For Stem Mastering, you would first organise your DAW channels into suitable groups that match (“stems”) then export the audio into different stereo files
Example 1: for an electronica track:
Stem 1: bassline
Stem 2: kick-drum
Stem 3: Other drums
Stem 4: Delicate synths
Stem 5: Rhythmic synths
Stem 6: Reverb
Example 2: for a rock track:
Stem 1: Vocals
Stem 2: Drums
Stem 3: Bass
Stem 4: Lead guitar & guitar effects
Stem 5: Rhythm guitars
Stem 6: Reverb
We mix the stems together then go back in and tweak each stem individually (e.g. EQ the vocals separately/run the drums through analogue processors/compress & EQ the bass…) so each stem is optimised in context of the track pre-master.
This neatly helps prevent things like the bassline & kick getting muddy, & allows us to EQ things separately that are in the same broad frequency range (e.g. on a regular stereo mix if you lower an EQ band on a synth sound then other sounds will also be affected, for example).
Then once that is sorted, we mix all the stems into a stereo bus & do a regular mastering job on it, but with the added luxury of being able to instantly tweak individual stems once the master is close to completion.
Stem mastering also gives you more flexibility – we’ve had several clients who have had a track they just couldn’t get the vocal to sit perfectly over the mix – very frustrating! So we can do a 2-stem job with the instrumental & the vocal stem, making everything sound sweet & balanced for only a few pounds on top – nice!