/beats?path= returns bpm, a phase-locked beat grid, downbeats (4/4) and an
honest confidence, using only stdlib + ffmpeg: decode to PCM, log-energy onset
flux, harmonic-enhanced autocorrelation with an octave check, comb phase
search, weighted LS grid refit. 4-min track analyses in ~0.5s, cached after.
Audio in/out now honoured at mux in both scene renders and film beds via one
resolve_audio() path. Found en route: on this ffmpeg build the obvious
atrim/asetpts + amix/apad chain yields an aac stream with ZERO samples, rc=0,
no warning - trim moved to the input side; and the test helper would have
passed on that empty stream, so it now refuses to answer on a window that
decodes nothing.
Tempo bias fixed after orchestrator ground-truth check: decoding at 22050
discarded the HF transient, and since the attack's HF share grew across a file
the surviving LF onset landed progressively later - a per-beat-varying
lateness that reads as tempo error. BEAT_SR now 44100/512 (same 86fps
envelope). Second bug found by sweeping: harmonics outside the lag table
counted as zero and the parabola was fitted on a slope, so 118 BPM read
114.79.
Verified vs independent ground truth: 120.030 vs 120.003 true, 128.020 vs
128.004; worst offset from the true grid 6.0/10.1ms over the whole file (was
45/52ms). Tests now assert whole-file DRIFT, not average BPM, incl. a 200s
track and hftick.wav (16kHz tick over a 60Hz body) which pins BEAT_SR.
18 groups green.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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