diff --git a/sdrbase/dsp/glspectrumsettings.cpp b/sdrbase/dsp/glspectrumsettings.cpp
index cc139ad14..9592815ef 100644
--- a/sdrbase/dsp/glspectrumsettings.cpp
+++ b/sdrbase/dsp/glspectrumsettings.cpp
@@ -60,7 +60,7 @@ QByteArray GLSpectrumSettings::serialize() const
 {
 	SimpleSerializer s(1);
 
-	s.writeU32(1, m_fftSize);
+	s.writeS32(1, m_fftSize);
 	s.writeS32(2, m_fftOverlap);
 	s.writeS32(3, (int) m_fftWindow);
 	s.writeReal(4, m_refLevel);
@@ -102,7 +102,7 @@ bool GLSpectrumSettings::deserialize(const QByteArray& data)
 
 	if (d.getVersion() == 1)
     {
-		d.readU32(1, &m_fftSize, 1024);
+		d.readS32(1, &m_fftSize, 1024);
 		d.readS32(2, &m_fftOverlap, 0);
         d.readS32(3, &tmp, (int) FFTWindow::Hanning);
 		m_fftWindow = (FFTWindow::Function) tmp;
diff --git a/sdrbase/dsp/glspectrumsettings.h b/sdrbase/dsp/glspectrumsettings.h
index 4bdfd08d4..a3a734c7a 100644
--- a/sdrbase/dsp/glspectrumsettings.h
+++ b/sdrbase/dsp/glspectrumsettings.h
@@ -36,7 +36,7 @@ public:
         AvgModeMax
     };
 
-	unsigned int m_fftSize;
+	int m_fftSize;
 	int m_fftOverlap;
 	FFTWindow::Function m_fftWindow;
 	Real m_refLevel;
diff --git a/sdrbase/dsp/spectrumvis.cpp b/sdrbase/dsp/spectrumvis.cpp
index 5b7f07211..7107bd7e8 100644
--- a/sdrbase/dsp/spectrumvis.cpp
+++ b/sdrbase/dsp/spectrumvis.cpp
@@ -123,9 +123,9 @@ void SpectrumVis::feed(const Complex *begin, unsigned int length)
 
     if (m_settings.m_averagingMode == GLSpectrumSettings::AvgModeNone)
     {
-        for (unsigned int i = 0; i < m_settings.m_fftSize; i++)
+        for (int i = 0; i < m_settings.m_fftSize; i++)
         {
-            if (i < length) {
+            if (i < (int) length) {
                 c = begin[i];
             } else {
                 c = Complex{0,0};
@@ -157,9 +157,9 @@ void SpectrumVis::feed(const Complex *begin, unsigned int length)
     }
     else if (m_settings.m_averagingMode == GLSpectrumSettings::AvgModeMoving)
     {
-        for (unsigned int i = 0; i < m_settings.m_fftSize; i++)
+        for (int i = 0; i < m_settings.m_fftSize; i++)
         {
-            if (i < length) {
+            if (i < (int) length) {
                 c = begin[i];
             } else {
                 c = Complex{0,0};
@@ -196,9 +196,9 @@ void SpectrumVis::feed(const Complex *begin, unsigned int length)
     {
         double avg;
 
-        for (unsigned int i = 0; i < m_settings.m_fftSize; i++)
+        for (int i = 0; i < m_settings.m_fftSize; i++)
         {
-            if (i < length) {
+            if (i < (int) length) {
                 c = begin[i];
             } else {
                 c = Complex{0,0};
@@ -241,9 +241,9 @@ void SpectrumVis::feed(const Complex *begin, unsigned int length)
     {
         double max;
 
-        for (unsigned int i = 0; i < m_settings.m_fftSize; i++)
+        for (int i = 0; i < m_settings.m_fftSize; i++)
         {
-            if (i < length) {
+            if (i < (int) length) {
                 c = begin[i];
             } else {
                 c = Complex{0,0};
@@ -690,22 +690,16 @@ void SpectrumVis::applySettings(const GLSpectrumSettings& settings, bool force)
 {
     QMutexLocker mutexLocker(&m_mutex);
 
-    unsigned int fftSize = settings.m_fftSize > MAX_FFT_SIZE ?
+    int fftSize = settings.m_fftSize > MAX_FFT_SIZE ?
         MAX_FFT_SIZE :
         settings.m_fftSize < 64 ?
             64 :
             settings.m_fftSize;
 
-    int overlapPercent = settings.m_fftOverlap > 100 ?
-        100 :
-        settings.m_fftOverlap < 0 ?
-            0 :
-            settings.m_fftOverlap;
-
     qDebug() << "SpectrumVis::applySettings:"
         << " m_fftSize: " << fftSize
         << " m_fftWindow: " << settings.m_fftWindow
-        << " m_fftOverlap: " << overlapPercent
+        << " m_fftOverlap: " << settings.m_fftOverlap
         << " m_averagingIndex: " << settings.m_averagingIndex
         << " m_averagingMode: " << settings.m_averagingMode
         << " m_refLevel: " << settings.m_refLevel
@@ -738,9 +732,10 @@ void SpectrumVis::applySettings(const GLSpectrumSettings& settings, bool force)
     }
 
     if ((fftSize != m_settings.m_fftSize)
-     || (overlapPercent != m_settings.m_fftOverlap) || force)
+     || (settings.m_fftOverlap != m_settings.m_fftOverlap) || force)
     {
-        m_overlapSize = (fftSize * overlapPercent) / 100;
+        m_overlapSize = settings.m_fftOverlap >= fftSize ? fftSize - 1 :
+            settings.m_fftOverlap < 0 ? 0 : settings.m_fftOverlap;
         m_refillSize = fftSize - m_overlapSize;
         m_fftBufferFill = m_overlapSize;
     }
@@ -762,7 +757,6 @@ void SpectrumVis::applySettings(const GLSpectrumSettings& settings, bool force)
 
     m_settings = settings;
     m_settings.m_fftSize = fftSize;
-    m_settings.m_fftOverlap = overlapPercent;
 }
 
 void SpectrumVis::handleConfigureDSP(uint64_t centerFrequency, int sampleRate)
diff --git a/sdrbase/dsp/spectrumvis.h b/sdrbase/dsp/spectrumvis.h
index cbe1facc1..ac26d4d7f 100644
--- a/sdrbase/dsp/spectrumvis.h
+++ b/sdrbase/dsp/spectrumvis.h
@@ -190,9 +190,9 @@ private:
 	std::vector<Real> m_powerSpectrum;
 
     GLSpectrumSettings m_settings;
-	std::size_t m_overlapSize;
-	std::size_t m_refillSize;
-	std::size_t m_fftBufferFill;
+	int m_overlapSize;
+	int m_refillSize;
+	int m_fftBufferFill;
 	bool m_needMoreSamples;
 
 	Real m_scalef;