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src/dsp/nbfm/FMDiscriminator.java

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+/*******************************************************************************
+ *     SDR Trunk 
+ *     Copyright (C) 2014 Dennis Sheirer
+ * 
+ *     This program is free software: you can redistribute it and/or modify
+ *     it under the terms of the GNU General Public License as published by
+ *     the Free Software Foundation, either version 3 of the License, or
+ *     (at your option) any later version.
+ * 
+ *     This program is distributed in the hope that it will be useful,
+ *     but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *     GNU General Public License for more details.
+ * 
+ *     You should have received a copy of the GNU General Public License
+ *     along with this program.  If not, see <http://www.gnu.org/licenses/>
+ ******************************************************************************/
+package dsp.nbfm;
+
+import sample.Listener;
+import sample.complex.ComplexSample;
+
+public class FMDiscriminator implements Listener<ComplexSample>
+{
+	private Listener<Float> mListener;
+	private ComplexSample mPreviousSample = new ComplexSample( 0.0f, 0.0f );
+	private double mGain;
+	
+	/**
+	 * Implements a polar discriminator with ArcTangent angle estimator to 
+	 * demodulate complex sampled frequency modulated signals.
+	 * 
+	 * @param gain - gain to be applied to the demodulated output - can be
+	 * dynamically applied to continuously adjust audio output
+	 */
+	public FMDiscriminator( double gain )
+	{
+		mGain = gain;
+	}
+	
+	public void dispose()
+	{
+		mListener = null;
+		mPreviousSample = null;
+	}
+
+	public synchronized void setGain( double gain )
+	{
+		mGain = gain;
+	}
+	
+	@Override
+    public void receive( ComplexSample currentSample )
+    {
+		/**
+		 * Multiply the current sample against the complex conjugate of the 
+		 * previous sample to derive the phase delta between the two samples
+		 * 
+		 * Negating the previous sample quadrature produces the conjugate
+		 */
+		double i = ( currentSample.inphase() * mPreviousSample.inphase() ) - 
+				( currentSample.quadrature() * -mPreviousSample.quadrature() );
+		double q = ( currentSample.quadrature() * mPreviousSample.inphase() ) + 
+				( currentSample.inphase() * -mPreviousSample.quadrature() );
+
+		double angle;
+
+		//Check for divide by zero
+		if( i == 0 )
+		{
+			angle = 0.0;
+		}
+		else
+		{
+			/**
+			 * Use the arcus tangent of imaginary (q) divided by real (i) to
+			 * get the phase angle (+/-) which was directly manipulated by the
+			 * original message waveform during the modulation.  This value now
+			 * serves as the instantaneous amplitude of the demodulated signal
+			 */
+			double denominator = 1.0d / i;
+			angle = Math.atan( (double)q * denominator );
+		}
+
+		if( mListener != null )
+		{
+			mListener.receive( (float)( angle * mGain ) );
+		}
+
+		/**
+		 * Store the current sample to use during the next iteration
+		 */
+		mPreviousSample = currentSample;
+    }
+
+    public void setListener( Listener<Float> listener )
+    {
+		mListener = listener;
+    }
+}