Pulse Oximeter
Explore how red and infrared PPG channels are separated into DC and pulsatile AC components before a ratio-of-ratios is formed.
Signal source
Channel controls
Why these controls?
These sliders build the two synthetic PPG signals: DC baseline, AC pulsatile amplitude, heart rate and noise. Change them to see how each ingredient moves the ratio-of-ratios R — and when the signal becomes too poor to trust.
Two-wavelength PPG
Clean reference and observed signals
Red AC / DC
0.0200
2.00 / 100.0
Infrared AC / DC
0.0273
3.00 / 110.0
Clean reference R
0.733
Known synthetic inputs
Estimated R
0.780
Observed signal estimate
Signal quality: sufficient
Pulsatile components are distinguishable in both channels.
Core calculation
Ratio-of-ratios
R = (ACred / DCred) / (ACIR / DCIR)
DC is the mean signal level. AC is defined here as half the peak-to-peak pulsatile excursion, using the same definition for both channels.
Optional teaching line
R–SpO₂ calibration
This user-defined line is not a clinical calibration or a measured SpO₂ value. Real conversion is experimentally calibrated for a specific device and sensor; there is no universal conversion equation.
Student checks
- Scale AC and DC proportionally in both channels and confirm that R is unchanged.
- Change only one channel’s DC baseline and observe the change in R.
- Use low perfusion to explain why a near-zero AC component makes the ratio unreliable.
- Compare clean and observed estimates under motion and noise.