Filter: added 2nd-order butterworth filters.

libraries/Filter/Butter.h

@@ -0,0 +1,103 @@
+template <typename Coefficients>
+class Butter2
+{
+public:
+  float filter(float input)
+  {
+        float newhist = input + Coefficients::A1*hist[1] + Coefficients::A2*hist[0];
+        float ret = (newhist + 2*hist[1] + hist[0])/Coefficients::GAIN;
+        hist[0] = hist[1]; hist[1] = newhist;
+        return ret;
+  }
+private:
+    float hist[2];
+};
+
+struct butter100_025_coeffs
+{
+  static const float A1 = 1.9777864838;
+  static const float A2 = -0.9780305085;
+  static const float GAIN = 1.639178228e+04;
+};
+typedef Butter2<butter100_025_coeffs> butter100hz0_25; //100hz sample, 0.25hz fcut
+typedef Butter2<butter100_025_coeffs> butter50hz0_125; //50hz sample, 0.125hz fcut
+typedef Butter2<butter100_025_coeffs> butter10hz0_025; //10hz sample, 0.025hz fcut
+
+struct butter100_05_coeffs
+{
+  static const float A1 = 1.9555782403;
+  static const float A2 = -0.9565436765;
+  static const float GAIN = 4.143204922e+03;
+};
+typedef Butter2<butter100_05_coeffs> butter100hz0_5; //100hz sample, 0.5hz fcut
+typedef Butter2<butter100_05_coeffs> butter50hz0_25; //50hz sample, 0.25hz fcut
+typedef Butter2<butter100_05_coeffs> butter10hz0_05; //10hz sample, 0.05hz fcut
+
+struct butter100_1_coeffs
+{
+  static const float A1 = 1.9111970674;
+  static const float A2 = -0.9149758348;
+  static const float GAIN = 1.058546241e+03;
+};
+typedef Butter2<butter100_1_coeffs> butter100hz1_0; //100hz sample, 1hz fcut
+typedef Butter2<butter100_1_coeffs> butter50hz0_5; //50hz sample, 0.5hz fcut
+typedef Butter2<butter100_1_coeffs> butter10hz0_1; //10hz sample, 0.1hz fcut
+
+struct butter100_1_5_coeffs
+{
+  static const float A1 = 1.8668922797;
+  static const float A2 = -0.8752145483;
+  static const float GAIN = 4.806381793e+02;
+};
+typedef Butter2<butter100_1_5_coeffs> butter100hz1_5; //100hz sample, 1.5hz fcut
+typedef Butter2<butter100_1_5_coeffs> butter50hz0_75; //50hz sample, 0.75hz fcut
+typedef Butter2<butter100_1_5_coeffs> butter10hz0_15; //10hz sample, 0.15hz fcut
+
+struct butter100_2_coeffs
+{
+  static const float A1 = 1.8226949252;
+  static const float A2 = -0.8371816513;
+  static const float GAIN = 2.761148367e+02;
+};
+typedef Butter2<butter100_2_coeffs> butter100hz2_0; //100hz sample, 2hz fcut
+typedef Butter2<butter100_2_coeffs> butter50hz1_0; //50hz sample, 1hz fcut
+typedef Butter2<butter100_2_coeffs> butter10hz0_2; //10hz sample, 0.2hz fcut
+
+struct butter100_3_coeffs
+{
+  static const float A1 = 1.7347257688;
+  static const float A2 = -0.7660066009;
+  static const float GAIN = 1.278738361e+02;
+};
+typedef Butter2<butter100_3_coeffs> butter100hz3_0; //100hz sample, 3hz fcut
+typedef Butter2<butter100_3_coeffs> butter50hz1_5; //50hz sample, 1.5hz fcut
+typedef Butter2<butter100_3_coeffs> butter10hz0_3; //10hz sample, 0.3hz fcut
+
+struct butter100_4_coeffs
+{
+  static const float A1 = 1.6474599811;
+  static const float A2 = -0.7008967812;
+  static const float GAIN = 7.485478157e+01;
+};
+typedef Butter2<butter100_4_coeffs> butter100hz4_0; //100hz sample, 4hz fcut
+typedef Butter2<butter100_4_coeffs> butter50hz2_0; //50hz sample, 2hz fcut
+typedef Butter2<butter100_4_coeffs> butter10hz0_4; //10hz sample, .4hz fcut
+
+struct butter100_8_coeffs
+{
+  static const float A1 = 1.3072850288;
+  static const float A2 = -0.4918122372;
+  static const float GAIN = 2.167702007e+01;
+};
+typedef Butter2<butter100_8_coeffs> butter100hz8_0; //100hz sample, 8hz fcut
+typedef Butter2<butter100_8_coeffs> butter50hz4_0; //50hz sample, 4hz fcut
+typedef Butter2<butter100_8_coeffs> butter10hz0_8; //10hz sample, .8hz fcut
+
+struct butter50_8_coeffs
+{
+  static const float A1 = 0.6710290908;
+  static const float A2 = -0.2523246263;
+  static const float GAIN = 6.881181354e+00;
+};
+typedef Butter2<butter50_8_coeffs> butter50hz8_0; //50hz sample, 8hz fcut
+typedef Butter2<butter50_8_coeffs> butter10hz1_6; //10hz sample, 1.6hz fcut

libraries/Filter/Filter.h

@@ -10,6 +10,7 @@
 #include "FilterWithBuffer.h"
 #include "LowPassFilter.h"
 #include "ModeFilter.h"
+#include "Butter.h"
 
 #endif //__FILTER_H__