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Mastering is the process of preparing and transferring recorded audio to a data storage device that will be used in the production of copies. 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 medium, chosen based on replication factory requirements and/or record label security concerns. The chosen medium is then used as the source from which all copies will be made (via methods such as pressing, duplication or replication).
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 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 late 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electrically driven mastering lathes came into use for cutting master discs (the cylinder format by then having been superseded).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 as the performance took place. Only a small minority of recordings were mastered using previously recorded material sourced from other discs.
Advances
The recording industry was revolutionised 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 78rpm disc and the later 7" single and LP record -- meant that the audio quality, dynamic range and running time of master discs was 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 1980s, the mastering process typically went through several stages. Once the studio recording on multitrack tape was complete, a final mix was prepared and dubbed down to the master tape -- usually either a single-track monophonic 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 'optimised' by making fine adjustments to the balance and equalisation prior to the cutting of the master disc.
Mastering became a highly skilled craft and it was widely recognised 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 the 1970s and beyond, one of the most sought-after mastering engineers in the world was Bob Ludwig. The New York based engineer began his audio career at A&R Studios in New York as assistant to producer Phil Ramone after which he spent many years based Sterling Sound, and then Masterdisk, mastering recordings in all genres by artists from all over the world.
Many artists found that their recordings were significantly degraded by sub-standard mastering, so some of the more technically adept pop musicians -- notably Frank Zappa and Todd Rundgren -- learned this process and became highly skilled mastering engineers in their own right in addition to their musical accomplishments.
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 multitrack 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-Sixties, 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 consumers.
Digital technology
In the 1990s, the old electro-mechanical processes were largely superseded by digital technology, with digital recordings tranferred 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).
Process
The process of audio mastering varies depending on the specific needs of the audio to be processed. Steps of the process typically include:
- Load the recorded audio tracks into the DAW.
- Correct any problems with the audio, such as volume level, tonal balance, or undesirable artifacts.
- Sequence the separate songs or tracks as it will appear on the final product (for example, a CD).
- Transfer the audio to the final master format (i.e., Redbook, CD-R, etc.).
Examples of possible actions taken during mastering:
- Apply noise reduction to eliminate hum and hiss.
- Limit the tracks to set the highest peaks in audio volume to a preset level; the overall audio should never exceed 0 dBFS.
- Equalize audio between tracks to ensure there are no jumps in bass, treble, midrange, volume or pan.
- Apply a compressor (for example, 1.5:1 starting at -10 dB) to compress the peaks making the softer parts sound relatively louder.
- In the case of mastering for broadcast, the bandwidth of the signal has to be reduced. For example, for TV broadcast: apply a high-pass filter at 80 Hz with -18 dB/octave to filter out low frequencies and apply a low-pass filter at 12 kHz with -9 dB/octave to filter out high frequencies.
- In the case of CD or Vinyl or similar media, arrange the tracks in the desired order and make sure the pauses or crossfades between tracks are as they should be.
The guidelines above are not specific instructions but processes that may or may not be applied in a given situation. Mastering needs 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.
RMS in music, average loudness
The Root Mean Square (RMS) in audio production terminology is a measure of average level and is found widely in software tools. In practice, a larger RMS number means higher average level; i.e. -9 dBFSD RMS is 2 dB louder than -11 dBFSD RMS. The maximum value for the RMS number is therefore zero. The loudest records of modern music are -7 to -9 dBFSD RMS, the softest -12 to -16 dBFSD RMS. The RMS level is no absolute guarantee of loudness, however; perceived loudness of signals of similar RMS level can vary widely since perception of loudness is dependent on several factors, including the spectrum of the sound (see Fletcher-Munson) and the density of the music (e.g., slow ballad versus fast rock).
Compressed higher RMS vs clipped higher RMS, density
Some experienced listeners feel that around -12 dBFSD RMS in general or during loud parts and -14 to -16 dBFSD RMS during soft parts is a "sweet spot" for optimal punch and loudness, neither too loud nor too soft. This perception is still valid considering that the extra loudness (usually 1-3 dB) has often been achieved by simply clipping the smoothly curved tops of the waveforms resulting in flat topped square waves, which may or may not result in a subjective improvement of the sound. Prior to clipping, usually the last procedure in audio production, the "natural" RMS of many songs is in fact just around -12 dBFSD RMS. Thus, in many cases where the final RMS is -8 to -11 dBFSD, the RMS has not really been increased over the -12 dBFSD RMS "sweet spot"; only the tops of the waveforms have been clipped by 1-3 dB. The music is not any thicker or denser, merely played louder with less punch and more distortion.
In contrast, a "true" higher RMS is achieved by increasing the density (usually by compression) of the sounds contributing most to the average level (i.e., everything but the drums), so that their volume as a group can be lower in relation to (usually drum) peaks. This retains the same RMS and perceived average loudness as the clipped mix, often with a stronger sense of density and pressure. However, in practice, this too would probably be subjected to some clipping, resulting in even higher loudness and pressure than one that was merely clipped.
In summary, what is important is not how loud the song is made but how the song is made loud.
Bit rate, sample rate, and dithering
Since the onset of digital recording, another job of the mastering engineer is to make higher resolution recordings into CD quality. For example, professional digital audio is almost always recorded at 24 bits, whereas a CD quality is 16 bits. Similarly, some projects may be recorded at a higher sample rate, such as 96kHz, whereas CD quality is 44.1kHz. While downsampling the audio is a relatively simple task, bit reduction has more blatant consequences. While reducing the bit depth from 24 bits to 16 bits, if one simply truncates the lower 8 bits it will lead to distortion and other subjectively ugly artifacts, called quantization error. The solution, or perhaps compromise, is to dither the signal. This process will add lower level white noise instead of the distortion - a subjectively more pleasing sound. Some even believe proper dithering can make 16 bit audio sound as though it actually has the dynamic range of 19 or 20 bit audio. Dithering noise can also be shaped in a way that makes it less audible by placing most of the noise in a higher frequency range, by principle of the Fletcher-Munson curve. The sound is not dissimilar to audio tape noise. Different dithering processes have different sounds, and thus the mastering engineer will choose the dither that he believes to be most appropriate for the type of music.
New trends
Artmastering is a process of mastering audio material with special emphasis on the artistic content of the music.
During the process of artmastering, an engineer together with the artist and his/her creative team focus on the artistic elements of an audio material, and try to devise a method, technique or process needed to accomplish the sonic goals by all means necessary whether subtle or radical.
As in traditional mastering, artmastering tries to achieve the best sonic quality of the material as defined by the artistic sense of human beings rather then by some rigid technical specifications such as frequency-response or signal-to-noise-ratio. During artmastering, an engineer has wider latitude and may either stay within the traditional boundaries of the mastering art or step beyond them according to the artist's wishes.
Software tools for mastering
Digital Audio Workstations
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