docs: accuracy experiment log E1-E4

Co-Authored-By: Claude Opus 5.5 <[email protected]>
2026-10-30 21:00:45 +01:00
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+# Accuracy experiments — 2026-10-31
+
+Branch `exp/accuracy`. Every experiment is judged on **both** the synthetic tiers
+(median merged-guitar pitch F1, ±50 ms) and the real reference song
+*Last Night on Earth* (`lnoe`, the only reference with a trustworthy alignment:
+constant +0.03 s, precision 93%). Swimming/Holes alignments are still flagged and
+are not used for decisions.
+
+Baseline before these experiments: synthetic full-mode guitar F1
+t1a 0.43 · t1b 0.51 · t1c 0.39 · t1d 0.60; lnoe guitar F1 0.390.
+
+| # | Experiment | Synthetic (median guitar F1) | lnoe guitar F1 | Verdict |
+|---|---|---|---|---|
+| E1 | Transcribe guitar from Demucs guitar + piano + other (instead of guitar stem only) | t1a 0.87 · t1b 0.81 · t1c 0.73 · t1d 0.72 | 0.403 | **Adopted** (commit 0f14fbc). Big synthetic gain is mostly a SONiVOX-timbre artefact (Demucs routes the synth guitar to piano/other); real gain is small but positive. t1a now passes. |
+| E2 | Basic Pitch thresholds grid (onset 0.3–0.6, frame 0.2–0.4, min note 58/100 ms) | best ≈ default (0.5/0.3/58) | best 0.411 vs 0.403 | Not adopted — defaults are at the optimum on both. |
+| E3 | Error taxonomy | — | fp: 46% no reference note near, 16% octave, 21% 50–150 ms timing; fn: 47% different pitch at the onset (partial chords), 25% octave, 24% missed | Octave fixing upper bound only 0.40 → 0.46 (pitch-class F1) with no systematic direction. |
+| E4 | Complete partial strummed chords to standard guitar voicings (open + E/A-shape barres; maj/min/5/7/m7/sus/maj7) | −0.03…+0.03 | +0.016 (min 3 notes) | Not adopted yet — small, single-song evidence. |
+
+## Learnings
+
+- Synthetic gains do not transfer 1:1 to real recordings while the synthetic guitar
+  is the SONiVOX GM patch; a more realistic guitar sound in the renderer would make
+  the synthetic benchmark more predictive.
+- On real audio the limits are Basic Pitch's partial-chord recall on strummed
+  rhythm guitar and octave confusions, not thresholds.
+- An undocumented MT3 ONNX export exists (HF `ModernMube/HTDemucs_onnx`, ~300 MB);
+  not pursued — input/token format would need reverse-engineering and MT3 is weak
+  on real recordings.
+
+Experiment scripts live in the git-ignored `.scratch/exp/`.