Suppose you’re learning your part (guitar, keyboards,…) in one of the more challenging rock or metal sounds, and you don’t just want to “play the song” — you want to hear exactly what your hero plays and reproduce it to the best of your ability.
There are a number of YouTube videos floating around of isolated vocal, guitar, bass, keyboard… parts of various classic rock and metal tracks. For some instruments, they can be extracted from the stereo mix by EQing and stereo manipulation. Cranking up bass in an EQ while rolling off midrange and treble, for example, will leave you with mostly bass guitar and kick drum, while doing the opposite may be handy if you’re trying to learn a complex bass part. However, if the instrument ranges cross, this doesn’t work — bass and guitar parts in Tool songs often cross, for instance, and Yes bassist Chris Squire (RIP) never stayed in his lane.
Also, as vocals tend to be placed in the center of the stereo image, mixing the left and right channels in counterphase will reduce the vocal a lot, allowing a “poor man’s karaoke”.
A “stem track”, on the other hand, is derived from the original multitrack recording: it is best defined as the submix from a single instrument group. A “guitar stem” would be all the guitar parts of a track (mixed, among them, like in the final master), a “drum stem” the final submix of all the drum channels, and the like.
In a way it’s an inversion of those old “Music Minus One” recordings for classical musicians: those were ensemble or orchestral recordings with one solo instrument omitted, sold together with sheet music for that solo part.
Here are a couple of examples:
- a stem of John Petrucci’s guitar parts on Dream Theater’s “Pull Me Under”
- a stem of Ritchie Blackmore’s guitar parts on Deep Purple’s “Highway Star”. This isolate of Jon Lord’s organ parts appears to have been extracted by EQing and stereo manipulation instead.
- John Entwistle’s bass part on The Who’s “The Real Me”, which is basically one continuous bass solo under the song.
It is very educational sometimes to hear how parts interlock, how minor imperfections in an individual part get buried in the overall mix, and how the whole of a song can be greater than the sum of the parts. Compare, for example, Steve Harris’s bass part on “Two Minutes To Midnight” with the final complete song: