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Audio mastering is the process of preparing audio for playback on a wide range of playback devices. The main changes to a track during mastering are: tonal balancing and level adjustment. The former is primarily done with equalization while the latter is primarily achieved with compression. Other effects that can be used during mastering are: audio enhancers, audio exciters and stereo expansion.
The usual steps taken in mastering a number of tracks are:
- Timeline all the tracks in the desired order. Leave a 2 second gap between tracks when producing a CD.
- 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.
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