Audio Input: quarter sample rate heterodyne for mono signals

plugins/samplesource/audioinput/audioinput.cpp

@@ -287,6 +287,7 @@ void AudioInput::applySettings(const AudioInputSettings& settings, bool force, b
     if ((m_settings.m_iqMapping != settings.m_iqMapping) || force)
     {
         reverseAPIKeys.append("iqMapping");
+        forwardChange = true;
 
         if (m_worker) {
             m_worker->setIQMapping(settings.m_iqMapping);
@@ -306,7 +307,11 @@ void AudioInput::applySettings(const AudioInputSettings& settings, bool force, b
 
     if (forwardChange)
     {
-        DSPSignalNotification *notif = new DSPSignalNotification(m_settings.m_sampleRate/(1<<m_settings.m_log2Decim), 0);
+        qint64 dF =
+            ((m_settings.m_iqMapping == AudioInputSettings::IQMapping::L) ||
+            (m_settings.m_iqMapping == AudioInputSettings::IQMapping::R)) ?
+                m_settings.m_sampleRate / 4 : 0;
+        DSPSignalNotification *notif = new DSPSignalNotification(m_settings.m_sampleRate/(1<<m_settings.m_log2Decim), dF);
         m_deviceAPI->getDeviceEngineInputMessageQueue()->push(notif);
     }
 }

plugins/samplesource/audioinput/audioinputgui.cpp

@@ -37,7 +37,8 @@ AudioInputGui::AudioInputGui(DeviceUISet *deviceUISet, QWidget* parent) :
     m_deviceUISet(deviceUISet),
     m_forceSettings(true),
     m_settings(),
-    m_sampleSource(NULL)
+    m_sampleSource(nullptr),
+    m_centerFrequency(0)
 {
     m_sampleSource = (AudioInput*) m_deviceUISet->m_deviceAPI->getSampleSource();
 
@@ -130,6 +131,7 @@ void AudioInputGui::handleInputMessages()
         {
             DSPSignalNotification* notif = (DSPSignalNotification*) message;
             m_sampleRate = notif->getSampleRate();
+            m_centerFrequency = notif->getCenterFrequency();
             qDebug("AudioInputGui::handleInputMessages: DSPSignalNotification: SampleRate: %d", notif->getSampleRate());
             updateSampleRateAndFrequency();
 
@@ -160,7 +162,7 @@ void AudioInputGui::updateSampleRateAndFrequency()
 */
     {
         m_deviceUISet->getSpectrum()->setSampleRate(m_sampleRate);
-        m_deviceUISet->getSpectrum()->setCenterFrequency(0);
+        m_deviceUISet->getSpectrum()->setCenterFrequency(m_centerFrequency);
         m_deviceUISet->getSpectrum()->setSsbSpectrum(false);
         m_deviceUISet->getSpectrum()->setLsbDisplay(false);
     }

plugins/samplesource/audioinput/audioinputgui.h

@@ -57,6 +57,7 @@ private:
     QTimer m_updateTimer;
     DeviceSampleSource* m_sampleSource;
     int m_sampleRate;
+    qint64 m_centerFrequency;
 
     MessageQueue m_inputMessageQueue;
 

plugins/samplesource/audioinput/audioinputgui.ui

@@ -210,6 +210,13 @@
        </property>
       </spacer>
      </item>
+     <item>
+      <widget class="QLabel" name="decimLabel">
+       <property name="text">
+        <string>Dec</string>
+       </property>
+      </widget>
+     </item>
      <item>
       <widget class="QComboBox" name="decim">
        <property name="maximumSize">
@@ -243,13 +250,6 @@
        </item>
       </widget>
      </item>
-     <item>
-      <widget class="QLabel" name="decimLabel">
-       <property name="text">
-        <string>Dec</string>
-       </property>
-      </widget>
-     </item>
     </layout>
    </item>
    <item>
@@ -336,12 +336,12 @@
        </property>
        <item>
         <property name="text">
-         <string>I=L, Q=0</string>
+         <string>Mono L</string>
         </property>
        </item>
        <item>
         <property name="text">
-         <string>I=R, Q=0</string>
+         <string>Mono R</string>
         </property>
        </item>
        <item>

plugins/samplesource/audioinput/audioinputworker.cpp

@@ -33,7 +33,8 @@ AudioInputWorker::AudioInputWorker(SampleSinkFifo* sampleFifo, AudioFifo *fifo,
     m_log2Decim(0),
     m_iqMapping(AudioInputSettings::IQMapping::L),
     m_convertBuffer(m_convBufSamples),
-    m_sampleFifo(sampleFifo)
+    m_sampleFifo(sampleFifo),
+    m_quNCOPhase(0)
 {
 }
 
