NFM demodulation without using atan and smooth squelch with AGC suppressing most clicks on low level signals and hiss on carrier tails. Only useful modulation comes through

Readme.md

@@ -80,6 +80,7 @@ Done since the fork
   - Added AM demod so now you can listen to air traffic!
   - Added the possibility to change the brightness and/or color of the grid.
   - Make the low cutoff frequency of the SSB filter variable so it can be used for CW also.
+  - NFM demodulation without using atan and smooth squelch with AGC suppressing most clicks on low level signals and hiss on carrier tails. Only useful modulation comes through.
     
 =====
 To Do

include-gpl/dsp/agc.h

@@ -18,26 +18,33 @@ public:
 		m_squelch(false),
 		m_fill(0),
 		m_cutoff(0),
+		m_clip(0),
 		m_moving_average()
 	{}
 
-	SimpleAGC(int historySize, Real initial, Real cutoff) :
+	SimpleAGC(int historySize, Real initial, Real cutoff=0, Real clip=0) :
 		m_squelch(false),
 		m_fill(initial),
 		m_cutoff(cutoff),
+		m_clip(clip),
 		m_moving_average(historySize, initial)
 	{}
 
-	void resize(int historySize, Real initial, Real cutoff)
+	void resize(int historySize, Real initial, Real cutoff=0, Real clip=0)
 	{
 		m_fill = initial;
 		m_cutoff = cutoff;
+		m_clip = clip;
 		m_moving_average.resize(historySize, initial);
 	}
 
 	Real getValue()
 	{
-		return m_moving_average.average();
+		if (m_moving_average.average() > m_clip) {
+			return m_moving_average.average();
+		} else {
+			return m_clip;
+		}
 	}
 
 	void feed(Real value)
@@ -60,10 +67,11 @@ public:
 	}
 
 private:
-	bool m_squelch;
-	Real m_fill;
-	Real m_cutoff;
-	MovingAverage m_moving_average;
+	bool m_squelch; // open for processing
+	Real m_fill;    // refill average at this level
+	Real m_cutoff;  // consider samples only above this level
+	Real m_clip;    // never go below this level
+	MovingAverage m_moving_average; // Averaging engine. The stack length conditions the smoothness of AGC.
 };
 
 

plugins/channel/nfm/nfmdemod.cpp

@@ -43,6 +43,8 @@ NFMDemod::NFMDemod(AudioFifo* audioFifo, SampleSink* sampleSink) :
 	m_audioBufferFill = 0;
 
 	m_movingAverage.resize(16, 0);
+	m_agcLevel = 0.003;
+	m_AGC.resize(4096, m_agcLevel, 0, 0.1*m_agcLevel);
 }
 
 NFMDemod::~NFMDemod()
@@ -108,6 +110,9 @@ void NFMDemod::feed(SampleVector::const_iterator begin, SampleVector::const_iter
 				if(m_squelchState > 0) {
 					m_squelchState--;
 
+					m_AGC.feed(abs(ci));
+					ci *= (m_agcLevel / m_AGC.getValue());
+
 					// demod
 					/*
 					Real argument = arg(ci);
@@ -115,9 +120,12 @@ void NFMDemod::feed(SampleVector::const_iterator begin, SampleVector::const_iter
 					m_lastArgument = argument;
 					*/
 
+					/*
+					// Original NFM
 					Complex d = conj(m_m1Sample) * ci;
 					Real demod = atan2(d.imag(), d.real());
 					demod /= M_PI;
+					*/
 
 					/*
 					Real argument1 = arg(ci);//atan2(ci.imag(), ci.real());
@@ -126,6 +134,14 @@ void NFMDemod::feed(SampleVector::const_iterator begin, SampleVector::const_iter
 					m_lastSample = Complex(argument1, 0);
 					*/
 
+					// Alternative without atan - needs AGC
+					// http://www.embedded.com/design/configurable-systems/4212086/DSP-Tricks--Frequency-demodulation-algorithms-
+					Real ip = ci.real() - m_m2Sample.real();
+					Real qp = ci.imag() - m_m2Sample.imag();
+					Real h1 = m_m1Sample.real() * qp;
+					Real h2 = m_m1Sample.imag() * ip;
+					Real demod = (h1 - h2) * 10000;
+
 					m_m2Sample = m_m1Sample;
 					m_m1Sample = ci;
 
@@ -133,15 +149,18 @@ void NFMDemod::feed(SampleVector::const_iterator begin, SampleVector::const_iter
 
 					demod = m_lowpass.filter(demod);
 
+					/*
 					if(demod < -1)
 						demod = -1;
 					else if(demod > 1)
 						demod = 1;
+					*/
 
 					demod *= m_running.m_volume;
 					sample = demod * 32700;
 
 				} else {
+					m_AGC.close();
 					sample = 0;
 				}
 

plugins/channel/nfm/nfmdemod.h

@@ -24,6 +24,7 @@
 #include "dsp/interpolator.h"
 #include "dsp/lowpass.h"
 #include "dsp/movingaverage.h"
+#include "dsp/agc.h"
 #include "audio/audiofifo.h"
 #include "util/message.h"
 
@@ -119,6 +120,9 @@ private:
 	Complex m_m1Sample;
 	Complex m_m2Sample;
 	MovingAverage m_movingAverage;
+	SimpleAGC m_AGC;
+	Real m_agcLevel; // AGC will aim to  this level
+	Real m_agcFloor; // AGC will not go below this level
 
 	AudioVector m_audioBuffer;
 	uint m_audioBufferFill;