Warp and Tempo Sync

Drop a drum loop recorded at 90 BPM into a project running at 120 BPM and it will not line up — it plays at its own speed while your grid moves faster. Warping is the feature that makes an audio clip bend to the project's tempo so everything stays locked together.

One-line definition

Warp = time-stretching an audio clip so it follows the project tempo instead of its own original tempo.

The tempo map: the project's grid

Before warping makes sense, you need the thing a clip warps to: the project's tempo map.

The tempo map is a list of tempo segments. Each segment says "from this point on the timeline, the tempo is this many BPM". A song can have one constant tempo, or many segments — a slow intro, a faster verse, a ritardando at the end. A segment can also ramp: glide smoothly from one BPM to another instead of jumping.

Alongside the tempo map are time signatures (4/4, 3/4, 6/8…), which set how many beats make a bar. Together the tempo map and the time signatures define the musical grid — the bars-and-beats ruler that every clip position from Clips and Tracks is measured against.

seconds  0        2        4        6        8        10
tempo    │ 90 BPM          │ ramp 90→120     │ 120 BPM        │
bars     1        2        3        4        5        6
         └ a clip warped to this map stretches to match each region

The grid is the thing a clip warps to, so it helps to see how tempo turns musical positions into seconds. Drag the tempo below and watch the bars compress or spread against a fixed seconds ruler — that is exactly the mapping a clip has to follow.

TIMING · TEMPO GRIDIDLE

Tempo and the grid — how BPM maps musical time to seconds

The seconds axis on top never moves; the bar-and-beat grid below is computed from the tempo. Drag BPM and the grid slides — a faster tempo packs more bars into the same six seconds, a slower one spreads them out. The readout turns the tempo into the durations the engine actually counts in: one bar, one beat, and one tick at PPQ 480, the resolution libsonare uses for MIDI timing. Change the beats per bar to reshape where the accented downbeats land. Press play for a metronome at the chosen tempo — the grid you see is the click you hear.

Tempo

120 BPM

Beats / bar

The three warp modes

Every audio clip has a warp mode that decides how (or whether) it follows the grid.

off vs repitch vs tempo-sync

• off — no stretching. The clip plays at its native speed and pitch, ignoring the project tempo. Use this for one-shots and sound effects that should not bend.
• repitch — like changing the speed of a tape. The clip is sped up or slowed down to fit the tempo, and the pitch moves with it: faster means higher, slower means lower. Simple, CPU-cheap, and musically useful when you want that vintage varispeed character.
• tempo-sync — follow the tempo while keeping the original pitch. The clip is time-stretched so its timing matches the grid, but a phase vocoder holds the pitch constant. This is what you want for vocals, melodic loops, and anything where the notes must stay in tune.

PARAM SWEEP · TIME STRETCHIDLE

Time stretch — changing length, not pitch

Time stretching is pitch shift's exact opposite: it changes how long the audio lasts while leaving the pitch alone. Drag the rate and the drum hits spread out or bunch up — the waveform fills more or less of the panel — but the spectrum below barely moves. Below 1.0 the clip slows down and grows; above 1.0 it speeds up and shrinks. Press play to hear the groove change tempo with no chipmunk effect.

Rate

1 ×

Mode	Timing follows tempo?	Pitch stays the same?	Typical use
off	No	Yes	One-shots, SFX
repitch	Yes	No (moves with speed)	Tape/varispeed feel, drums
tempo-sync	Yes	Yes	Vocals, melodic loops

Warp anchors: pinning the audio to the grid

A tempo change tells the clip how fast to play, but real performances are not perfectly even — a drummer pushes and pulls, a phrase rushes slightly. Warp anchors let you pin specific points inside the audio to specific points on the timeline.

Each anchor ties a position in the source audio to a position on the project timeline. The downbeat of the loop snaps to bar 1, the snare hit snaps to beat 2, and the stretching between anchors is computed automatically. With enough anchors you can straighten a loose performance onto the grid, or deliberately bend a tight one to groove.

tempo-sync is real time-stretching, with limits

Because tempo-sync changes duration while preserving pitch, it uses a phase vocoder under the hood — the same family of algorithm described in Phase Vocoder Stretch. Small stretches are transparent; very large ones can smear transients or add a "phasey" quality. On stereo and multichannel clips the stretch is phase-locked across channels, so the stereo image does not drift between left and right. repitch has no such artifacts (it is just resampling) but it moves the pitch, so the two modes trade different costs.

How playback and bounce stay consistent

A subtle but important guarantee: a clip warps the same way whether you are auditioning it live or rendering the final file. The tempo map, the warp mode, and the anchors are part of the project's edit model, so real-time playback and the offline bounce produce matching timing and pitch. What you hear while editing is what you get on render.

How libsonare implements this

Warp lives on the Project editing model. A clip's mode is set with setClipWarpMode(clipId, mode) where the mode is one of 'off' | 'repitch' | 'tempo-sync'. Warp anchors are first-class warp maps: setWarpMap({ id, name?, anchors }), with each anchor as { warpSample, sourceSample }, and a clip references one via setClipWarpRef(clipId, warpRefId) (removeWarpMap clears it). The grid comes from setTempoSegments (each { startPpq, bpm, endBpm? }, where endBpm drives a ramp) and setTimeSignatures (each { startPpq, numerator, denominator }). For tempo-sync, the stretch is performed by StreamingPhaseVocoder so the pitch is preserved, and the same path is used in both realtime playback and offline bounce() — repitch, by contrast, resamples and lets pitch track speed. All of warp mode, anchors, tempo, and time signatures round-trip through project JSON via toJson() / Project.fromJson(json).

Related: Project Editing, Phase Vocoder Stretch, Project Bounce