mirror of
https://github.com/DSheirer/sdrtrunk.git
synced 2026-11-12 08:37:25 -05:00
[WIP] Updates the LTR-Standard and LTR-Net decoders to use a new LTR
decoder with a zero-crossing symbol timing error detector. The new decoder is optimized for 8 kHz FM demodulated audio produced by the AudioModule.
This commit is contained in:
Denny
221
src/main/java/io/github/dsheirer/dsp/fsk/LTRDecoder.java
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src/main/java/io/github/dsheirer/dsp/fsk/LTRDecoder.java
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/*******************************************************************************
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* sdr-trunk
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* Copyright (C) 2014-2018 Dennis Sheirer
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*
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* This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public
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* License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any
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* later version.
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*
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* This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
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* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along with this program.
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* If not, see <http://www.gnu.org/licenses/>
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*
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******************************************************************************/
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package io.github.dsheirer.dsp.fsk;
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import io.github.dsheirer.dsp.filter.dc.IIRSinglePoleDCRemovalFilter;
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import io.github.dsheirer.dsp.filter.design.FilterDesignException;
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import io.github.dsheirer.dsp.filter.fir.FIRFilterSpecification;
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import io.github.dsheirer.dsp.filter.fir.real.RealFIRFilter2;
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import io.github.dsheirer.dsp.filter.fir.remez.RemezFIRFilterDesigner;
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import io.github.dsheirer.dsp.symbol.ISyncDetectListener;
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import io.github.dsheirer.sample.Listener;
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import io.github.dsheirer.sample.buffer.ReusableBuffer;
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import org.slf4j.Logger;
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import org.slf4j.LoggerFactory;
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/**
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* Logic Trunked Radio (LTR) 300-baud FSK decoder is designed to operate against an 8 kHz FM demodulated audio stream.
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*
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* This decoder employs a zero-crossing timing error detector with adjustable symbol timing error gain. This decoder
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* also employs a synchronization monitor that automatically adjusts symbol timing error gain according to the quantity
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* of successful message sync pattern detections. This is optimized for LTR signals that periodically transmit idle
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* bursts and transmit continuous message streams during voice transmissions. The synchronization monitor adjusts to an
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* aggressive gain value when sync is lost and automatically reduces timing error gain as successive sync pattern
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* detections occur.
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*
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* FM demodulated sample buffers should not be filtered prior to this decoder. An internal DC-removal filter coupled
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* with a low-pass filter removes any tuning offset and/or audio components above 300 Hertz. The DC removal filter is
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* adjusted for a slower offset removal in order to prevent overly distorting the FSK signalling component. This mild
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* adjustment may not fully compensate for mis-tuned signals and may result in missed decodes of idle bursts. However,
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* the DC filter will track a mis-tuned signal to baseband quickly upon a continuous transmission.
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*/
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public class LTRDecoder implements Listener<ReusableBuffer>, ISyncDetectListener, ISyncStateListener
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{
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private final static Logger mLog = LoggerFactory.getLogger(LTRDecoder.class);
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public static final float SAMPLES_PER_SYMBOL = 8000.0f / 300.0f;
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protected static final float COARSE_TIMING_GAIN = 1.0f / 3.0f;
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protected static final float MEDIUM_TIMING_GAIN = 1.0f / 4.0f;
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protected static final float FINE_TIMING_GAIN = 1.0f / 5.0f;
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private static float[] sLowPassFilterCoefficients;
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static
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{
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FIRFilterSpecification specification = FIRFilterSpecification.lowPassBuilder()
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.sampleRate(8000)
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.gridDensity(16)
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.oddLength(true)
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.passBandCutoff(300)
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.passBandAmplitude(1.0)
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.passBandRipple(0.01)
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.stopBandStart(500)
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.stopBandAmplitude(0.0)
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.stopBandRipple(0.03) //Approximately 60 dB attenuation
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.build();
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try
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{
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RemezFIRFilterDesigner designer = new RemezFIRFilterDesigner(specification);
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if(designer.isValid())
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{
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sLowPassFilterCoefficients = designer.getImpulseResponse();
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}
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}
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catch(FilterDesignException fde)
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{
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mLog.error("Filter design error", fde);
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}
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}
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protected float mSymbolTimingGain = COARSE_TIMING_GAIN;
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protected SampleBuffer mSampleBuffer;
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protected ZeroCrossingErrorDetector mTimingErrorDetector = new ZeroCrossingErrorDetector(SAMPLES_PER_SYMBOL);
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protected SynchronizationMonitor mSynchronizationMonitor;
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private IIRSinglePoleDCRemovalFilter mDCFilter = new IIRSinglePoleDCRemovalFilter(0.99999f);
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private RealFIRFilter2 mLowPassFilter = new RealFIRFilter2(sLowPassFilterCoefficients);
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private Listener<Boolean> mSymbolDecisionListener;
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private boolean mSampleDecision;
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/**
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* Implements a Logic Trunked Radio sub-audible 300 baud FSK signaling decoder
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*
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* @param messageLength in symbols
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*/
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public LTRDecoder(int messageLength)
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{
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this(messageLength, new SampleBuffer(SAMPLES_PER_SYMBOL, COARSE_TIMING_GAIN),
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new ZeroCrossingErrorDetector(SAMPLES_PER_SYMBOL));
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}
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/**
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* Implements a Logic Trunked Radio sub-audible 300 baud FSK signaling decoder.
