Drop a beat. Get the tempo and the key.

How it works. The browser decodes your audio via the Web Audio API and downmixes to mono. BPM: we build a coarse energy envelope (squared samples, box-filtered) and autocorrelate it across lag values that correspond to 60–200 BPM, picking the strongest periodic match. Key: we slide a 4096-sample Hann-windowed FFT across the first ~30s and bin power into a 12-element pitch-class chroma vector, then correlate against all 24 (12 major + 12 minor) Krumhansl- Schmuckler key profiles to pick the best fit. Tradeoffs: works best on tracks with a clear pulse and a stable tonal center — drum-driven beats with a consistent harmony are ideal. Ambient or modulating music can shift the chromagram around. Half/double-time tempo ambiguity is shown alongside; relative-major/minor key ambiguity (a classic Krumhansl-Schmuckler weakness) is the most common key-detection error. Piano roll → scale detector and guitar tuner are the companion tools.