hard-coded calibration coeffs for MAX30101

Author: jaketheduque

EmotiBit SpO2 to EB Oscilloscope.ahk

@@ -1,5 +1,5 @@
 # Description
-This algorithm is derived from the Brainflow SpO2 algorithm, and uses PPG-R and PPG-IR data from the EmotiBit to calculate SPO2 level.
+This algorithm is derived from the Brainflow SpO2 algorithm, and uses PPG-R and PPG-IR data from the EmotiBit to calculate SPO2 level. As a starting point, `run.py` splits PPG-R and PPG-IR signals into buffers of length 64, which are then passed to the algorithm. It is recommended for the buffers to contain **at least** 64 samples for accurate results.
 
 # Required Hardware
 - Emotibit

README.md

@@ -9,30 +9,18 @@ PYBIND11_MODULE(SPO2Algorithm, m) {
   m.def(
     "get_oxygen_level",
     [](py::array_t<float, py::array::c_style | py::array::forcecast> ppg_ir,
-       py::array_t<float, py::array::c_style | py::array::forcecast> ppg_red,
-       int sampling_rate,
-       float callib_coef1,
-       float callib_coef2,
-       float callib_coef3)
+       py::array_t<float, py::array::c_style | py::array::forcecast> ppg_red)
     {
         float oxygen_level = 0.0;
         get_oxygen_level(
             const_cast<float*>(ppg_ir.data()),
             const_cast<float*>(ppg_red.data()),
             static_cast<int>(ppg_ir.size()),
-            sampling_rate,
-            callib_coef1,
-            callib_coef2,
-            callib_coef3,
             &oxygen_level
         );
         return oxygen_level;
     },
     py::arg("ppg_ir"),
-    py::arg("ppg_red"),
-    py::arg("sampling_rate"),
-    py::arg("callib_coef1"),
-    py::arg("callib_coef2"),
-    py::arg("callib_coef3")
+    py::arg("ppg_red")
   );
 }

src/bindings.cpp

@@ -26,13 +26,7 @@ def split_into_chunks(data, chunk_size):
 
 def main():
   # define constants
-  chunk_size = 200
-  sampling_rate = 10
-
-  # default calibration coefficients from Table 1 (https://www.analog.com/en/resources/technical-articles/guidelines-for-spo2-measurement--maxim-integrated.html)
-  coeff_1 = 1.5958422 
-  coeff_2 = -34.6596622
-  coeff_3 = 112.6898759
+  chunk_size = 64
 
   parser = argparse.ArgumentParser(description='Run SPO2 algorithm on PPG data.')
   parser.add_argument('data_folder', nargs='?', default=os.path.join(cur_dir, 'data'),
@@ -74,7 +68,7 @@ def main():
   o2_ts = []
   index = 0
   for red_chunk, ir_chunk in zip(ppg_red_chunks, ppg_ir_chunks):
-    o2 = SPO2Algorithm.get_oxygen_level(ir_chunk, red_chunk, sampling_rate, coeff_1, coeff_2, coeff_3)
+    o2 = SPO2Algorithm.get_oxygen_level(ir_chunk, red_chunk)
     o2_levels.append(o2)
     o2_ts.append(ppg_red_ts[index])
     index += chunk_size

src/run.py

@@ -2,7 +2,18 @@
 
 #define MAX_FILTER_ORDER 8
 
-void get_oxygen_level(float *ppg_ir, float *ppg_red, int data_size, int sampling_rate, float callib_coef1, float callib_coef2, float callib_coef3, float *oxygen_level)
+/**
+ * @brief Calculates the oxygen saturation level (SpO2) from PPG IR and RED signals.
+ *
+ * This function processes infrared (IR) and red photoplethysmogram (PPG) signals to estimate
+ * the blood oxygen saturation level using filtering, detrending, and a calibration formula.
+ *
+ * @param ppg_ir         Pointer to the array of IR PPG signal samples.
+ * @param ppg_red        Pointer to the array of RED PPG signal samples.
+ * @param data_size      Number of samples in the PPG signal arrays.
+ * @param oxygen_level   Pointer to a float where the calculated SpO2 value will be stored (0-100).
+ */
+void get_oxygen_level(float *ppg_ir, float *ppg_red, int data_size, float *oxygen_level)
 {
     float *red_raw = new float[data_size];
     float *ir_raw = new float[data_size];
@@ -20,8 +31,8 @@ void get_oxygen_level(float *ppg_ir, float *ppg_red, int data_size, int sampling
     // filtering(full size)
     detrend (red_raw, data_size, (int)DetrendOperations::CONSTANT);
     detrend (ir_raw, data_size, (int)DetrendOperations::CONSTANT);
-    perform_bandpass (red_raw, data_size, sampling_rate, 0.7, 1.5, 4, (int)FilterTypes::BUTTERWORTH, 0.0);
-    perform_bandpass (ir_raw, data_size, sampling_rate, 0.7, 1.5, 4, (int)FilterTypes::BUTTERWORTH, 0.0);
+    perform_bandpass (red_raw, data_size, FILTER_SAMPLING_RATE, 0.7, 1.5, 4, (int)FilterTypes::BUTTERWORTH, 0.0);
+    perform_bandpass (ir_raw, data_size, FILTER_SAMPLING_RATE, 0.7, 1.5, 4, (int)FilterTypes::BUTTERWORTH, 0.0);
 
     // calculate AC & DC components using mean & rms:
     float redac = rms (new_red_raw, new_size);
@@ -31,7 +42,7 @@ void get_oxygen_level(float *ppg_ir, float *ppg_red, int data_size, int sampling
 
     // https://www.maximintegrated.com/en/design/technical-documents/app-notes/6/6845.html
     float r = (redac / reddc) / (irac / irdc);
-    float spo2 = callib_coef1 * r * r + callib_coef2 * r + callib_coef3;
+    float spo2 = (CALIB_COEFF_1 * r * r) + (CALIB_COEFF_2 * r) + (CALIB_COEFF_3);
     if (spo2 > 100.0)
     {
         spo2 = 100.0;

src/spo2_algorithm.cpp

@@ -5,6 +5,13 @@
 #include <stdlib.h>
 #include "DspFilters/Dsp.h"
 
+// These constants are from Table 1 (https://www.analog.com/en/resources/technical-articles/guidelines-for-spo2-measurement--maxim-integrated.html)
+#define CALIB_COEFF_1 1.5958422
+#define CALIB_COEFF_2 -34.6596622
+#define CALIB_COEFF_3 112.6898759
+
+#define FILTER_SAMPLING_RATE 10 // Hz
+
 enum class FilterTypes : int
 {
     BUTTERWORTH = 0,
@@ -52,7 +59,7 @@ inline void reverse_array (float data[], int len)
     }
 }
 
-void get_oxygen_level (float *ppg_ir, float *ppg_red, int data_size, int sampling_rate, float callib_coef1, float callib_coef2, float callib_coef3, float *oxygen_level);
+void get_oxygen_level (float *ppg_ir, float *ppg_red, int data_size, float *oxygen_level);
 void detrend (float *data, int data_len, int detrend_operation);
 void perform_bandpass (float *data, int data_len, int sampling_rate, float start_freq, float stop_freq, int order, int filter_type, float ripple);