What are stems? (and how they differ from multitracks)

A stem is a group of audio sources mixed down together and then handled as a single unit: all the drums as one file, all the backing vocals as another. Wikipedia puts it as "a discrete or grouped collection of audio sources mixed together", dealt with "downstream as one unit".

Stems vs multitracks

Multitracks are the individual tracks of a session: kick, snare, overheads, each vocal take, each guitar. Stems sit one level up: several of those tracks summed into a group.

iZotope's stem mastering guide describes stems as "stereo mixes of multiple individual tracks, delivered without any master bus effects", which keep the processing on those tracks. It sums up the trade-off: with stems "you don't have quite the level of freedom you would with multitracks (as a mixer would), but you have more freedom than in a stereo mastering setting."

So the ladder runs: multitracks (most control), stems (some control), stereo mix (least).

Where stems turn up

• Mastering: a mastering engineer can rebalance groups without a full remix (iZotope, above).
• Film and TV: dialogue, music and effects are kept as separate stems, so dialogue can be swapped for a foreign-language version (Wikipedia).
• Remixing: Wikipedia notes stems can be used for remixing songs.
• DJing: Native Instruments' Stems format is "an open, multi-channel audio file", with the example of a track split into four elements: bass, drums, vocals and melody.

Which to send

Mixer: multitracks. Mastering engineer offering stem mastering, remix contest, or sync library: stems, unless they ask otherwise. Always ask for their spec first.

What we think

"Send me the stems" is one of the most misused phrases in music. Half the time the person means multitracks, and the other half they mean a different grouping from the one you made. The fix is boring: agree the group list (drums, bass, music, vocals, FX), start every file at the same point, export at the session's sample rate, and leave the master bus off. Getting that right once saves a round of emails on every collaboration after.