Analyze the whole track.
rbxport’s analysis engine, rbl-analysis, turns decoded audio into BPM, beat timestamps, bar numbers, key, and a three-band waveform. The same audio and settings produce the same result. It is tuned around electronic music in 4/4: four beats per bar, numbered 1–4.
Kick attacks place the markers; changes in the arrangement help decide which marker is beat 1. Whole-track analysis also detects tempo changes after a steady intro.
How the analysis pipeline works.
Decoded audio passes through BPM detection, kick alignment, tempo and phase checks, downbeat selection, file-start handling, key detection, and waveform generation.
Four-stage pipeline
¹ Kick-attack placement requires High precision. ² Internal phrase-start markers do not include intro, verse, or chorus labels.
Choose what to analyze.
Choose Analyze Track from a track or player menu. The Analysis Setting dialog applies to the selected batch; changing your selection afterward does not change the queued tracks.
| Setting | What changes |
|---|---|
| BPM / Grid | Updates BPM and beat markers and regenerates waveforms. |
| High precision | Uses attack placement; enabled by default. Without it, placement uses the onset envelope. |
| BPM Range | 70–180 by default; also 98–195, 118–236, and 58–115. |
| KEY | Updates key metadata. Key-only analysis leaves BPM, grids, and waveform files untouched. |
Unchecked results are preserved. Analysis locks prevent changes and a refused track does not stop the rest of the batch. If key detection returns no result, the existing key stays.
Presets and automatic imports
The documented Normal and RBXport presets currently select the same underlying analysis options; they are not two independent engines. Automatic imports use full analysis, high precision, and a 70–180 range. Manual choices belong to that batch rather than overwriting global preferences.
Estimate BPM from onset patterns.
The first pass measures onsets: moments when sound energy rises. Sustained notes and fading tails contribute less than a fresh percussive hit. Two measurements work together: how often that pattern repeats, and how strongly it supports each rhythmic rate.
For octave-related candidates such as 174 and 87 BPM, the faster interpretation wins when the onset pattern supports it. A broad prior centered near 132 BPM resolves close ties.
The tempo calculation
A 1,024-sample spectrum advances by 256 samples. Positive spectral differences form a normalized onset envelope. Candidates are scored using autocorrelation × √Fourier magnitude × a broad tempo prior. The standalone documentation describes a 70–200 search; the app constrains it to your selected BPM range.
Align markers to kick attacks.
BPM gives the spacing between beats. A rough grid estimates their positions, then high-precision analysis looks near each marker for the beginning of a kick. It follows the nearby strong rise rather than the loudest part of the kick’s body.
A weighted timing line is fitted through reliable attacks, then fitted again with outliers removed. Missing kicks do not pull the line away from the steady sections. The grid extends to the start and end of the file.
Precision and whole-number tempos
The attack detector uses the 900–9,000 Hz band, RMS in 1 ms steps, and a ±15 ms search around each prediction. It compares two phases half a beat apart. A steady fitted tempo within 0.1 BPM of a whole number is snapped to that number and re-aligned through the same attacks. Ramps keep their measured per-beat intervals.
Detect and follow tempo changes.
The engine estimates tempo in 16-second windows. A tempo change is accepted when at least three windows agree within 1% and the candidate differs from the current tempo by more than 2%.
Illustrative intervals: 128 → 136 → 146 → 160 → 174 BPM. Each measured interval gets its own BPM; there is no single jump during the ramp. The final grid resumes after four consecutive intervals match its period within 1% and its phase within 2 ms. Blue, taller markers indicate beat 1; coral markers indicate beats 2–4. Schematics, not track measurements.
Illustrative mid-bar cut: the old grid holds until the first supported new-grid beat. Reliable kick runs are preferred; if none qualifies, transient support helps choose the cut. The yellow dashed line marks the BPM decision, not necessarily a downbeat. Blue, taller markers indicate beat 1; coral markers indicate beats 2–4. Schematics, not track measurements.
Gradual changes: follow the hits toward the next stable section and keep every measured interval, including curved ramps. Abrupt changes: if the walk cannot settle, make a cut where the new grid has reliable support. Beat numbers continue 1–4 across the join.
