SSB demod: better algorithm for AGC limitation. For #1918

plugins/channelrx/demodssb/readme.md

@@ -143,7 +143,7 @@ This AGC is based on the calculated magnitude (square root of power of the filte
 
 <h4>13.3: AGC clamping</h4>
 
-When on this clamps signal at the maximum amplitude. Normally this is not needed for most signals as the AGC amplitude order is quite conservative at 10% of the maximum. You may switch it on if you notice a loud click when a transmission starts.
+This limits AGC gain when signal rises up sharply avoiding peak audio overload. You may switch it on if you notice a loud click on strong signals after silences.
 
 <h4>13.4: Noise Reduction</h4>
 

plugins/channelrx/demodssb/ssbdemodsink.cpp

@@ -73,8 +73,6 @@ SSBDemodSink::SSBDemodSink() :
 	m_magsqSum = 0.0;
 	m_magsqPeak = 0.0;
 	m_magsqCount = 0;
-    m_magsqCur = 0.0;
-    m_magsqPrev = 0.0;
 
 	SSBFilter = new fftfilt(m_LowCutoff / m_audioSampleRate, m_Bandwidth / m_audioSampleRate, m_ssbFftLen);
 	DSBFilter = new fftfilt((2.0f * m_Bandwidth) / m_audioSampleRate, 2 * m_ssbFftLen);
@@ -175,24 +173,20 @@ void SSBDemodSink::processOneSample(Complex &ci)
         float agcVal = m_agcActive ? m_agc.feedAndGetValue(sideband[i]) : 1.0;
         fftfilt::cmplx& delayedSample = m_squelchDelayLine.readBack(m_agc.getStepDownDelay());
         m_audioActive = delayedSample.real() != 0.0;
-        m_magsqCur = std::norm(sideband[i]*agcVal);
 
         // Prevent overload based on squared magnitude variation
         // Only if AGC is active
-        if (m_agcActive && m_agcClamping && (std::abs(m_magsqCur - m_magsqPrev) > m_agcTarget*m_agcTarget*5.0) & (agcVal > 100.0))
+        if (m_agcActive && m_agcClamping && agcVal > 100.0)
         {
-            float target = m_agcTarget*sqrt(agcVal); // Quench AGC depending on previous value
+            qDebug("SSBDemodSink::processOneSample: %f", agcVal);
-            m_agc.reset(target);
+            m_agc.reset(m_agcTarget*m_agcTarget);
-            m_squelchDelayLine.write(fftfilt::cmplx{target, 0.0});
+            m_squelchDelayLine.write(fftfilt::cmplx{0.0, 0.0});
-            m_magsqCur = target*target;
         }
         else
         {
             m_squelchDelayLine.write(sideband[i]*agcVal);
         }
 
-        m_magsqPrev = m_magsqCur;
-
         if (m_audioMute)
         {
             m_audioBuffer[m_audioBufferFill].r = 0;
@@ -200,7 +194,6 @@ void SSBDemodSink::processOneSample(Complex &ci)
         }
         else
         {
-            // fftfilt::cmplx z = m_agcActive ? delayedSample * m_agc.getStepValue() : delayedSample;
             fftfilt::cmplx z = (m_agcActive && m_agcClamping) ?
                 fftfilt::cmplx{m_lowpassI.filter(delayedSample.real()), m_lowpassQ.filter(delayedSample.imag())}
                 : delayedSample;

plugins/channelrx/demodssb/ssbdemodsink.h

@@ -102,8 +102,6 @@ private:
 	double m_magsqSum;
 	double m_magsqPeak;
     int  m_magsqCount;
-    double m_magsqCur;
-    double m_magsqPrev;
     MagSqLevelsStore m_magSqLevelStore;
     MagAGC m_agc;
     bool m_agcActive;