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Mastering is the process of preparing and transferring recorded audio to a medium 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 (see Pressing, Duplication and Replication).
History/Overview
Traditionally, mastering was the process of transferring audio recordings on magnetic tape to a phonograph lathe for the production of vinyl records. With the advent of the Compact Disc, the lathe was replaced with a digital encoder and recording device. In the 1990's, 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).
There is a modern trend toward producing increasingly "loud" CD's—a phenomenon driven by today's hyper-competitive music market and commonly referred to as the "loudness war." Carried to an extreme, this process can produce obvious mix changes resulting from the significant reduction of peaks from musical parts/instruments with high dynamic ranges and the amplification of softer parts. As some parts are squashed, others can bloom up in volume around them, requiring mastering engineers to balance the effects of increased loudness while minimizing apparent changes to the mix. This is an area of sonic compromise that requires a trained ear and considerable technical knowledge.
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 but to expand the softer parts.
- 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.
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 adjectives 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 smaller 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.
Software tools for mastering
Digital Audio Workstations
- Sonic Solutions
- Sonic Studio
- SaDiE
- Seqouia
- Audio Cube
- Pyramix
- Sony Sonoma
- Nuendo
- Pro Tools
- SteinbergWavelab
- Cakewalk Sonar
- JAMin