When kicks and usable clicks disappear, quiet full-band transients can carry the transition. The engine boosts their rises internally by 4× with a 20 ms release. This changes the analysis signal, not your audio. Original hit timestamps are retained; silence and release tails cannot invent beats.
When a new tempo qualifies
Familiar rhythm ratios have a 1% exclusion zone. A walked transition allows up to 5% period drift per beat and rejoins the steady grid after four consecutive intervals agree within 1% in period and 2 ms in phase. Full-band transient support is used only inside transitions.
Preserve phase through breakdowns.
After two bars without supporting hits, the engine checks how the music returns. If the hits land on the original grid, that grid holds through the breakdown. If they return at the same BPM but shifted in time, each side gets its own alignment and a cut joins them.
The gap walker follows a slowdown or speed-up when the returning hits establish a new tempo or phase. If the music returns to the original grid, the grid stays unchanged. The gap walker uses its original envelope and thresholds without the transition-emphasis fallback.
Choose beat 1 from arrangement changes.
A loud off-beat hat, clap, or reverse bass can fool a simple hit detector. rbxport compares the music before and after possible bar positions, looking for drops, breakdowns, and new bass lines. Those repeated changes help identify the downbeat.
It examines eight positions per bar: four beats and their midpoints. Evidence is combined over one-, two-, four-, and eight-bar views. If the winning position lies between the current markers, the grid shifts half a beat.
When the opening tempo segment has at least 32 beats, its own music decides the opening bar position. Later sections do not change that choice. If arrangement scoring finds no clear change, the first beat remains the fallback.
Handle a kick at the file start.
A fitted marker just before zero can leave the first visible marker almost a beat late. If audio starts immediately, a grid beat is close to the boundary, and the downbeat decision agrees, rbxport can anchor beat 1.1 at exactly zero.
If strictly less than 20 ms of the opening beat has been cut away, only that opening marker is added at zero; later markers and BPM values keep their fitted positions. Silence at the start, a clearly later downbeat, or a cut of 20 ms or more does not qualify.
When the boundary adjustment applies
The first segment and boundary are checked within 15 ms. Audio must already be present at the beginning, with the first millisecond carrying at least 10% of the local first-15-ms peak. An uncut boundary beat can re-anchor the first segment; a clipped opening uses a shortened first interval. Later tempo segments are preserved. The downbeat decision is made separately from this boundary timing check.
Build a twelve-class pitch profile.
Key detection uses Ángel Faraldo’s edmkey approach with electronic-music key profiles. It gathers spectral peaks, balances their influence, and folds their evidence into twelve pitch classes. Octaves contribute to the same note class.
The pitch profile is compared with major and minor templates at all twelve tonics: 24 candidates. Harmonic weighting, a per-frame gate, and detuning correction help the profile reflect tonal content rather than raw loudness.
Read the paper: Key Estimation in Electronic Dance Music by Ángel Faraldo, Emilia Gómez, Sergi Jordà, and Perfecto Herrera (2016).
From spectrum to key name
The documented front end uses 4,096-sample Hann-windowed frames
with no overlap, up to 60 spectral peaks between 25 and 3,500 Hz,
whitening, and a harmonic pitch-class profile. Four harmonic
contributions have weights 1, 0.6, 0.36, and 0.22. Bins below 0.2
of each frame’s strongest bin are gated out. The averaged profile
is compared using Pearson correlation against the
edma templates. Results use rekordbox-compatible
names such as Fm, Db, and F#m.
Add 0.1 to minor-key scores.
The documented default applies PreferMinor before
selecting the highest of the 24 candidate scores. The 0.1 bias can
change a close major/minor result.
How this relates to the current analysis rules
The current default does not automatically change this track from A minor to D minor.
The source also contains experimental BassRoot and
BassVote rules, but they are
not enabled in the documented default. Their test
windows include the first and last 45 seconds, bar starts, and
selected parts of beats or phrases. The reviewed docs and default
rules do not establish a shipped first-and-last-24-bars fallback.
