Library & files
BPM detection
BPM detection finds the repeating pulse in a track by measuring where energy rises and looking for the interval that best explains those onsets. It returns a tempo and a grid, and both can be right about the wrong thing.
In practice
Check the octave before you trust the number. Half and double are the errors that survive looking correct.
After a batch analysis, sort by BPM and look at the extremes — a cluster of drum and bass sitting in the eighties is the octave error, not a genre of slow records. Fix the tempo first, then check that beat one is on the first kick rather than on whatever transient preceded it. Both corrections are permanent once written, which is why this is a job for an evening at home rather than a moment in a booth.
½ / 2×
The characteristic error
Placement
The other half of the job
Sort by BPM
How to find them fast
Where these come from: How onset-based tempo detection fails — established signal-processing behaviour. The catalog reading of the half-time problem is on the linked article. For what we can measure, see the half-time correction, measured.
Why it matters
The characteristic error is not a few beats out — it is exactly half or exactly double, because a pulse at 87 explains a 174 BPM record perfectly well. Drum and bass, footwork and half-time productions are where this bites, and the number in the field can be honest and useless at once.
The second error is placement. A detector that finds the right tempo can still put beat one on the wrong transient — an intro sweep, a reversed cymbal — which produces a grid that drifts nowhere but is a beat off everywhere. That is a different problem from a wrong tempo and it needs a different fix.
Frequently asked
Why does my drum and bass show as 87 BPM?+
Because a pulse at half the playing tempo explains the record just as well mathematically. It is the classic octave error and it is fixed by doubling the tempo and re-gridding.
The BPM is right but the grid is wrong — why?+
The detector found the right interval and the wrong starting point, usually an intro transient. Move beat one to the first real kick and the grid falls into place.
Does a fixed BPM mean the grid is fixed?+
No. Tempo and grid placement are separate, and a track can have the right tempo with beat one in the wrong place — which sounds like everything being consistently a beat off.
Related terms
Beatgrid
A beatgrid is the map your DJ software lays over a track marking where every beat falls. Once it is right, sync, looping and quantise all work; once it is wrong, everything built on top of it is wrong too.
Beatgrid drift
Beatgrid drift is a grid that lines up at the start of a track and slides out of alignment later — the tempo the software stored is close to the real one but not equal to it.
Key detection
Key detection estimates a track’s musical key from the audio by measuring which pitches dominate and matching that profile against major and minor templates. It is an estimate, and it is confident even when wrong.
Batch analysis
Batch analysis runs the tempo, key, grid and waveform detection across many tracks at once rather than as each one loads. It is processor-heavy, takes hours on a large library, and only has to happen once per track.