The EEG–ECG offset is fixed by the SNUADC correction mode¶

After the stamp model, BIS EEG still lags the SNUADC ECG by a constant: −804 ms in step mode and −660 ms in frequent-correction mode over the whole release, tight within a port or unit. The mode, a property of the setup, sets the constant.


Status	established: shown on many cases, with a control.
Clocks	SNUADC sample clock, BIS clock
Devices	SNUADC, BIS monitor
Variables	BIS EEG, ECG lead II (SNUADC)

What it means¶

One EEG–ECG constant for all recordings is wrong by about 110 ms for the minority in frequent-correction mode.

What is done about it¶

The stamp model can apply the constant of the recording's mode instead of one for all; the setup register says which setups use frequent-correction mode.

Detail¶

Pilot, 117 step-mode and 29 frequent-mode cases: using the constant of the case's mode (leave-one-out) lines a typical window up to 33 ms (median; p90 116 ms), against 38 ms (p90 154 ms) with one constant for all recordings and 16 ms with the case's own constant from cardiac artefact bursts. Whole release (2,710 recordings, 19 October): −804 ms in step mode (1,526), −660 ms in frequent-correction mode (78) and −766 ms with no SNUADC correction (1,099); the mode's constant lines a window up to 42 ms (p90 150 ms), the recording's own to 17 ms (p90 79 ms). Why the constant differs between modes is open (Closing loops in the timing graph).

Evidence¶

Study	Date	Status	Summary
Closing loops in the timing graph	2026-10-13	current	Where two independent routes reach the same clock, the offsets around the loop must sum to zero. Three measured loops close within their noise; the BIS loop closes up to a constant latency; the proposed EMR loop is not a loop at all.
The stamp sawtooth model for SNUADC and BIS	2026-10-12	current	Fitting each track's record stamps against its sample count as a drift with corrections (a sawtooth) puts SNUADC and BIS on one timeline without any artefact. Checked against shared bursts, the EEG–ECG timing error falls from about 790 ms on the recorded stamps to 33 ms with one constant per SNUADC mode in the pilot, and 42 ms over the whole release.