#35 WIP Updated AM decoder to adjust demodulator gain and implemented…

… new Gain and DC removal control that attempts to normalize audio gain toward an objective value.

src/main/java/io/github/dsheirer/dsp/gain/AudioGainAndDcFilter.java

@@ -0,0 +1,140 @@
+/*
+ * *****************************************************************************
+ * Copyright (C) 2014-2023 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.gain;
+
+import org.apache.commons.math3.util.FastMath;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Audio gain that normalizes audio amplitude against an objective amplitude value where the audio amplitude values
+ * normally fall in the range of -1.0 to 1.0.
+ *
+ * Removes DC bias on a per audio sample buffer basis.
+ *
+ * This control is designed to work on a per-call basis by monitoring the maximum observed amplitude and adjusting the
+ * gain to normalize the amplitude toward the objective amplitude value.  This control is designed to be reset at the
+ * beginning/end of each call segment so that the max observed amplitude is relevant to the current call segment.
+ *
+ * Min and max gains should be sized to constrain the applied value during periods of extended silence at the
+ * beginning of an audio segment or where a single amplitude spike might erroneously affect gain across the segment.
+ */
+public class AudioGainAndDcFilter
+{
+    private static final Logger mLog = LoggerFactory.getLogger(AudioGainAndDcFilter.class);
+    private float mMinGain;
+    private float mMaxGain;
+    private float mCurrentGain;
+    private float mObjectiveGain;
+    private float mObjectiveAmplitude;
+    private float mMaxObservedAmplitude;
+    private static float MAX_AMPLITUDE = 0.95f;
+    //Stabilizes gain after 2x buffers of 2048 samples
+    private static final float GAIN_LOOP_BANDWIDTH = 0.0015f;
+
+    /**
+     * Constructs an instance
+     * @param minGain to apply to the incoming sample stream.
+     * @param maxGain to apply
+     * @param objectiveAmplitude to achieve by adjusting gain between min and max.
+     */
+    public AudioGainAndDcFilter(float minGain, float maxGain, float objectiveAmplitude)
+    {
+        mMinGain = minGain;
+        mMaxGain = maxGain;
+        mObjectiveAmplitude = objectiveAmplitude;
+        reset();
+    }
+
+    /**
+     * Resets this gain control to prepare for the next audio call/segment.
+     */
+    public void reset()
+    {
+        mMaxObservedAmplitude = 0.0f;
+        mObjectiveGain = 1.0f;
+        mCurrentGain = 1.0f;
+    }
+
+    /**
+     * Process a buffer of audio samples and apply gain.
+     * @param samples to adjust.
+     * @return amplified audio samples
+     */
+    public float[] process(float[] samples)
+    {
+        float currentAmplitude;
+        float dcAccumulator = 0.0f;
+
+        //Decay the max observed value by 10% each buffer so that an initial spike doesn't carry across all buffers
+        mMaxObservedAmplitude *= 0.9f;
+
+        for(float sample: samples)
+        {
+            dcAccumulator += sample;
+            currentAmplitude = FastMath.min(Math.abs(sample), 1.2f);
+
+            if(currentAmplitude > mMaxObservedAmplitude)
+            {
+                mMaxObservedAmplitude = currentAmplitude;
+            }
+        }
+
+        mObjectiveGain = mObjectiveAmplitude / mMaxObservedAmplitude;
+
+        if(mObjectiveGain > mMaxGain)
+        {
+            mObjectiveGain = mMaxGain;
+        }
+        else if(mObjectiveGain < mMinGain)
+        {
+            mObjectiveGain = mMinGain;
+        }
+
+        float dcOffset = dcAccumulator / samples.length;
+        float gain = mCurrentGain;
+        float objective = mObjectiveGain;
+        float[] processed = new float[samples.length];
+        boolean objectiveAchieved = (gain == objective);
+        float amplified;
+
+        for(int x = 0; x < samples.length; x++)
+        {
+            if(!objectiveAchieved)
+            {
+                gain += ((objective - gain) * GAIN_LOOP_BANDWIDTH);
+                if(Math.abs(objective - gain) < 0.00005f)
+                {
+                    gain = objective;
+                    objectiveAchieved = true;
+                }
+            }
+
+            amplified = (samples[x] - dcOffset) * gain;
+            amplified = FastMath.min(amplified, MAX_AMPLITUDE);
+            amplified = FastMath.max(amplified, -MAX_AMPLITUDE);
+            processed[x] = amplified;
+        }
+
+        mCurrentGain = gain;
+        mObjectiveGain = objective;
+        return processed;
+    }
+}

src/main/java/io/github/dsheirer/gui/playlist/alias/identifier/TalkgroupEditor.java

@@ -1,6 +1,6 @@
 /*
  * *****************************************************************************
- *  Copyright (C) 2014-2020 Dennis Sheirer
+ * Copyright (C) 2014-2023 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
@@ -27,6 +27,8 @@
 import io.github.dsheirer.preference.UserPreferences;
 import io.github.dsheirer.preference.identifier.IntegerFormat;
 import io.github.dsheirer.protocol.Protocol;
+import java.util.ArrayList;
+import java.util.List;
 import javafx.beans.value.ChangeListener;
 import javafx.beans.value.ObservableValue;
 import javafx.geometry.HPos;
@@ -38,9 +40,6 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-import java.util.ArrayList;
-import java.util.List;
-
 /**
  * Editor for talkgroup alias identifiers
  */
@@ -238,6 +237,10 @@ private TalkgroupDetail getTalkgroupDetail(Protocol protocol, IntegerFormat inte
     private void loadTalkgroupDetails()
     {
         mTalkgroupDetails.clear();
+        mTalkgroupDetails.add(new TalkgroupDetail(Protocol.AM, IntegerFormat.DECIMAL, new IntegerFormatter(1,0xFFFF),
+                "Format: 1 - 65535"));
+        mTalkgroupDetails.add(new TalkgroupDetail(Protocol.AM, IntegerFormat.HEXADECIMAL, new HexFormatter(1,0xFFFF),
+                "Format: 1 - FFFF"));
         mTalkgroupDetails.add(new TalkgroupDetail(Protocol.APCO25, IntegerFormat.DECIMAL, new IntegerFormatter(0,65535),
                 "Format: 0 - 65535"));
         mTalkgroupDetails.add(new TalkgroupDetail(Protocol.APCO25, IntegerFormat.HEXADECIMAL, new HexFormatter(0,65535),

