#36 NBFM squelch control. Updates NBFM decoder to incorporate squelch control. Squelch is a configuration item and also available to adjust at runtime with new 'Channel' tab in Now Playing window. All decoders were updated to produce continuous channel power (dB) measurements that are displayed in the Channel tab's new Power Meter. (#1100)

Co-authored-by: Dennis Sheirer <dsheirer@github.com>

src/main/java/io/github/dsheirer/dsp/fm/SquelchingFMDemodulator.java

@@ -0,0 +1,146 @@
+/*******************************************************************************
+ *     SDR Trunk 
+ *     Copyright (C) 2014,2015 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 io.github.dsheirer.dsp.fm;
+
+import io.github.dsheirer.dsp.squelch.PowerSquelch;
+import io.github.dsheirer.sample.Listener;
+import io.github.dsheirer.sample.buffer.ReusableBufferQueue;
+import io.github.dsheirer.sample.buffer.ReusableComplexBuffer;
+import io.github.dsheirer.sample.buffer.ReusableFloatBuffer;
+import io.github.dsheirer.source.SourceEvent;
+
+/**
+ * FM Demodulator for demodulating complex samples and producing demodulated floating point samples.
+ *
+ * Implements listener of source events to process runtime squelch threshold change request events
+ * which are forwarded to the power squelch control.
+ */
+public class SquelchingFMDemodulator extends FMDemodulator implements Listener<SourceEvent>
+{
+    private ReusableBufferQueue mReusableBufferQueue = new ReusableBufferQueue("SquelchingFMDemodulator");
+    private PowerSquelch mPowerSquelch;
+    private boolean mSquelchChanged = false;
+
+    /**
+     * Creates an FM demodulator instance with a default gain of 1.0.
+     */
+    public SquelchingFMDemodulator(double alpha, double threshold, int ramp)
+    {
+        mPowerSquelch = new PowerSquelch(alpha, threshold, ramp);
+    }
+
+    /**
+     * Registers the listener to receive notifications of squelch change events from the power squelch.
+     */
+    public void setSourceEventListener(Listener<SourceEvent> listener)
+    {
+        mPowerSquelch.setSourceEventListener(listener);
+    }
+
+    /**
+     * Demodulates the complex baseband sample buffer and returns a demodulated reusable buffer with the user count
+     * set to 1.  The complex baseband buffer's user count is decremented after demodulation.
+     *
+     * @param basebandSampleBuffer containing samples to demodulate
+     * @return demodulated sample buffer.
+     */
+    @Override
+    public ReusableFloatBuffer demodulate(ReusableComplexBuffer basebandSampleBuffer)
+    {
+        setSquelchChanged(false);
+
+        ReusableFloatBuffer demodulatedBuffer = mReusableBufferQueue.getBuffer(basebandSampleBuffer.getSampleCount());
+
+        float[] basebandSamples = basebandSampleBuffer.getSamples();
+        float[] demodulatedSamples = demodulatedBuffer.getSamples();
+        float i,q;
+
+        for(int x = 0; x < basebandSamples.length; x += 2)
+        {
+            i = basebandSamples[x];
+            q = basebandSamples[x + 1];
+
+            mPowerSquelch.process(i, q);
+
+            if(mPowerSquelch.isUnmuted() || mPowerSquelch.isDecay())
+            {
+                demodulatedSamples[x / 2] = demodulate(i, q);
+            }
+            else
+            {
+                demodulatedSamples[x / 2] = 0.0f;
+            }
+
+            if(mPowerSquelch.isSquelchChanged())
+            {
+                setSquelchChanged(true);
+            }
+        }
+
+        basebandSampleBuffer.decrementUserCount();
+
+        return demodulatedBuffer;
+    }
+
+    /**
+     * Sets the threshold for squelch control
+     * @param threshold (dB)
+     */
+    public void setSquelchThreshold(double threshold)
+    {
+        mPowerSquelch.setSquelchThreshold(threshold);
+    }
+
+    /**
+     * Indicates if the squelch state has changed during the processing of buffer(s)
+     */
+    public boolean isSquelchChanged()
+    {
+        return mSquelchChanged;
+    }
+
+    /**
+     * Sets or resets the squelch changed flag.
+     */
+    private void setSquelchChanged(boolean changed)
+    {
+        mSquelchChanged = changed;
+    }
+
+    /**
+     * Indicates if the squelch state is currently muted
+     */
+    public boolean isMuted()
+    {
+        return mPowerSquelch.isMuted();
+    }
+
+    @Override
+    public void receive(SourceEvent sourceEvent)
+    {
+        //Only forward squelch threshold change request events
+        if(sourceEvent.getEvent() == SourceEvent.Event.REQUEST_CHANGE_SQUELCH_THRESHOLD)
+        {
+            mPowerSquelch.receive(sourceEvent);
+        }
+        else if(sourceEvent.getEvent() == SourceEvent.Event.REQUEST_CURRENT_SQUELCH_THRESHOLD)
+        {
+            mPowerSquelch.receive(sourceEvent);
+        }
+    }
+}