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*
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* @param messageLength in symbols
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* @param sampleBuffer to use for storing sample decisions
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* @param detector for symbol timing errors
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*/
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public LTRDecoder(int messageLength, SampleBuffer sampleBuffer, ZeroCrossingErrorDetector detector)
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{
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mSynchronizationMonitor = new SynchronizationMonitor(messageLength);
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mSynchronizationMonitor.setListener(this);
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mTimingErrorDetector = detector;
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mSampleBuffer = sampleBuffer;
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mSampleBuffer.setTimingGain(mSymbolTimingGain);
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}
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/**
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* Implements the ISyncDetectedListener interface to be notified of message sync detection events.
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*
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* This allows the internal timing error detector to adjust symbol timing error gain levels according to the
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* message synchronization state to quickly adjust to initial signal streams or to reduce gain levels once
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* synchronization has been achieved.
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*/
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@Override
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public void syncDetected()
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{
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mSynchronizationMonitor.syncDetected();
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}
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/**
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* Processes the buffer samples by converting all samples to boolean values reflecting if the sample value is
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* greater than zero (or not). Average symbol timing offset is calculated for the full buffer and the offset is
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* adjusted and then each of the symbols are decoded using a simple majority decision.
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*
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* @param buffer containing 8.0 kHz unfiltered FM demodulated audio samples with sub-audible LTR signalling.
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*/
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@Override
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public void receive(ReusableBuffer buffer)
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{
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ReusableBuffer dcFiltered = mDCFilter.filter(buffer);
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ReusableBuffer lowPassFiltered = mLowPassFilter.filter(dcFiltered);
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for(float sample : lowPassFiltered.getSamples())
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{
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mSampleDecision = sample > 0.0;
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mSampleBuffer.receive(mSampleDecision);
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mTimingErrorDetector.receive(mSampleDecision);
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if(mSampleBuffer.hasSymbol())
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{
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if(mSymbolDecisionListener != null)
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{
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mSymbolDecisionListener.receive(mSampleBuffer.getSymbol());
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}
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mSampleBuffer.resetAndAdjust(-mTimingErrorDetector.getError());
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mSynchronizationMonitor.increment();
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}
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}
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lowPassFiltered.decrementUserCount();
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}
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/**
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* Registers a listener to receive decoded LTR symbols.
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*
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* @param listener to receive symbols.
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*/
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public void setListener(Listener<Boolean> listener)
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{
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mSymbolDecisionListener = listener;
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}
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/**
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* Removes the symbol listener from receiving decoded LTR symbols.
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*/
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public void removeListener()
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{
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mSymbolDecisionListener = null;
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}
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/**
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* Implements the ISyncStateListener interface to receive synchronization state events and adjust the symbol timing
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* error gain levels on the internal timing error detector.
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*
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* Gain levels are defined relative to a unit gain of 1.0 over a number of symbol periods. LTR uses an initial
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* ramp-up symbol reversal pattern of 0101 which provides three zero crossing detection opportunities for the timing
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* error detector, therefore we react to a COARSE gain state to average timing error over 3 symbols.
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*
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* @param syncState from an external synchronization state monitor
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*/
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@Override
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public void setSyncState(SyncState syncState)
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{
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switch(syncState)
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{
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case FINE:
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mSymbolTimingGain = FINE_TIMING_GAIN;
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break;
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case MEDIUM:
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mSymbolTimingGain = MEDIUM_TIMING_GAIN;
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break;
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case COARSE:
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mSymbolTimingGain = COARSE_TIMING_GAIN;
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break;
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default:
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throw new IllegalArgumentException("Unrecognized sync state level: " + syncState.name());
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}
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mSampleBuffer.setTimingGain(mSymbolTimingGain);
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}
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}
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