#2212 Introduce carrier offset measurement processor to monitor a 12.5 kHz channel and provide estimated carrier offset measurements that allow the tuner's frequency error monitor to correct for the average error reported from each channel. Uses a 15 dB threshold where the signal must be at least 15 dB above the noise floor before it will calculate and broadcast the estimate. Stores the updated PPM in the tuner configuration after each update to ensure the value persists across sessions. When selected in the tuner panel, updates the displayed PPM value after each PPM autocorrection. (#2218)

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

src/main/java/io/github/dsheirer/module/decode/DecoderFactory.java

@@ -1,6 +1,6 @@
 /*
  * *****************************************************************************
- * Copyright (C) 2014-2024 Dennis Sheirer
+ * Copyright (C) 2014-2025 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

src/main/java/io/github/dsheirer/module/decode/dmr/DMRDecoder.java

@@ -28,6 +28,7 @@ import io.github.dsheirer.dsp.psk.dqpsk.DQPSKDemodulatorFactory;
 import io.github.dsheirer.dsp.squelch.PowerMonitor;
 import io.github.dsheirer.message.IMessage;
 import io.github.dsheirer.message.SyncLossMessage;
+import io.github.dsheirer.module.carrier.CarrierOffsetProcessor;
 import io.github.dsheirer.module.decode.Decoder;
 import io.github.dsheirer.module.decode.DecoderType;
 import io.github.dsheirer.module.decode.dmr.audio.DMRAudioModule;
@@ -84,6 +85,7 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
     private static final Logger LOGGER = LoggerFactory.getLogger(DMRDecoder.class);
     private static final DecimalFormat DECIMAL_FORMAT = new DecimalFormat("#.##");
     private static final int SYMBOL_RATE = 4800;
+    private static final float MAXIMUM_CARRIER_OFFSET = 5000.0f; //Threshold for retuning the signal.
     private static final Map<Double,float[]> BASEBAND_FILTERS = new HashMap<>();
     private DQPSKDemodulator mDemodulator;
     private final DMRMessageFramer mMessageFramer;
@@ -92,6 +94,7 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
     private IRealFilter mIBasebandFilter;
     private IRealFilter mQBasebandFilter;
     private final PowerMonitor mPowerMonitor = new PowerMonitor();
+    private final CarrierOffsetProcessor mCarrierOffsetProcessor = new CarrierOffsetProcessor();
 
     /**
      * Constructs an instance
@@ -137,6 +140,7 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
         mSymbolProcessor.setSamplesPerSymbol(mDemodulator.getSamplesPerSymbol());
         mMessageFramer.setListener(mMessageProcessor);
         mMessageProcessor.setMessageListener(getMessageListener());
+        mCarrierOffsetProcessor.setSampleRate(sampleRate);
     }
 
     /**
@@ -147,6 +151,7 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
     public void receive(ComplexSamples samples)
     {
         mMessageFramer.setTimestamp(samples.timestamp());
+
         float[] i = mIBasebandFilter.filter(samples.i());
         float[] q = mQBasebandFilter.filter(samples.q());
 
@@ -155,6 +160,25 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
 
         float[] demodulated = mDemodulator.demodulate(i, q);
         mSymbolProcessor.receive(demodulated);
+
+        //Estimate carrier offset and broadcast at each update. This value is used in the channel spectral display,
+        // and it's also processed by the tuner's PPM error monitor to auto-adjust the tuner PPM value.
+        if(mCarrierOffsetProcessor.process(samples))
+        {
+            //Tuner PPM Monitor - negate the value to indicate channel error from tuner's PPM that's causing the offset
+            mPowerMonitor.broadcast(SourceEvent.frequencyErrorMeasurement(-mCarrierOffsetProcessor.getEstimatedOffset()));
+
+            //Channel spectral display - when there's a carrier send the estimate, otherwise send a zero to cause the
+            //display to blank the carrier offset measurement indicator line
+            if(mCarrierOffsetProcessor.hasCarrier())
+            {
+                mPowerMonitor.broadcast(SourceEvent.carrierOffsetMeasurement(mCarrierOffsetProcessor.getEstimatedOffset()));
+            }
+            else
+            {
+                mPowerMonitor.broadcast(SourceEvent.carrierOffsetMeasurement(0));
+            }
+        }
     }
 
     /**
@@ -262,9 +286,17 @@ public class DMRDecoder extends Decoder implements IByteBufferProvider, IComplex
      */
     private void process(SourceEvent sourceEvent)
     {
-        if(sourceEvent != null && sourceEvent.getEvent() == SourceEvent.Event.NOTIFICATION_SAMPLE_RATE_CHANGE)
+        if(sourceEvent != null)
         {
-            setSampleRate(sourceEvent.getValue().doubleValue());
+            switch(sourceEvent.getEvent())
+            {
+                case NOTIFICATION_SAMPLE_RATE_CHANGE:
+                    setSampleRate(sourceEvent.getValue().doubleValue());
+                    break;
+                case NOTIFICATION_FREQUENCY_CORRECTION_CHANGE:
+                    mCarrierOffsetProcessor.reset();
+                    break;
+            }
         }
     }
 

src/main/java/io/github/dsheirer/module/decode/dmr/DMRDecoderState.java

@@ -223,7 +223,7 @@ public class DMRDecoderState extends TimeslotDecoderState
      */
     private boolean isValid(IMessage message)
     {
-        return mIgnoreCRCChecksums || message.isValid();
+        return message != null && (mIgnoreCRCChecksums || message.isValid());
     }
 
     /**