SSB demod: use delay line to squeeze squelch tail

plugins/channelrx/demodssb/ssbdemod.cpp

@@ -51,6 +51,7 @@ SSBDemod::SSBDemod(DeviceSourceAPI *deviceAPI) :
         m_agcNbSamples(12000),
         m_agcPowerThreshold(1e-2),
         m_agcThresholdGate(0),
+        m_squelchDelayLine(2*48000),
         m_audioActive(false),
         m_sampleSink(0),
         m_audioFifo(24000),
@@ -207,8 +208,10 @@ void SSBDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
                 m_sum.imag(0.0);
 			}
 
-            double agcVal = m_agcActive ? m_agc.feedAndGetValue(sideband[i]) : 10.0; // 10.0 for 3276.8, 1.0 for 327.68
-            m_audioActive = agcVal != 0.0;
+            float agcVal = m_agcActive ? m_agc.feedAndGetValue(sideband[i]) : 10.0; // 10.0 for 3276.8, 1.0 for 327.68
+            fftfilt::cmplx& delayedSample = m_squelchDelayLine.readBack(m_agc.getStepDownDelay());
+            m_audioActive = delayedSample.real() != 0.0;
+            m_squelchDelayLine.write(sideband[i]*agcVal);
 
 			if (m_audioMute)
 			{
@@ -217,23 +220,25 @@ void SSBDemod::feed(const SampleVector::const_iterator& begin, const SampleVecto
 			}
 			else
 			{
+			    fftfilt::cmplx z = delayedSample * m_agc.getStepDownValue();
+
 				if (m_audioBinaual)
 				{
 					if (m_audioFlipChannels)
 					{
-						m_audioBuffer[m_audioBufferFill].r = (qint16)(sideband[i].imag() * m_volume * agcVal);
-						m_audioBuffer[m_audioBufferFill].l = (qint16)(sideband[i].real() * m_volume * agcVal);
+						m_audioBuffer[m_audioBufferFill].r = (qint16)(z.imag() * m_volume);
+						m_audioBuffer[m_audioBufferFill].l = (qint16)(z.real() * m_volume);
 					}
 					else
 					{
-						m_audioBuffer[m_audioBufferFill].r = (qint16)(sideband[i].real() * m_volume * agcVal);
-						m_audioBuffer[m_audioBufferFill].l = (qint16)(sideband[i].imag() * m_volume * agcVal);
+						m_audioBuffer[m_audioBufferFill].r = (qint16)(z.real() * m_volume);
+						m_audioBuffer[m_audioBufferFill].l = (qint16)(z.imag() * m_volume);
 					}
 				}
 				else
 				{
-					Real demod = (sideband[i].real() + sideband[i].imag()) * 0.7;
-					qint16 sample = (qint16)(demod * m_volume * agcVal);
+					Real demod = (z.real() + z.imag()) * 0.7;
+					qint16 sample = (qint16)(demod * m_volume);
 					m_audioBuffer[m_audioBufferFill].l = sample;
 					m_audioBuffer[m_audioBufferFill].r = sample;
 				}

plugins/channelrx/demodssb/ssbdemod.h

@@ -29,6 +29,7 @@
 #include "dsp/agc.h"
 #include "audio/audiofifo.h"
 #include "util/message.h"
+#include "util/doublebufferfifo.h"
 
 #include "ssbdemodsettings.h"
 
@@ -259,6 +260,7 @@ private:
     int m_agcNbSamples;         //!< number of audio (48 kHz) samples for AGC averaging
     double m_agcPowerThreshold; //!< AGC power threshold (linear)
     int m_agcThresholdGate;     //!< Gate length in number of samples befor threshold triggers
+    DoubleBufferFIFO<fftfilt::cmplx> m_squelchDelayLine;
     bool m_audioActive;         //!< True if an audio signal is produced (no AGC or AGC and above threshold)
 
 	NCOF m_nco;

plugins/channelrx/demodssb/ssbplugin.cpp

@@ -8,7 +8,7 @@
 
 const PluginDescriptor SSBPlugin::m_pluginDescriptor = {
 	QString("SSB Demodulator"),
-	QString("3.14.0"),
+	QString("3.14.4"),
 	QString("(c) Edouard Griffiths, F4EXB"),
 	QString("https://github.com/f4exb/sdrangel"),
 	true,

sdrbase/dsp/agc.cpp

@@ -176,3 +176,15 @@ double MagAGC::feedAndGetValue(const Complex& ci)
         return m_u0;
     }
 }
+
+float MagAGC::getStepDownValue() const
+{
+    if (m_count <  m_stepDownDelay)
+    {
+        return 1.0f;
+    }
+    else
+    {
+        return StepFunctions::smootherstep(m_stepDownCounter * m_stepDelta);
+    }
+}

sdrbase/dsp/agc.h

@@ -50,6 +50,9 @@ public:
     void setStepDownDelay(int stepDownDelay) { m_stepDownDelay = stepDownDelay; }
     void setClamping(bool clamping) { m_clamping = clamping; }
     void setClampMax(double clampMax) { m_clampMax = clampMax; }
+    int getStepDownDelay() const { return m_stepDownDelay; }
+    float getStepDownValue() const;
+
 private:
     bool m_squared;        //!< use squared magnitude (power) to compute AGC value
 	double m_magsq;        //!< current squared magnitude (power)