SNUADC (frequent-correction mode) against the recorder PC¶

In frequent-correction mode the SNUADC stamps follow the PC, with small forward steps; the −0.95 ppm they show against the sample count is stamp arithmetic.


Clock	SNUADC sample clock
Measured against	Recorder PC clock
Modes	frequent
Status	measured
Drift	−0.954 ppm (n = 6321; census of 6,321 files (17 October): median −0.9535 ppm; 89.5% of recording hours within 0.05 ppm of −0.954; 747 files (12%) between −0.90 and −0.55 ppm)
Correction size	+5 to +15 ms (median +10), about one an hour (median 1.1 per hour per recording)

Correction¶

Small forward steps of +5 to +15 ms, about one an hour; never the 0.48 s pull-back of step mode.

Parameters¶

Parameter	Value	Status	From
SNUADC frequent-correction steps	+5 to +15 ms (median +10), about one an hour (median 1.1 per hour per recording)	measured	Clock corrections in every file
SNUADC stamp arithmetic	−0.954 ppm (n = 6321; census of 6,321 files (17 October): median −0.9535 ppm; 89.5% of recording hours within 0.05 ppm of −0.954; 747 files (12%) between −0.90 and −0.55 ppm)	measured	SNUADC stamp arithmetic

Evidence¶

Study	Date	Status	Summary
SNUADC stamp arithmetic	2026-10-17	current	Between corrections the SNUADC block stamps run −0.95 ppm against the sample count: each 0.2 s increment is about 0.19 µs short. This, not clock drift, is the "about −1 ppm" seen in frequent-correction mode.
Clock corrections in every file	2026-10-11	current	A scan of all 6,388 files for the jumps where the recorder re-synchronises a device's stamps. Each device type has its own correction signature; only SNUADC and BIS correct clearly enough to measure their drift.