Stems and Rebalance

Nebula can take a finished song apart into its parts, called stems: the vocals, the drums, the bass and the rest, each on its own track. It can also turn one part up or down inside a finished mix without giving you separate tracks, which is Rebalance. Both run on this Mac, with stem models you install once.

Install the stem models

Nebula never comes with the stem models: they are large, and their licences are their authors'. They are for your own use. Until they are installed, the Stem Splitter says which ones are missing, for example "Stem models not installed: …", and will not start.

To install them, run tools/stems/install.sh from Nebula's source folder in Terminal. It downloads each model, checks it, converts it for this Mac and tests it. It needs the uv tool. The models go into the folder Library/Application Support/Nebula/Stem Models in your home folder, where Nebula finds them. Rebalance uses the same models.

There are five models; a split uses only the ones its stems need:

  • Six stems (BS-RoFormer SW): vocals, drums, bass, guitar, piano and other. Every split uses it.
  • Vocals (unwa Leap Xe): the vocals alone, used by the Best quality.
  • Lead and backing vocals (becruily): lead vocals out of the vocals.
  • Drum kit pieces (DrumSep): kick, snare, toms, hi-hat and cymbals out of the drums.
  • Vocal de-reverb (anvuew): the dry voice, with its reverb as a stem of its own.

Split a song into stems

  1. Choose an audio region: a whole song, a loop or a recording.
  2. Choose Track ▸ Split to Stems… (⌃⌥⌘S). It is also on the region's right-click menu in the audio editor and in the arrangement's Functions menu. The Stem Splitter opens.
  3. Choose which stems you want (see the next section) and a Quality: Best takes the vocals out first with a model that does only that; Fast uses the six-stem model alone, in about a third of the time.
  4. Click Separate into … Stems (it shows how many). A progress bar shows how far it has gone.
  5. When it is done, listen, choose which tracks to keep, and click Add … Tracks to Project.

Splitting takes a while. Click Run in Background to close the window and keep working; the job indicator brings it back when the stems are ready. Cancel Separation stops it and changes nothing.

Which stems you can split

The presets across the top choose for you: 4 stems (vocals, drums, bass, other), 6 stems (adds guitar and piano) and Full separation (all of them). Change any choice and the preset shows Custom.

  • Vocals, with two options: Separate lead and backing vocals gives Lead Vocals and Backing Vocals instead; Separate vocal reverb adds a Vocal Reverb track and leaves the voice dry.
  • Drums, with Separate kit pieces: Kick, Snare, Toms, Hi-Hat and Cymbals instead of one Drums track.
  • Bass, Guitar and Piano.
  • Other is always made: whatever is not in the stems you chose. So the stems always add back up to the song.

Full separation gives eleven stems.

Listen before you add stems

When the split is done, each stem has a lane in the Stem Splitter with its own S (solo) and M (mute) buttons. They are for listening only and change nothing in the song. Click the waveform to move the playhead, drag to loop a part, and double-click to clear the loop.

Compare switches between Stems and Original without losing your place. Untick a stem's box to leave it out.

If you change the settings after a split, Nebula says the tracks are from the previous separation. Click Separate Again, or Add Previous to keep the ones you have. Discard forgets the stems.

What you get

Each stem becomes a new track under the region's own, grouped together, lined up to the sample. Each holds a copy of the region, with the same place, length, loop, fades and stretch, playing its stem. Mute original after insertion, on unless you turn it off, mutes the original region; it is never removed. Adding the stems is one undo step.

Claude can do this for you: ask it to split a region into four, six or all stems, with the vocal reverb on its own if you like.

Rebalance a finished mix

Rebalance turns a part up or down inside a finished stereo mix, for a bounce where there are no separate tracks.

  1. Choose the audio region.
  2. Choose Track ▸ Rebalance… (also on the region's right-click menu and in the Functions menu).
  3. Move the faders: Kick, Bass, Drums, Vocals and Everything Else, each from −6 to +6 dB in half-decibel steps. Double-click a value for 0 dB.
  4. Click Apply.

The first Apply separates the region, which takes a while; Stop cancels it. After that, a fader moved again renders in moments. Kick moves only the kick's body, below 400 Hz; its click and the hi-hats stay as they are.

Whatever the models cannot place stays exactly as it was, and every fader at 0 dB is the region as it was. A second Rebalance starts from the original, so changes never pile up. Revert puts the region back on its original audio. Each is one undo step.

When the whole song is one audio region and its kick is weaker against the bass than your references, the Mastering Assistant suggests a Rebalance and how much. It never applies it for you.

Claude can do this for you: ask it to rebalance a region (for example the vocals up 2 dB), or to revert it.