Rekordbox isn't loved.
It's load-bearing.

Serato, VirtualDJ and djay all separate stems in real time — on a laptop. Rekordbox does too, in Performance mode, which means the CDJ-3000 gets stems only while a computer is running. Engine DJ took the other route: render the stems at home, play them standalone. It works, and it's the only route that works on club hardware.

Demucs is a masking separator: every stem comes back with the same sample rate, channel count and length as the mix it came from. That one property is the whole trick. The beatgrid, the phrase boundaries and all eight hot cues of the parent track are valid verbatim on the acapella and the instrumental. Rendering a stem becomes a file operation; the musical preparation is inherited, and the two decks can never disagree about where bar one is.

The rendering pipeline already exists in this repository — demucs, format-matched output, tags copied from the parent with the stem name appended. What the spec adds around it is the part that makes a stem play well in a booth: gain applied to the samples rather than written as a ReplayGain tag players ignore, clipping prevented rather than reported, isolation quality scored per track, and companions sorted directly beneath their parent in the exported playlist.

Rekordbox splits into Export mode and Performance mode — two mental models over one library, a seam that exists for licensing reasons rather than for anything a DJ wants. Here there is one view. The prep editor sits under the browser rather than replacing it, so arrow-keying down a crate moves the waveform, and every prep action is one keystroke from the list you're digging in.

The query bar is the browser

Everything the list can be filtered by is expressible as text, which means it is also scriptable, shareable and diffable. A saved query is a smart playlist — there is no second concept, no separate rule editor, and no 1,000-track ceiling of the kind rekordbox's Intelligent Playlists impose.

The one screen that must never be wrong

The preflight runs before a byte is copied; the verification runs after, using a parser that shares no code with the writer. The sheet does not report success until the second one passes — because a writer that validates its own output is a writer that validates its own bugs.

Gathered from AlphaTheta's own community forums, the DJ trade press and the Mixxx issue tracker. Each complaint on the left generates the requirement on the right. Forum reports are cited as evidence that a complaint is common, not that any particular number is exact.

Everything else in this spec is ordinary software. This is the part that decides whether the project is possible at all, so it comes first. A prepared drive holds a database of tracks and playlists plus one analysis file per track — and there are three generations of that database.

The analysis files carry everything a CDJ-3000 needs and all of it is public: PQTZ beat grids with per-beat tempo, PCO2 cues and loops with RGB colours and UTF-16 comments, PWV5 colour waveforms, PWV6/7 the CDJ-3000's three-band waveforms, and PSSI song-structure phrases. A drive a CDJ-3000 will play can be written today, from Rust, with nobody's permission.

The CDJ-3000X is the other half of the story, and it requires OneLibrary, which is closed. Lexicon already ships both databases on one drive, so third-party implementation is evidently possible — by licence, by reverse engineering, or both. The honest plan is to be valuable before that partnership exists: v1 writes the legacy database and hands off to rekordbox's free Export mode for OneLibrary hardware. Clunky, and it works on day one.

The honest reading: rekordbox wins on ecosystem breadth and on OneLibrary hardware until phase 3 lands. This wins on everything a DJ does between buying a track and playing it.

Still open

Can a third-party export populate Related Tracks and My Tag on a CDJ-3000, or are those tables written but ignored? Which .2EX fields does the three-band waveform actually require, and what happens when they're malformed? Which models and firmwares really accept exFAT? How many bytes does a rendered stem cost across genres? Every one of these is answered by writing a drive and plugging it into a player — which is why phase 0 exists.