BIS against the recorder PC¶

The BIS EEG stamps drift about +25 ppm against the PC (median from the correction steps of 1,939 recordings; a fit over every recording's stamps gives +21 ppm in 4,728) and are pulled forward on a fixed 8,192 s timer of the BIS's own, which does not start with the recording.


Clock	BIS clock
Measured against	Recorder PC clock
Modes	all
Status	measured
Drift	25 ppm (p10–p90 12 to 50; 5th–95th 7.3–51.1 ppm; census fit of 4,728 recordings median 21.1 ppm)
Corrected every	8,192 s (2 h 16.5 min), ± 20 s in 95% of intervals
Correction size	0.17 s median

Correction¶

On a fixed timer: every 8,192 s the run is cut and restarted forward by whatever drift has built up. A correction moves every BIS track together. In all recordings (When each device's clock is corrected, in every recording), 95% of corrections after the first are 8,192 ± 20 s apart, and the first comes at any time after the BIS starts: the timer is the BIS's own. The rate is relative to the recorder PC, whose own corrections are part of what it measures.

Parameters¶

Parameter	Value	Status	From
BIS drift against the recorder PC	25 ppm (p10–p90 12 to 50; 5th–95th 7.3–51.1 ppm; census fit of 4,728 recordings median 21.1 ppm)	measured	The stamp sawtooth model for SNUADC and BIS
BIS correction interval	8,192 s (2 h 16.5 min), ± 20 s in 95% of intervals	measured	When each device's clock is corrected, in every recording
BIS correction size	0.17 s median	measured	When each device's clock is corrected, in every recording

Evidence¶

Study	Date	Status	Summary
When each device's clock is corrected, in every recording	2026-10-19	draft	From the steps between records in all 6,388 recordings: the BIS is corrected every 8,192 s and the Primus every 7,230 s, on their own timers (the first correction comes at any time after the device starts); the SNUADC is reset when 0.49 s of drift has built up. With the full rerun of the correction fit, it gives each device's drift against the recorder PC in 4,728 BIS and 2,728 SNUADC recordings, and shows that the Primus's 20–23 s AWP pauses belong to its correction cycle. Every rate here is relative to the recorder PC, whose own clock drifts and is corrected.
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.