P4: NOTES — broadband hypothesis tested NEGATIVE (per-bin reduction confirmed) + LUT re-cut rejected

Windowed-FFT of dual ref: gain@500=0.309, gain@2000=0.292 but gain@1000 (empty bin)=0.995
=> reduction is per-bin (am_k/res_k), NOT broadband — validates model structure.
16-knot monotone LUT (floor .35/cap .70) regressed corpus mean 0.77->1.09; reverted.
t1k_b1f/al residuals rooted in am-normalization / input-level differences, not LUT shape.

handoff/NOTES_LEVEL.md

@@ -970,3 +970,25 @@ multiband interplay absent) -> keep Pchip baseline committed; A/B/gamma = runtim
 BandConfig ctx+0x188, NOT live-captured (SET TO: static path uses preset curve).
 Structural next: decode BandConfig ctx+0x188 writers (FUN_1804835d0/487200 at
 decomp_funs2.txt:21937/21962), wire combine 0x5407c8 + FFT-conv 0x535a70.
+
+## ============ UPDATE 2026-08-20s: BROADBAND HYPOTHESIS TESTED (NEGATIVE) + LUT re-cut attempt ============
+Tested the mask
+
+## ============ UPDATE 2026-08-20s: BROADBAND HYPOTHESIS TESTED (NEGATIVE) + LUT re-cut attempt ============
+Tested the "mask uses scalar broadband level" hypothesis (NOTES:637) directly: windowed
+FFT of dual_b1q_1.0 ref vs dual.wav input (2.5-3.5s steady):
+  gain@500 = 0.309 (-10.2dB), gain@2000 = 0.292 (-10.7dB), gain@1000 (NO tone) = 0.995 (-0.05dB),
+  gain@1500/3000/5000 = 0.85/1.00/1.00.
+=> REDUCTION IS PER-BIN: only bins *with a tone* are cut, empty bins pass through (~1.0).
+   So the level path is am_k/res_k (per-bin), NOT a scalar broadband level. Model structure correct.
+LUT re-cut attempt (floor 0.35/cap 0.70 from t1k_b1f+al_ extraction, monotone 16 knots):
+  REGRESSED whole corpus (mean|err| 0.77 -> 1.09, dual/t1kq/local worse). Reverted to the
+  joint-fit 12-knot Pchip (dual<=0.72, fc-scan<=0.59, corpus mean 0.77, max 2.12) = KEEP.
+Root cause of t1k_b1f residual (+2.1 loud) and al low-level (-1.8): NOT LUT cap shape alone;
+  al/t1k use DIFFERENT input (0dBFS vs -18dBFS) and the am normalization (pipeline am vs
+  tone_amp metric) differs; the joint-fit nodes were fit AT the al_* pipeline xv, so they
+  don't extrapolate cleanly to t1k_b1f's xv~0.85. Structural A/B/gamma (BandConfig ctx+0x188)
+  would give the *curve*, but writers are JUCE param plumbing (FUN_1804835d0/487200 = UI curve
+  setters, not DSP scalar setters) - static decode exhausted for now.
+NEXT: wire combine/accumulator 0x5407c8 + FFT-conv 0x535a70 into the chain (structural, may
+  close the t1k_b1f/al tail by smoothing the mask) - todo #3.