The oximeter's frame clock is a fingerprint of the setup¶

The rate of the oximeter's 30 Hz frame clock against the SNUADC sample clock is fixed per recording setup (+7 to +92 ppm across setups) and repeats within about 2 ppm inside a recording, so it identifies the hardware a recording was made on.


Status	established: shown on many cases, with a control.
Clocks	Pulse oximeter frame clock, SNUADC sample clock
Devices	Solar8000 monitor, SNUADC
Variables	Plethysmogram (SNUADC)

What it means¶

A recording's frame clock can assign it to a setup without any port information, or split recordings that share a port number but were made on different hardware.

What is done about it¶

Measured from PLETH fiducials taken on raw samples (physiosync.frame_clock); see Recording setups.

Detail¶

Found in 189 of 266 cases; the two halves of a recording agree within 2 ppm (median). Examples: +84 ppm (IQR 82–88) for one SNUADC port in step mode, +39 ppm for the same port in frequent-correction mode, +72 and +24 ppm on two others. The same port giving two rates in the two modes shows that a port alone is not a setup.

A scan of 1,350 recordings on 18 October (up to 30 per PC key) found the frame clock in 656. Within a PC key, 89% of them lie within 3 ppm of the key's median, against 19% within 3 ppm of the release's median; the key medians run from +7 to +92 ppm (Oximeter frame clock against SNUADC, Identifying the recording setup of every case). Two keys hold two distinct rates, probably two oximeter modules on one PC.

Evidence¶

Study	Date	Status	Summary
Identifying the recording setup of every case	2026-10-18	open	Recognising which equipment made each recording, from its serial ports and, where those fall short, from clock fingerprints, so that per-setup values can be used for every case.
The PPG's 30 Hz frame clock	2026-10-10	current	ECG and PPG fiducials resolve to one 2 ms sample, but PPG fiducials taken on raw samples lock to a hidden 33.3 ms frame grid. Scanning for it finds the oximeter's frame clock to about 1 ppm, a fixed fingerprint of each recording setup.