Reorganized settable parameters and fixed minor bugs.

Placed all crucial and adjustable parameters in a single spot in
algorithm_by_RF.h. Changing sampling frequency and/or sampling time
should now be more straightforward. Changed the type of “i” index in
RD117_ARDUINO.ino in anticipation of higher sampling rates.

RD117_ARDUINO.ino

@@ -142,7 +142,7 @@ void setup() {
   dataFile.print(F("Time[s]\tSpO2\tHR\tClock\tRatio\tCorr"));
 #endif
 #ifdef SAVE_RAW_DATA
-  int8_t i;
+  int32_t i;
   // These are headers for the red signal
   for(i=0;i<BUFFER_SIZE;++i) {
     dataFile.print("\t");
@@ -158,17 +158,17 @@ void setup() {
   timeStart=millis();
 }
 
-//Continuously taking samples from MAX30102.  Heart rate and SpO2 are calculated every 4 seconds
+//Continuously taking samples from MAX30102.  Heart rate and SpO2 are calculated every ST seconds
 void loop() {
   float n_spo2,ratio,correl;  //SPO2 value
   int8_t ch_spo2_valid;  //indicator to show if the SPO2 calculation is valid
   int32_t n_heart_rate; //heart rate value
   int8_t  ch_hr_valid;  //indicator to show if the heart rate calculation is valid
-  int8_t i;
+  int32_t i;
   char hr_str[10];
 
-  //buffer length of 100 stores 4 seconds of samples running at 25sps
-  //read 100 samples, and determine the signal range
+  //buffer length of BUFFER_SIZE stores ST seconds of samples running at FS sps
+  //read BUFFER_SIZE samples, and determine the signal range
   for(i=0;i<BUFFER_SIZE;i++)
   {
     while(digitalRead(oxiInt)==1);  //wait until the interrupt pin asserts
@@ -183,7 +183,7 @@ void loop() {
 #endif
   }
 
-  //calculate heart rate and SpO2 after 100 samples (4 seconds of samples) using Robert's method
+  //calculate heart rate and SpO2 after BUFFER_SIZE samples (ST seconds of samples) using Robert's method
   rf_heart_rate_and_oxygen_saturation(aun_ir_buffer, BUFFER_SIZE, aun_red_buffer, &n_spo2, &ch_spo2_valid, &n_heart_rate, &ch_hr_valid, &ratio, &correl); 
   elapsedTime=millis()-timeStart;
   millis_to_hours(elapsedTime,hr_str); // Time in hh:mm:ss format
@@ -202,7 +202,7 @@ void loop() {
 #endif
 
 #ifdef TEST_MAXIM_ALGORITHM
-  //calculate heart rate and SpO2 after 100 samples (4 seconds of samples) using MAXIM's method
+  //calculate heart rate and SpO2 after BUFFER_SIZE samples (ST seconds of samples) using MAXIM's method
   float n_spo2_maxim;  //SPO2 value
   int8_t ch_spo2_valid_maxim;  //indicator to show if the SPO2 calculation is valid
   int32_t n_heart_rate_maxim; //heart rate value

algorithm_by_RF.cpp

@@ -53,7 +53,7 @@ void rf_heart_rate_and_oxygen_saturation(uint32_t *pun_ir_buffer, int32_t n_ir_b
 */
 {
   int32_t k;  
-  static int32_t n_last_peak_interval=FS; // Initialize it to 25, which corresponds to heart rate of 60 bps, RF
+  static int32_t n_last_peak_interval=INIT_INTERVAL;
   float f_ir_mean,f_red_mean,f_ir_sumsq,f_red_sumsq;
   float f_y_ac, f_x_ac, xy_ratio;
   float beta_ir, beta_red, x;

