The plethysmogram is built in 30 Hz frames¶

The SNUADC PLETH waveform comes from the oximeter in 33.3 ms frames, smoothed to 500 Hz. Its content changes once per frame, on the oximeter's own clock, about 0.4 s after the finger.


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

What it means¶

Pulse arrival time from VitalDB's PLETH is about 0.4 s longer than physiology and carries a sawtooth of frame slips; raw-sample PPG fiducials lock to the frame grid.

What is done about it¶

physiosync removes the sawtooth; the planner shifts PLETH by its chain delay.

physiosync: physiosync/frames.py

Evidence¶

Study	Date	Status	Summary
Monitor clocks in the waveforms' spectral lines	2026-10-17	open	Differences of the raw converter counts show lines from the monitor's analogue outputs: 240 Hz in arterial pressure and plethysmogram, and a 30 Hz family in the plethysmogram alone. Each measures a monitor clock against SNUADC.
The PPG hardware delay and the adopted pulse arrival time	2026-10-12	current	VitalDB's raw ECG→PPG pulse arrival time is about 0.4 s longer than physiology. Splitting it into a hardware delay (one value for the release; One device delay for the PLETH across the release) and a physiological part needs an adopted value from studies that sampled ECG and PPG on one clock: about 270 ms for anaesthetised adults.
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.
The PPG sawtooth and its correction	2026-10-10	current	The ECG→PPG delay creeps at 18–52 ppm and jumps up by one 33.3 ms frame at each slip, every 11–31 minutes. A tooth-by-tooth model removes it; where the correction is confident it is essentially exact on injected test data.