added filter doc and cleaned up a bit

preload/apps/ecg/__init__.py

@@ -51,12 +51,13 @@ last_sample_count = 1
 leds.dim_top(1)
 COLORS = [((23 + (15 * i)) % 360, 1.0, 1.0) for i in range(11)]
 
-
 # variables for high-pass filter
+# note: corresponds to 1st order hpf with -3dB at ~18.7Hz
+# general formula: f(-3dB)=-(sample_rate/tau)*ln(1-betadash)
 moving_average = 0
 alpha = 2
 beta = 3
-
+betadash = beta/(alpha+beta)
 
 def update_history(datasets):
     global history, moving_average, alpha, beta, last_sample_count
@@ -64,7 +65,8 @@ def update_history(datasets):
     for val in datasets:
         if current_mode == MODE_FINGER:
             history.append(val - moving_average)
-            moving_average = (alpha * moving_average + beta * val) / (alpha + beta)
+            moving_average += betadash*(val-moving_average)
+            # identical to: moving_average = (alpha * moving_average + beta * val) / (alpha + beta)
         else:
             history.append(val)