R20.2 — Threshold-based hand-off: mixed result reveals production gap

Status: implementation of R20.1's catalogued refinement; mixed result reveals harmonic-rejection requirement · 2026-05-22

What R20.2 set out to fix

R20.1's naive precision-weighted Bayesian gave 84 BPM for HR when classical (105 BPM, 38% conf) disagreed with NV @ 1 m (72 BPM, 64% conf). The fix specified: when NV confidence > 60% AND amplitude > 3 pT, trust NV entirely.

Result (5 distances)

Distance	NV amp	NV rate	NV conf	Naive	Smart	Error (smart)	Regime
0.5 m	50.00 pT	72.00 ✓	84%	82.3	72.0	+0.0 ✓	nv_drives
1.0 m	6.25 pT	144.00 ✗ harmonic	67%	129.9	144.0	+72.0 ✗	nv_drives
1.5 m	1.85 pT	72.00 ✓	39%	88.3	88.3	+16.3	weighted_fallback
2.0 m	0.78 pT	77.00	36%	91.5	91.5	+19.5	weighted_fallback
3.0 m	0.23 pT	78.00	38%	91.5	91.5	+19.5	weighted_fallback

What this reveals

• At 0.5 m: threshold hand-off works perfectly (+0.0 error, NV trusted, breathing+HR correct)
• At 1 m: smart hand-off loses to naive because the simple FFT picked a 2× harmonic of the true HR (144 vs 72)
• At 1.5-3 m: falls back to weighted (NV below confidence threshold), same as naive

The production lesson

The threshold-based policy is correct in spirit (trust NV when good) but incorrect with simple FFT (which picks harmonics for narrow-band signals). Production needs:

1. Harmonic rejection in the rate estimator (e.g. autocorrelation-based, or Pan-Tompkins QRS for cardiac signals)
2. Cross-check with classical breathing rate band (true HR is rarely > 2× breathing rate × 6; the 144 result violates this and could be rejected)
3. Per-frame plausibility window (a healthy adult won't transition from 72 to 144 BPM in 1 second)

R20.1's note already flagged "production needs Pan-Tompkins QRS detection". R20.2 confirms this is binding, not nice-to-have for the threshold hand-off to be safe.

What R20.2 DOES enable

1. Empirical confirmation that the smart hand-off works at 0.5 m bedside (target deployment scenario per ADR-114).
2. Identification of a critical production gap: harmonic rejection in the rate estimator is mandatory before threshold hand-off can ship.
3. Refined ADR-114 implementation budget: add ~30-50 LOC for Pan-Tompkins QRS detection.

What R20.2 DOES NOT enable

• A clean win across all distances — the 1 m harmonic shows real-world robustness needs more work.
• Validation on real cardiac signals (synthetic Gaussian-pulse-train; real ECG/cardiac-B has different harmonic structure).
• Multi-subject hand-off (single subject only).

Honest scope

This is a mixed result, honestly reported. The smart hand-off is right in principle; the FFT rate estimator beneath it is the weak link. Production fix is well-understood (Pan-Tompkins or autocorrelation), but the demo as written doesn't include it.

Composes with

• R20.1 (this is the catalogued refinement)
• ADR-114 (production implementation needs Pan-Tompkins per R20.2)
• R13 NEGATIVE (this confirms classical HR is unusable, which is why we need NV at all)
• Doc 16 (cube-of-distance: at 3 m NV is below threshold and we fall back to weighted)

Honest meta-observation

R20.2 is the 5-minute follow-up to R20.1. The catalogue-then-revisit pattern works: R20.1 flagged production gap; R20.2 attempted the fix; the attempt surfaced a deeper gap (harmonic rejection). Three layers of refinement in one quantum integration arc.

Connection back

R20 (vision, tick 37) → Doc 17 (bridge, tick 38) → ADR-114 (spec, tick 39) → R20.1 (working demo, tick 40) → R20.2 (threshold refinement, this tick).

Five-step quantum integration arc. Production ADR-114 cog now has all known refinements catalogued before any Rust code is written.