Mastering is usually done from a stereo or dual-mono mixdown of the final mix sessions. However, in some cases, if there are some uncertainties about the mix on the band or mixing engineer’s part, stem mastering can be beneficial.
Stem mastering is a process where the mixes are separated into groups (or stems) of instruments at the levels they were mixed so that when the stems are loaded into a new session, the mastering engineer has access to the individual instruments in the context of the mix. For example, if the drums need a boost to their low frequencies, the necessary changes can be made clinically without affecting areas such as the bass guitar, which will usually sit adjacent in the mix. Stem mastering can also be beneficial to increasing the perceived volume of a master in some instances. It should be stressed, however, that this isn’t a fix-all solution to problematic mixes, but it certainly gives the mastering engineer a much broader scope in which to work.
Ideally, the stems should be separated as below, but feel free to send an email if your band uses any unorthodox instrumentation, and you’re unsure as to how to proceed.
Again, please make sure there is headroom in the stems, and files should be submitted as .wav or .aiff files – all starting from the same zero point in your mix session.
