as pressing, duplication orreplication). In recent years,digital mastershave 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.

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 engineersoften applyequalizationanddynamic range compressionin order to optimize sound translation on all playback systems.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.

History

Pre-1940s

In the earliest days of the recording industry, all phases of the recording and mastering process were entirely achieved by mechanical processes. Performers sang or played into a largeacoustic hornand the master recording was created by the direct transfer of acoustic energy from thediaphragmof the recording horn to themastering 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.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.

After the introduction of themicrophoneandelectronic amplifierin 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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).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.Only a small minority of recordings were mastered using previously recorded material sourced from other discs.

Emergence of magnetic tape

In the late 1940s, the recording industry was revolutionized by the introduction ofmagnetic tape.Magnetic tape was invented for recording sound byFritz Pfleumerin 1928 in Germany, based on the invention ofmagnetic wire recordingbyValdemar Poulsenin 1898. Not until the end ofWorld War IIcould 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.

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,and running timeof master discs were still limited compared to later media such as thecompact disc.

Electro-mechanical mastering 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 finalmixwas prepared and dubbed down to the master tape, usually either a single-trackmonoor two-trackstereotape. Prior to the cutting of the master disc, the master tape was often subjected to further electronic treatment by a specialistmastering 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 recordingand did not install 8-track recorders in theirAbbey Road Studiosuntil the late 1960s, more than a decade after the first commercial 8-track recorders were installed by American independent studios.

Digital technology

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

In the 1990s, electro-mechanical processes were largely superseded by digital technology, withdigital recordingsstored onhard disk drivesor 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 technologyare 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.

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 enoughheadroomto process and produce a final master.Mastering engineers recommend leaving enough headroom on the mix to avoid distortion.The reduction of dynamics by the mix or mastering engineer has resulted in aloudness warin commercial recordings.

Process

A common mastering processor fordynamic range compression

The source material, ideally at the originalresolution,is processed usingequalization,compression,limitingand other processes. Additional operations, such asediting,specifying the gaps between tracks, adjusting level, fading in and out,noise reductionand other signal restoration and enhancement processes can also be applied as part of the mastering stage.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,andindexesare defined for playback navigation along withInternational Standard Recording Code(ISRC) and other information necessary to replicate aCD.Vinyl LP andcassetteshave 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 masterthat 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:

1. Transferring the recorded audio tracks into theDigital Audio Workstation(DAW)
2. Sequence the separate songs or tracks as they will appear on the final release
3. Adjust the length of the silence between songs
4. Process orsweetenaudio to maximize the sound quality for the intended medium (e.g. applying specific EQ for vinyl)
5. Transfer the audio to the final master format (CD-ROM, half-inch reel tape,PCM 1630 U-matic tape,etc.)

Examples of possible actions taken during mastering:

1. Editing minor flaws
2. Applyingnoise reductionto eliminate clicks, dropouts, hum and hiss
3. Adjusting stereo width
4. Equalize audio across tracks for the purpose of optimized frequency distribution
5. Adjust volume
6. Dynamic range compression or expansion
7. Peak limit
8. InsertingISRCcodes and CD text
9. Arranging tracks in their final sequential order
10. Fading out the ending of each song
11. Dither

Notable audio mastering engineers

• Brad Blackwood
• Greg Calbi
• Tony Dawsey
• P. A. Deepak
• Brian Gardner
• Chris Gehringer
• Kevin Gray
• Bernie Grundman
• Steve Hoffman
• Ted Jensen
• Bob Katz
• Heba Kadry
• Emily Lazar
• Bob Ludwig
• Stephen Marcussen
• George Marino
• Randy Merrill
• Mandy Parnell
• George "Porky" Peckham
• Eric Pillai
• Shadab Rayeen
• Doug Sax
• H. Sridhar
• Ray Staff
• Rudy Van Gelder
• Howie Weinberg
• Leon Zervos

See also

• Album era
• Remaster
• 2008 Universal Studios fire