# Detecting musical key and tempo on-device: the DSP behind BeatScope

> Source: <https://dev.to/conkormedia/detecting-musical-key-and-tempo-on-device-the-dsp-behind-beatscope-31me>
> Published: 2026-07-14 21:36:50+00:00

I needed to read a track's **BPM** and **musical key** in seconds, offline, on a

phone. No ML black box, no server. Here's the signal-processing pipeline I ended

up with — hand-rolled, since the usual packages weren't an option.

##
Tempo

Tempo is periodicity in the track's energy. The steps:

-
**Spectral flux → onsets.** Sum the positive frame-to-frame magnitude increases
of the STFT. Peaks are onsets (kick, snare, synth attacks). That's the onset
envelope.

-
**Autocorrelation.** The lag where the onset envelope best matches itself is the
beat period.
-
**Comb over multiples.** Weight candidate periods by their multiples (½, 2×) so
tempo isn't confused with its harmonics.
-
**Octave folding + prior.** Fold candidates into 60–180 BPM and softly prefer
typical dance tempos — kills the "174 vs 87" error.
-
**Median smoothing** for stability.

For the beat **grid**, I use dynamic programming (Ellis-style): reward beats that

land on onsets, penalize deviation from a steady interval. The DP gives a grid

that doesn't drift.

##
Key

-
**Chromagram.** Fold the spectrum (~55–2000 Hz) into 12 pitch classes,
accumulated with a leaky integrator (~8 s memory) → an averaged chroma vector.
-
**Krumhansl–Schmuckler correlation.** Correlate against 24 reference key
profiles (12 major + 12 minor); best match wins.

-
**Map to Camelot** for harmonic mixing. This lineage goes back to libKeyFinder.

##
Honest limits

- Atonal/dense harmony blurs the chroma vector → less certain key.
- Mid-track modulation → it reports the dominant key.
- Weak percussion/rubato → fewer onsets, shakier BPM.

##
Sources

- Krumhansl,
*Cognitive Foundations of Musical Pitch* (1990)
- Sha'ath,
*Estimation of Key in Digital Music Recordings* (2011)
- Ellis,
*Beat Tracking by Dynamic Programming* (2007)

The pipeline ships in **BeatScope**, a free offline analyzer for macOS/iPhone:

[https://beatscope.pro](https://beatscope.pro) — happy to answer DSP questions in the comments.