src/main/java/io/github/dsheirer/module/decode/am/AMDecoder.java

@@ -20,23 +20,22 @@
 package io.github.dsheirer.module.decode.am;
 
 import io.github.dsheirer.dsp.am.SquelchingAMDemodulator;
-import io.github.dsheirer.dsp.gain.AutomaticGainControl;
-import io.github.dsheirer.dsp.gain.ObjectiveGainControl;
+import io.github.dsheirer.dsp.gain.AudioGainAndDcFilter;
 import io.github.dsheirer.module.decode.DecoderType;
 import io.github.dsheirer.module.decode.analog.SquelchingAnalogDecoder;
-import io.github.dsheirer.source.wave.RealWaveSource;
-
-import java.io.File;
 
 /**
- * Decoder module with integrated squelching AM demodulator
+ * Analog AM radio decoder module with integrated squelching control
  */
 public class AMDecoder extends SquelchingAnalogDecoder
 {
-    private static final float DEMODULATOR_GAIN = 400.0f; //400 seems about right for a strong local signal
+    private static final float DEMODULATOR_GAIN = 250.0f;
     private static final float SQUELCH_ALPHA_DECAY = 0.0004f;
     private static final float SQUELCH_THRESHOLD_DB = -78.0f;
-    private AutomaticGainControl mAGC = new AutomaticGainControl();
+    private static final float MINIMUM_GAIN = 0.5f;
+    private static final float MAXIMUM_GAIN = 16.0f;
+    private static final float OBJECTIVE_AUDIO_AMPLITUDE = 0.75f;
+    private AudioGainAndDcFilter mAGC = new AudioGainAndDcFilter(MINIMUM_GAIN, MAXIMUM_GAIN, OBJECTIVE_AUDIO_AMPLITUDE);
 
     /**
      * Constructs an instance
@@ -60,8 +59,8 @@ public DecoderType getDecoderType()
     @Override
     protected void broadcast(float[] demodulatedSamples)
     {
-        float[] amplified = mAGC.process(demodulatedSamples);
-        super.broadcast(amplified);
+        //Apply audio gain and rebroadcast
+        super.broadcast(mAGC.process(demodulatedSamples));
     }
 
     /**
@@ -73,30 +72,4 @@ protected void notifyCallStart()
         mAGC.reset();
         super.notifyCallStart();
     }
-
-    public static void main(String[] args)
-    {
-        ObjectiveGainControl agc = new ObjectiveGainControl(1.0f, 4.0f, 0.7f);
-
-        String path = "C:\\Users\\sheirerd\\SDRTrunk\\recordings\\20230430_052528Air_Boston_Center__TO_3.wav";
-        File file = new File(path);
-
-        try
-        {
-            RealWaveSource source = new RealWaveSource(file);
-            source.setListener(floats -> agc.process(floats));
-            source.open();
-
-            while(true)
-            {
-                source.next(256, true);
-            }
-
-        }
-        catch(Exception e)
-        {
-//            mLog.error("Error", e);
-        }
-        System.out.println("Finished.");
-    }
 }

src/main/java/io/github/dsheirer/module/decode/am/AMDecoderState.java

@@ -24,6 +24,7 @@
 import io.github.dsheirer.identifier.IdentifierClass;
 import io.github.dsheirer.identifier.Role;
 import io.github.dsheirer.identifier.string.SimpleStringIdentifier;
+import io.github.dsheirer.module.decode.DecoderType;
 import io.github.dsheirer.module.decode.analog.AnalogDecoderState;
 
 /**
@@ -47,6 +48,12 @@ public AMDecoderState(String channelName, DecodeConfigAM decodeConfig)
         mTalkgroupIdentifier = new AMTalkgroup(decodeConfig.getTalkgroup());
     }
 
+    @Override
+    public DecoderType getDecoderType()
+    {
+        return DecoderType.AM;
+    }
+
     @Override
     protected Identifier getChannelNameIdentifier()
     {

src/main/java/io/github/dsheirer/module/decode/analog/AnalogDecoderState.java

@@ -25,7 +25,6 @@
 import io.github.dsheirer.identifier.Identifier;
 import io.github.dsheirer.identifier.IdentifierCollection;
 import io.github.dsheirer.message.IMessage;
-import io.github.dsheirer.module.decode.DecoderType;
 import io.github.dsheirer.module.decode.event.DecodeEvent;
 import io.github.dsheirer.module.decode.p25.identifier.channel.StandardChannel;
 import io.github.dsheirer.sample.Listener;
@@ -142,12 +141,6 @@ private void endCallEvent()
         resetState();
     }
 
-    @Override
-    public DecoderType getDecoderType()
-    {
-        return DecoderType.NBFM;
-    }
-
     @Override
     public void start()
     {