Every time is quantised, and fiducials inherit it¶
A time on a grid of step Δ carries an error of Δ/√12 (1 SD). In VitalDB that is 0.58 ms for a 500 Hz sample, 2.3 ms for BIS EEG, 0.8 ms for the arrhythmia annotations' 1/360 s grid, and up to ±17 ms for PPG fiducials taken on raw samples, which lock to the 33.3 ms frame.
| Status | established: shown on many cases, with a control. |
| Variables | Plethysmogram (SNUADC), ECG lead II (SNUADC), BIS EEG |
What it means¶
Timing differences smaller than the grid's SD cannot be resolved from single events; averaging over n events divides the random part by √n. Sub-sample interpolation helps only where the signal is smooth and not frame-quantised.
What is done about it¶
The PPG correction uses the steepest upstroke on the filtered signal, which does not lock. The planner adds each signal's interval/√12 to its timing error.
Detail¶
| Time | Grid | 1 SD |
|---|---|---|
| SNUADC sample (500 Hz) | 2 ms | 0.58 ms |
| BIS EEG sample (128 Hz) | 7.8 ms | 2.3 ms |
| Arrhythmia annotations | 1/360 s | 0.80 ms |
| Numeric record stamps | 1 ms (0.1 ms in about 17% of files) | 0.29 ms |
| PPG fiducial on raw samples, locked to the frame | up to 33.3 ms | up to about 9.6 ms |
| Clinical-table entries | 1 or 5 min | 17 s or 87 s |
Which PPG fiducials lock depends on how flat the pulse is where they sit: the raw systolic maximum locks most (interval resultant at 33.3 ms: 0.38), the raw foot and steepest rise less (0.10), and the steepest upstroke on the filtered signal hardly at all (0.02). Consecutive inter-beat intervals show no coarser grid for any detector.
Evidence¶
| Study | Date | Status | Summary |
|---|---|---|---|
| Time zeros and loaded subsets | 2026-10-16 | current | A VitalDB time "from the start" depends on which tracks were loaded: loading a subset moves the zero to that subset's first record, seconds or hours later. The arrhythmia database's annotations count from the full file's zero. |
| 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. |
References¶
- Ruffolo I, Siddiqui A, Nguyen B, Dixon W, Assadi A, Greer R, Schwartz S, Brudno M, Mariakakis A, Goodwin A. High-fidelity measurement of pulse arrival time in critically ill children using standard bedside monitoring equipment. Physiol Meas. 2025;46(11):115006. ↗