@@ -56,20 +57,40 @@ void AudioInputWorker::stopWork()
 void AudioInputWorker::workIQ(unsigned int nbRead)
 {
     // Map between left and right audio channels and IQ channels
-    if (m_iqMapping == AudioInputSettings::IQMapping::L)
+    if ((m_iqMapping == AudioInputSettings::IQMapping::L) || // mono
-    {
+        (m_iqMapping == AudioInputSettings::IQMapping::R))
-        for (uint32_t i = 0; i < nbRead; i++)
-            m_buf[i*2+1] = 0;
-    }
-    else if (m_iqMapping == AudioInputSettings::IQMapping::R)
     {
         for (uint32_t i = 0; i < nbRead; i++)
         {
-            m_buf[i*2] = m_buf[i*2+1];
+            qint16 r = m_buf[i*2 + (m_iqMapping == AudioInputSettings::IQMapping::R ? 1 : 0)]; // real sample
-            m_buf[i*2+1] = 0;
+
+            if (m_quNCOPhase == 0) // 0
+            {
+                m_buf[i*2]   = r;  // 1
+                m_buf[i*2+1] = 0;  // 0
+                m_quNCOPhase = 1;  // next phase
+            }
+            else if (m_quNCOPhase == 1) // -pi/2
+            {
+                m_buf[i*2]   = 0;  // 0
+                m_buf[i*2+1] = -r; // -1
+                m_quNCOPhase = 2;  // next phase
+            }
+            else if (m_quNCOPhase == 2) // pi or -pi
+            {
+                m_buf[i*2]   = -r; // -1
+                m_buf[i*2+1] = 0;  // 0
+                m_quNCOPhase = 3;  // next phase
+            }
+            else if (m_quNCOPhase == 3) // pi/2
+            {
+                m_buf[i*2]   = 0;  // 0
+                m_buf[i*2+1] = r;  // 1
+                m_quNCOPhase = 0;  // next phase
+            }
         }
     }
-    else if (m_iqMapping == AudioInputSettings::IQMapping::LR)
+    else if (m_iqMapping == AudioInputSettings::IQMapping::LR) // stereo - reverse
     {
         for (uint32_t i = 0; i < nbRead; i++)
         {
@@ -79,30 +100,35 @@ void AudioInputWorker::workIQ(unsigned int nbRead)
         }
     }
 
+    decimate(m_buf, nbRead);
+}
+
+void AudioInputWorker::decimate(qint16 *buf, unsigned int nbRead)
+{
     SampleVector::iterator it = m_convertBuffer.begin();
 
     switch (m_log2Decim)
     {
     case 0:
-        m_decimatorsIQ.decimate1(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate1(&it, buf, 2*nbRead);
         break;
     case 1:
-        m_decimatorsIQ.decimate2_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate2_cen(&it, buf, 2*nbRead);
         break;
     case 2:
-        m_decimatorsIQ.decimate4_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate4_cen(&it, buf, 2*nbRead);
         break;
     case 3:
-        m_decimatorsIQ.decimate8_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate8_cen(&it, buf, 2*nbRead);
         break;
     case 4:
-        m_decimatorsIQ.decimate16_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate16_cen(&it, buf, 2*nbRead);
         break;
     case 5:
-        m_decimatorsIQ.decimate32_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate32_cen(&it, buf, 2*nbRead);
         break;
     case 6:
-        m_decimatorsIQ.decimate64_cen(&it, m_buf, 2*nbRead);
+        m_decimatorsIQ.decimate64_cen(&it, buf, 2*nbRead);
         break;
     default:
         break;

plugins/samplesource/audioinput/audioinputworker.h

@@ -51,8 +51,10 @@ private:
     SampleVector m_convertBuffer;
     SampleSinkFifo* m_sampleFifo;
     Decimators<qint32, qint16, SDR_RX_SAMP_SZ, 16, true> m_decimatorsIQ;
+    int m_quNCOPhase; //!< Quarter sample rate pseudo NCO phase index (0, 90, 180, 270)
 
     void workIQ(unsigned int nbRead);
+    void decimate(qint16 *buf, unsigned int nbRead);
 
 private slots:
     void handleAudio();

plugins/samplesource/audioinput/readme.md

@@ -38,9 +38,9 @@ A control to set the input volume. This is not supported by all input audio devi
 
 <h3>7: Channel Map</h3>
 
-This controls how the left and right stereo audio channels map on to the IQ channels.
+This controls how the left and right audio channels map on to the IQ channels.
 
-* I=L, Q=0 - The left audio channel is driven to the I channel. The Q channel is set to 0.
+* Mono L - Real samples are taken from the left audio channel and are heterodyned by the fourth of the sample rate (fs/4) to obtain complex samples. Therefore the spectrum of the complex baseband is centered at the fourth of the sample rate (fs/4). As per Nyquist rule only a bandwidth of half of the sample rate (fs/2) is available for real signals. Frequencies outside the [0, fs/2] interval are artefacts and can be eliminated by decimating by a factor of 2.
-* I=R, Q=0 - The right audio channel is driven to the I channel. The Q channel is set to 0.
+* Mono R - Same as above but takes the right audio channel for the real signal.
-* I=L, Q=R - The left audio channel is driven to the I channel. The right audio channel is driven to the Q channel.
+* I=L, Q=R - The left audio channel is driven to the I channel. The right audio channel is driven to the Q channel for a complex (analytic signal)input.
-* I=R, Q=L - The right audio channel is driven to the I channel. The left audio channel is driven to the Q channel.
+* I=R, Q=L - The right audio channel is driven to the I channel. The left audio channel is driven to the Q channel for a complex (analytic signal)input.