Epiphany
Most revelations seem trivial or obvious after the fact. This is one of those moments.
For a long time, mixing and mastering seemed like the same thing, or rather, too similar to be truly different things. It first hit me when I realized what automation (see previous post here) actually automates—the movement of someone’s hands as they used a mixer!
Mixing
In the older days, mixing was done in realtime. Talk about pressure! Literally someone moved their fingers adjusting settings on a soundboard while playing back all the tracks. Similar to live sound in a way. This process reduces N tracks into 2 track stereo (typically).1
Sure, there may be channel EQ and compression settings during the mixing phase (like in live sound), but for the most part, the focus during mixing is on the relative volume of the tracks to each other.
Mastering
Mastering, then, applies final EQ and compression to the stereo mix so that it provides an overall style/color and conforms to broadcast standards.2 There are so many different directions one can go in after the mix is done which is why mastering is its own phase.
References
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I learned this from talking with an older recording engineer who recorded abroad. He brough his own mics but recorded and mixed tracks using other peoples’ equipment on-site. He dubbed the stereo mix output to his portable recorder and took it back with him to master. ↩︎
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Slate’s new Virtu mastering engine has good documentation on common mastering styles. https://support.slatedigital.com/hc/en-us/articles/17554883409171-Audio-Settings ↩︎