In the recorded edma run, the minor bias changed 51
tracks, fixed 44 reference disagreements, and introduced six.
Generate detail and overview waveforms.
The detail waveform has 150 columns per second, with low, mid, high, and overall peaks. The separate 1,200-bucket overview calculates bass RMS and mid/high energy, then normalizes each band.
The output approximates rekordbox waveform geometry. Calibration used nine reference tracks rather than a held-out accuracy set. Most measured shapes improved, while one track’s absolute-height error increased. Existing tracks require waveform regeneration to receive the updated output.
Phrase starts versus phrase detection
The downbeat stage can produce internal section-boundary candidates from strong four-bar changes on downbeats. The app does not implement user-facing phrase labels, vocal analysis, or automatic cues.
Measured results.
We wanted to at least match rekordbox’s key detection, then add adjustments for electronic music. We tested a reference playlist of tracks purchased from Beatport, synthetic tracks generated with known tempos, and unreleased DJ edits with no existing reference data.
Real-track reference baseline
The reference playlist contains 155 tracks in 4/4, spanning 123–178 BPM. These recorded results predate the transient-emphasis fallback and have not been remeasured for that change.
| Measure | Pass condition | Passed |
|---|---|---|
| BPM | Within 0.05 BPM | 155 / 155 |
| Downbeat | Within 25 ms, modulo a bar | 143 / 155 |
| Full grid | 98% of reference beats within 25 ms, same number | 142 / 155 |
| Key | Same key name as the reference | 141 / 155 |
The 99% figure in the engineering docs is a target, not an achieved result across every metric. Key agreement is agreement with the reference label, not proof of the only musically valid answer.
Known disagreements
Eleven older rekordbox 6 grids sit about 25 ms after the kick. Other differences include an early downbeat choice and hand grids that hold a tempo where rbxport follows a slowdown. The reported rekordbox-7-only subset counts differ between documents and are omitted here.
Saved synthetic grids
Seven two-minute fixtures cover a steady tempo, upward and downward cuts, a mid-bar change, linear ramps, and a curved ramp. The documented saved-grid run matched all 2,103 ticks within 2 ms, with no extra markers, correct numbering, and an exact-zero first marker. Worst error across those cases was 1.938 ms.
Separate regressions cover quiet transitions, release tails, silence, and preserving kick timestamps. The later fallback run records 41 passing unit and integration tests and a clean lint check. Saved library grids change only after reanalysis.
Nine real DJ edits
The older multi-tempo baseline passed BPM and key on 9/9 edits, downbeat on 8/9, and the strict full-grid metric on 4/9. Seven of ten tempo changes were within 3 ms. The difficult cuts were hand-placed at an early impact, well before the new kick pattern arrived. The newer fallback has not been scored against a fresh playlist.
How the validation tools work
The golden harness caches decoded audio and reference grids, then scores candidate analysis without writing to the library. Specialized runs isolate downbeat decisions, key profiles, and bass rules. The multi-tempo rig instead imports copies, writes analysis files, and checks whether the grids survive loading in rekordbox. Its format observations apply to that rig and version.
The synthetic fixtures use sample-exact timestamps generated independently of the analyzer. The checker reads persisted grids back from the app and requires a correctly numbered marker within 2 ms of every tick, with no extras. Rebuilding alone does not update saved grids; tracks must be analyzed again.
Limits and verification.
- The engine assumes 4/4 and is tuned to electronic music; the reference set is not evidence for every genre or meter.
- Ambiguous percussion, long beatless transitions, and creative edits can produce different valid interpretations.
- Whole-number snapping applies only to nearby steady tempos, not every measured interval.
- Key detection outputs one label even when the tonal evidence is ambiguous.
- Internal phrase boundaries are not user-facing phrase labels, vocal detection, or stems.
After analysis, check the first downbeat, a later steady section, and any transition. Report incorrect results with the section and expected behavior using the app’s Report bug action.
Open Source
This analysis pipeline is open source (GPL v3). You can explore the code and implementation details on GitHub ↗.
Figures are explanatory schematics, not captured track measurements. Reported test results come from the project’s recorded benchmarks; no new benchmark was run for this page.