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). 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.
Mastering requires critical listening; however, software tools exist to facilitate the process. Results depend upon the intent of the engineer, the skills of the engineer, the accuracy of the speaker monitors, and the listening environment. Mastering engineers often apply equalization, dynamic compression and limiting in order to optimise 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.
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. 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 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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).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. Only a small minority of recordings were mastered using previously recorded material sourced from other discs.
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In the 1990s, electro-mechanical processes were largely superseded by digital technology, with digital recordings stored on hard disk drives or digital tape and mastered to CD.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.
Digital systems have higher performance and allow mixing to be performed at lower maximum levels. With peaks between -3 and -9 dBFS on a mix, the mastering engineer has enough headroom to process and produce a final master. It is important to allow enough headroom for the mastering engineer’s work. Reduction of headroom by the mix or mastering engineer has resulted in a loudness war in commercial recordings.
The source material, ideally at the original resolution, is processed using equalization, compression, limiting and other processes. Additional operations, such as editing, 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. 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, and indexes are defined for playback navigation along with International Standard Recording Code (ISRC) and other information necessary to replicate a CD. Vinyl LP and cassettes 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 a Disc Description Protocol (DDP) file set or an ISO image. Regardless of what delivery method is chosen, the replicator factory 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. 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 the Digital 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 or sweeten audio 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.)
1-Editing minor flaws
2-Applying noise reduction to eliminate clicks, dropouts, hum and hiss
3-Adjusting stereo width
4-Equalize audio across tracks for the purpose of optimized frequency distribution
6-Dynamic range compression or expansion
8-Inserting ISRC codes and CD text
9-Arranging track in their final sequential order
10-Fading out the ending of each song (if required)