Filter: added auto-init to LowPassFilter

make sure initial filter returns are not a long way off if filtered
input is far from zero

libraries/Filter/LowPassFilter.cpp

@@ -27,12 +27,20 @@ T DigitalLPF<T>::apply(const T &sample, float cutoff_freq, float dt) {
     float rc = 1.0f/(M_2PI*cutoff_freq);
     alpha = constrain_float(dt/(dt+rc), 0.0f, 1.0f);
     _output += (sample - _output) * alpha;
+    if (!initialised) {
+        initialised = true;
+        _output = sample;
+    }
     return _output;
 }
 
 template <class T>
 T DigitalLPF<T>::apply(const T &sample) {
     _output += (sample - _output) * alpha;
+    if (!initialised) {
+        initialised = true;
+        _output = sample;
+    }
     return _output;
 }
 
@@ -54,6 +62,7 @@ const T &DigitalLPF<T>::get() const {
 template <class T>
 void DigitalLPF<T>::reset(T value) { 
     _output = value;
+    initialised = true;
 }
     
 ////////////////////////////////////////////////////////////////////////////////////////////

libraries/Filter/LowPassFilter.h

@@ -61,10 +61,14 @@ public:
     // get latest filtered value from filter (equal to the value returned by latest call to apply method)
     const T &get() const;
     void reset(T value);
+    void reset() {
+        initialised = false;
+    }
 
 private:
     T _output;
     float alpha = 1.0f;
+    bool initialised;
 };
 
 // LPF base class
@@ -85,7 +89,7 @@ public:
     T apply(T sample);
     const T &get() const;
     void reset(T value);
-    void reset(void) { reset(T()); }
+    void reset(void) { _filter.reset(); }
 
 protected:
     float _cutoff_freq;