Audio mastering
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Audio mastering is the process of preparing and transferring recorded audio to a medium for future duplication. The specific medium that receives the mastered audio varies, depending on the intended release format of the final product. This medium is then used as the master copy, from which all further production of the audio material will be based.
History
Traditionally, Audio Mastering was the process of transferring audio recordings on magnetic tape to a phonograph lathe for the production of vinyl records. The mastering process was performed in real-time to disk, with any mistakes appearing on the master laquer disk. 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 became common in many mastering facilites, 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:
- Sequence the recorded audio as it will appear on the final product.
- Correct any problems with the audio, such as volume level, tonal balance, or undesireable artifacts.
- Transfer the audio to the final master format.
Examples of possible actions taken during mastering:
- Apply noise reduction to eliminate hum and hiss.
- Normalize 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 two tracks to ensure there are no jumps in bass, treble, midrange, volume or pan.
- Apply a compressor (for example, 4:1 starting at -6 dB) to compress the peaks but to expand the softer parts.
- Apply a dynamics compressor to compress only specific frequencies that generate the audio peaks.
- 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.
Please note that the above are not specific instructions but some processes that may or may not be applied. Audio 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.