Monitor clocks in the waveforms' spectral lines¶

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.

Data¶

6 short-to-medium files (1.4–2.6 h), chosen at random; work of the R-peak timing session, consolidated here.

Method¶

Integer counts were recovered from each track and the spectrum of their sample-to-sample changes searched for lines. The 240 Hz line was demodulated in 10 s windows on the global sample index; its frequency came from a zero-padded FFT with parabolic peak interpolation. The phase difference of ART and PLETH at that line gave their skew.

Results¶

• 240 Hz in PLETH and ART (weakly in ECG in 3 cases): −25.4 to −28.7 ppm against the SNUADC.
• A 30 Hz family (30, 60, 90, 150 Hz) in PLETH only: −30 to −73 ppm in frequency, that is +30 to +73 ppm in period, matching the oximeter frame clock from fiducial locking (The PPG's 30 Hz frame clock).
• ART leads PLETH by 0.31–0.39 ms at 240 Hz in every case, consistent with the converter reading its channels one after another.
• Line frequencies group files by device: two of the six share every line and are probably the same monitor and converter.
• No frame-slip sawtooth was seen in the ART→PLETH lag of these six files, which conflicts with The PPG sawtooth and its correction; a few slips per short file barely show in a jump histogram, and many stretches have a paused sawtooth. To settle by running these files through physiosync.

Discussion¶

A third, independent clock (the monitor's output stage) measured from the waveforms alone, and a blind way to group recordings by device.

Observations it supports¶

Limitations¶

Six files.

Open questions¶

• Monitor clock lines in six files only (next step). The monitor clock lines were seen in six files; running these files through physiosync, and then a larger set, would settle whether they are general.