algorithm_by_RF.h

@@ -35,24 +35,45 @@
 #define ALGORITHM_BY_RF_H_
 #include <arduino.h>
 
-#define FS 25    //sampling frequency
-#define BUFFER_SIZE  (FS*4)
-#define FS60 (FS*60) // Conversion factor for heart rate
-#define MAX_HR 125 // Maximal heart rate
-#define MIN_HR 40 // Minimal heart rate
-
-const int32_t LOWEST_PERIOD = FS60/MAX_HR; // Minimal distance between peaks
-const int32_t HIGHEST_PERIOD = FS60/MIN_HR; // Maximal distance between peaks
-// RF: Sum of squares of numbers from 0 to 99, inclusively, centered around the 49.5 mean
-const float sum_X2 = 83325;
-const float mean_X = (float)(BUFFER_SIZE-1)/2.0; // 49.5
+/*
+ * Settable parameters 
+ * Leave these alone if your circuit and hardware setup match the defaults 
+ * described in this code's Instructable. Typically, different sampling rate
+ * and/or sample length would require these paramteres to be adjusted.
+ */
+#define ST 4      // Sampling time in s. WARNING: if you change ST, then you MUST recalcuate the sum_X2 parameter below!
+#define FS 25     // Sampling frequency in Hz. WARNING: if you change FS, then you MUST recalcuate the sum_X2 parameter below!
+// Sum of squares of ST*FS numbers from -mean_X (see below) to +mean_X incremented be one. For example, given ST=4 and FS=25,
+// the sum consists of 100 terms: (-49.5)^2 + (-48.5)^2 + (-47.5)^2 + ... + (47.5)^2 + (48.5)^2 + (49.5)^2
+// The sum is symmetrc, so you can evaluate it by multiplying its positive half by 2. It is precalcuated here for enhanced 
+// performance.
+const float sum_X2 = 83325; // WARNING: you MUST recalculate this sum if you changed either ST or FS above!
+#define MAX_HR 125  // Maximal heart rate. To eliminate erroneous signals, calculated HR should never be greater than this number.
+#define MIN_HR 40   // Minimal heart rate. To eliminate erroneous signals, calculated HR should never be lower than this number.
+// Typical heart rate. Set it to the upper value of the expected heart rate range in a given application. Obviously, it must be 
+// in between MIN_HR and MAX_HR. For example, if HR in an overnight measurement varies between 46 and 65, but 90% of the time 
+// stays between 50 and 60, then set it to 60.
+// WARNING: This is a CRUCIAL parameter! Proper HR evaluation depends on it.
+#define TYPICAL_HR 60
 // Minimal ratio of two autocorrelation sequence elements: one at a considered lag to the one at lag 0.
 // Good quality signals must have such ratio greater than this minimum.
 const float min_autocorrelation_ratio = 0.5;
 // Pearson correlation between red and IR signals.
 // Good quality signals must have their correlation coefficient greater than this minimum.
 const float min_pearson_correlation = 0.8;
 
+/*
+ * Derived parameters 
+ * Do not touch these! 
+ * 
+ */
+const int32_t BUFFER_SIZE = FS*ST; // Number of smaples in a single batch
+const int32_t FS60 = FS*60;  // Conversion factor for heart rate from bps to bpm
+const int32_t LOWEST_PERIOD = FS60/MAX_HR; // Minimal distance between peaks
+const int32_t HIGHEST_PERIOD = FS60/MIN_HR; // Maximal distance between peaks
+const int32_t INIT_INTERVAL = FS60/TYPICAL_HR; // Seed value for heart rate determination routine
+const float mean_X = (float)(BUFFER_SIZE-1)/2.0; // Mean value of the set of integers from 0 to BUFFER_SIZE-1. For ST=4 and FS=25 it's equal to 49.5.
+
 void rf_heart_rate_and_oxygen_saturation(uint32_t *pun_ir_buffer, int32_t n_ir_buffer_length, uint32_t *pun_red_buffer, float *pn_spo2, int8_t *pch_spo2_valid, int32_t *pn_heart_rate, 
                                         int8_t *pch_hr_valid, float *ratio, float *correl);
 float rf_linear_regression_beta(float *pn_x, float xmean, float sum_x2);