AI tasks
Follow the beat
Find the beats and downbeats in a recording so you can slice, sync and quantise to it: how beat tracking works, where it slips and what to build with it.
A beat tracker listens to a recording and marks where you would tap your foot, and which of those taps starts a bar. With that grid you can slice a loop on the beat, sync an effect to a song that was not made to a click, or line up a sample with the track it came from.
Try it
In StemSep, load a song, open Beat grid and choose Track beats in mix, or split the stems first and choose Track beats in drums. Export the grid as a MIDI file and drop it into your DAW to see it line up; the file holds metronome hits on the beats, not the song's drum notes.
How it works
A trained model listens for the patterns people tap along to: the strength and regularity of onsets, the kick and snare, harmonic changes at bar lines. It outputs a likelihood of a beat and of a downbeat over time, and the tracker picks the most consistent sequence. Beat and tempo tracking in Learn explains why trackers agree with listeners on common music and struggle elsewhere.
Simpler tools find onsets, the moments sounds start, without a model. They are good for slicing drums but do not know where the bar begins.
Ideas
- Slice on the beat. Chop a break into hits that fall exactly on the grid.
- Sync to a live recording. Drive a delay or a filter sweep from the beats of a band that drifted in tempo.
- Quantise with feel. Snap transcribed notes to the song's own beats rather than a fixed tempo.
- Find the one. Align a loop to the downbeat before you layer it.
- Make a DJ cue sheet. Export the grid and mark the drops.
What it gets wrong
- Half or double time. The tracker may tap twice as fast or half as fast as you would. Both can be musically right.
- Unusual meters and rubato. Odd time signatures, free-tempo passages and ambient music give weak or wrong grids.
- Beats, not tempo. You get beat times; a single tempo number is an average of them and can mislead on music that speeds up.
- Tracking the drum stem is usually steadier than the full mix.
Build your own
Ask your coding agent to add a beat grid to your module:
Read the Maru docs page "Beats and tempo in a module", then read StemSep's source with `maru module files stemsep`, looking for how its face calls trackRhythm. Add "slice on beats" to my sampler, falling back to onset detection when no beat tracker is available.
Developers can start from Beats and tempo in a module. The model directory describes the beat tracking models.