Audio mastering

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 equalisation 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 steps taken in mastering a number of tracks are:

1. timeline all the tracks in the desired order. Leave a 2 second gap between tracks when producing a CD.
2. apply noise reduction to eliminate hum and hiss
3. normalize the tracks to set the highest peaks in audio volume to a preset level; the overall audio should never exceed 0 dBfs
4. equalize audio between two tracks to ensure there are no jumps in bass, treble, midrange, volume or pan
5. apply a compressor (for example, 4:1 starting at -6 dB) to compress the peaks but to expand the softer parts
6. apply a dynamics compressor to compress only specific frequencies that generate the audio peaks
7. 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