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Pulse oximeter frame clock

The oximeter module plays the plethysmogram out in 33.3 ms (30 Hz) frames on its own clock, fixed per recording setup at +7 to +92 ppm against the SNUADC sample clock (medians over 42 PC keys).
Type oscillator
Nominal 30 Hz output frames

Description

Two rates live inside the oximeter. The frame clock sets when the output changes; the PPG content runs at a slightly different rate against those frames, so a frame of lag builds up and one frame is repeated: the sawtooth (PPG content against the oximeter frames (the sawtooth)). Raw-sample PPG fiducials lock to the frame grid and reveal it.

Route to UTC

The route the atlas trusts most (3 links; its weakest link is measured apart from the link to UTC that every clock shares):

Pulse oximeter frame clock
link measured Oximeter frames against SNUADC: drift +7 to +92 ppm by setup (medians over 42 PC keys); within 3 ppm of its setup's median in 89% of recordings
SNUADC sample clock
link measured SNUADC (frequent-correction mode) against the recorder PC: 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)
Recorder PC clock
link assumed Recorder PC against UTC: drift 0.1–10 ppm (assumed); corrected every 8–24 h (assumed)
UTC
Where every route ends.

Independent checks. 7 other routes reach UTC; where they meet, the offsets must close around the loop:

Plan. Measured against SNUADC two ways (fiducial frame locking, and the 30 Hz lines), then through SNUADC to the PC. The PPG content's own drift against the frames (the sawtooth) is corrected tooth by tooth.

Link Clock Against Modes Rate Corrected every Status
Oximeter frames against SNUADC Pulse oximeter frame clock SNUADC sample clock all +7 to +92 ppm by setup (medians over 42 PC keys); within 3 ppm of its setup's median in 89% of recordings — measured
PPG content against the oximeter frames (the sawtooth) Pulse oximeter frame clock SNUADC sample clock all 18–52 ppm by setup — measured

Observations

Observation Status Summary
The ECG→PPG delay is a sawtooth established Inside the oximeter the PPG content creeps 18–52 ppm against its 30 Hz frames; every 11–31 minutes a frame repeats and the delay jumps up by 33 ms. The rate is fixed per recording setup.
The oximeter's frame clock is a fingerprint of the setup established 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.
The PPG sawtooth sometimes stops open In some stretches, and some whole recordings, no frame slip comes although several were due. The delay there is flat and the frame clock keeps running; what stops the content's drift is not known.

Studies

Study Date Status Summary
One device delay for the PLETH across the release 2026-10-18 current The stored ECG-to-PLETH delay was measured in 5,536 recordings, to the foot and to the steepest upstroke of each pulse. It has one peak (655 ms to the foot, 715 ms to the upstroke, in the median recording), no clusters by recording setup or SNUADC port, and no shift along the case order: one global device delay, 385 ms with an adopted R-to-foot finger PAT of 270 ms, fits the whole release, as Ruffolo et al. found in their own data.
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.
Arterial pressure as a check on the PPG correction 2026-10-11 current Arterial pressure is sampled by the same converter but has no frame and no sawtooth, so every PPG slip leaves ECG→ABP unchanged. Against it, detected slips are 97–100% real and tier A leaves about 2 ms of drift mid-tooth.
Where the PPG sawtooth stops 2026-10-11 current In some stretches no slip comes although several were due. The PPG delay there is flat, not hidden: the monitor keeps framing, and only the content's drift against the frames has stopped.
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.