Extension of mix process. Brings more life and quality into mix with the use of industry standard audio software and analog audio equipment.
Stem mastering happens after mixing and before mastering. Stem mastering takes stems of audio tracks and processes them with more EQ, compression, and some automation.
Example: In mixing Vocal Track 1, Vocal Track 2, and Vocal Track 3 are all processed individually. Stem mastering would take all three of the Vocal Tracks, or one Vocal Stem, and processes them as one in order to glue them together.
Vocal 1 + Vocal 2 + Vocal 3 = Vocal Stem
Kick + Snare + Toms + Overheads = Drum Stem
Guitar 1 + Guitar 2 + Guitar 3 = Guitar Stem
Vocal Stem + Drum Stem + Guitar Stem = Stem Master
Stem are processed with audio software and are then run through analog equipment to give them that much more life.