SNUADC (step mode) against the recorder PC¶

In step mode the SNUADC stamps drift by tens of ppm and are pulled back by about half a second whenever the drift reaches that size, about every two hours.


Clock	SNUADC sample clock
Measured against	Recorder PC clock
Modes	step
Status	measured
Drift	−63 ppm (p10–p90 −79 to −44; 5th–95th −80.3 to −36.9 ppm; medians of −46 to −74 ppm by serial port)
Corrected every	2.08 h (5th–95th 1.65–3.57 h between resets; per-recording means p10–p90 1.68–3.07 h)
Correction size	about 0.498 s true (paced: 0.4981–0.4988 s); −0.480 s on the stamps

Correction¶

Triggered by drift, not by a timer: once the stamps are about 0.5 s ahead, the recorder pulls the track back. The stamps show only −0.480 s of the true correction of about 0.498 s, because about 18 ms of signal is never sampled (Signal lost at a SNUADC step-mode correction). In all recordings (When each device's clock is corrected, in every recording): 3,417 resets in 2,540 recordings, a median 2.08 h apart; the drift count usually starts with the recording (the first reset falls at 0.99 × the time 0.49 s of drift takes, interquartile 0.87–1.01), though not reliably enough to time drift from a single reset. Rates are relative to the recorder PC.

Parameters¶

Parameter	Value	Status	From
SNUADC step-mode drift	−63 ppm (p10–p90 −79 to −44; 5th–95th −80.3 to −36.9 ppm; medians of −46 to −74 ppm by serial port)	measured	The stamp sawtooth model for SNUADC and BIS
Signal lost at a SNUADC step-mode correction	17.7 ms (range 17.2 to 18.2; n = 3; 3 corrections in 2 paced cases; ECG–EEG bursts give 13.0 ± 1.7 ms)	measured	Paced ECG as an independent clock
SNUADC step-mode correction size	about 0.498 s true (paced: 0.4981–0.4988 s); −0.480 s on the stamps	measured	Paced ECG as an independent clock
SNUADC step-mode correction interval	2.08 h (5th–95th 1.65–3.57 h between resets; per-recording means p10–p90 1.68–3.07 h)	measured	The stamp sawtooth model for SNUADC and BIS

Evidence¶

Study	Date	Status	Summary
Paced ECG as an independent clock	2026-10-17	current	Fixed-rate pacing gives a clock the recording chain cannot touch. Timing paced beats on three timelines shows that each SNUADC step-mode correction loses about 18 ms of signal, and that nothing else breaks the sample stream.
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.