Fixed jitter in M17 baseband signal generation

openrtx/include/protocols/M17/M17Modulator.h

@@ -57,13 +57,17 @@ public:
 
     /**
      * Generate and transmit the baseband signal obtained by 4FSK modulation of
-     * a given block of data.
+     * a given block of data. When called for the first time, this function
+     * starts baseband transmission.
      *
      * @param sync: synchronisation word to be prepended to data block.
      * @param data: data block to be transmitted.
+     * @param isLast: flag signalling that current block is the last one being
+     * transmitted.
      */
     void send(const std::array< uint8_t, 2 >& sync,
-              const std::array< uint8_t, 46 >& data);
+              const std::array< uint8_t, 46 >& data,
+              const bool isLast = false);
 
 private:
 
@@ -110,6 +114,8 @@ private:
     int16_t      *baseband_buffer;    ///< Buffer for baseband audio handling.
     dataBuffer_t *activeBuffer;       ///< Half baseband buffer, in transmission.
     dataBuffer_t *idleBuffer;         ///< Half baseband buffer, free for processing.
+    bool         txRunning;           ///< Transmission running.
+    bool         stopTx;              ///< Stop transmission request.
 };
 
 #endif /* M17_MODULATOR_H */

openrtx/src/protocols/M17/M17Modulator.cpp

@@ -89,9 +89,10 @@ void M17Modulator::init()
      */
 
     baseband_buffer = new int16_t[2 * M17_FRAME_SAMPLES];
-    activeBuffer    = new (baseband_buffer) dataBuffer_t;
+    idleBuffer      = new (baseband_buffer) dataBuffer_t;
     int16_t *ptr    = baseband_buffer + activeBuffer->size();
-    idleBuffer      = new (ptr) dataBuffer_t;
+    activeBuffer    = new (ptr) dataBuffer_t;
+    txRunning       = false;
 }
 
 void M17Modulator::terminate()
@@ -103,7 +104,8 @@ void M17Modulator::terminate()
 }
 
 void M17Modulator::send(const std::array< uint8_t, 2 >& sync,
-                        const std::array< uint8_t, 46 >& data)
+                        const std::array< uint8_t, 46 >& data,
+                        const bool isLast)
 {
     auto sync1 = byteToSymbols(sync[0]);
     auto sync2 = byteToSymbols(sync[1]);
@@ -117,6 +119,9 @@ void M17Modulator::send(const std::array< uint8_t, 2 >& sync,
         it       = std::copy(sym.begin(), sym.end(), it);
     }
 
+    // If last frame, signal stop of transmission
+    if(isLast) stopTx = true;
+
     generateBaseband();
     emitBaseband();
 }
@@ -149,11 +154,29 @@ void M17Modulator::emitBaseband()
         idleBuffer->at(i) = shifted_sample;
     }
 
-    toneGen_waitForStreamEnd();
+    if(txRunning == false)
-    std::swap(idleBuffer, activeBuffer);
+    {
+        // First run, start transmission
+        toneGen_playAudioStream(reinterpret_cast< uint16_t *>(baseband_buffer),
+                                2*M17_FRAME_SAMPLES, M17_RTX_SAMPLE_RATE, true);
+        txRunning = true;
+        stopTx    = false;
+    }
+    else
+    {
+        // Transmission is ongoing, syncronise with stream end before proceeding
+        toneGen_waitForStreamEnd();
 
-    toneGen_playAudioStream(reinterpret_cast< uint16_t *>(activeBuffer->data()),
+        // Check if transmission stop is requested
-                            activeBuffer->size(), M17_RTX_SAMPLE_RATE);
+        if(stopTx == true)
+        {
+            toneGen_stopAudioStream();
+            stopTx    = false;
+            txRunning = false;
+        }
+    }
+
+    std::swap(idleBuffer, activeBuffer);
 }
 #elif defined(PLATFORM_LINUX)
 void M17Modulator::emitBaseband()

openrtx/src/protocols/M17/M17Transmitter.cpp

@@ -120,5 +120,6 @@ void M17Transmitter::send(const payload_t& payload, const bool isLast)
 
     interleave(frame);
     decorrelate(frame);
-    modulator.send(DATA_SYNC_WORD, frame);
+
+    modulator.send(DATA_SYNC_WORD, frame, isLast);
 }

platform/drivers/tones/toneGenerator_MDx.cpp

@@ -54,7 +54,8 @@ uint32_t beepTableIncr  = 0; // "beep" sine table index increment per tick
 uint32_t beepTimerCount = 0; // Downcounter for timed "beep"
 uint8_t  beepVolume     = 0; // "beep" volume level
 
-bool tonesLocked = false;    // If true tone channel is in use by FSK/playback
+bool tonesLocked  = false;   // If true tone channel is in use by FSK/playback
+bool circularMode = false;   // Circular mode enabled
 
 using namespace miosix;
 Thread *dmaWaiting = 0;
@@ -98,19 +99,22 @@ void __attribute__((used)) TIM8_TRG_COM_TIM14_IRQHandler()
  */
 void __attribute__((used)) DMA_Handler()
 {
-    DMA1->LIFCR |= DMA_LIFCR_CTCIF2         // Clear interrupt flags
+    DMA1->LIFCR |= DMA_LIFCR_CTCIF2             // Clear interrupt flags
+                | DMA_LIFCR_CHTIF2
                 | DMA_LIFCR_CTEIF2;
 
-    TIM7->CR1   = 0;                        // End of transfer, stop TIM7
+    if(circularMode == false)
-    TIM3->CCER &= ~TIM_CCER_CC3E;           // Turn off compare channel
+    {
 
-    RCC->AHB1ENR &= ~RCC_AHB1ENR_DMA1EN;    // Turn off DMA
+        TIM7->CR1     = 0;                      // End of transfer, stop TIM7
-    RCC->APB1ENR &= ~RCC_APB1ENR_TIM7EN;    // Turn off TIM7
+        TIM3->CCER   &= ~TIM_CCER_CC3E;         // Turn off compare channel
-    __DSB();
+        RCC->APB1ENR &= ~RCC_APB1ENR_TIM7EN;    // Turn off TIM7
+        __DSB();
 
-    tonesLocked = false;                    // Finally, unlock tones
+        tonesLocked = false;                    // Finally, unlock tones
+    }
 
-    if(dmaWaiting == 0) return;             // Wake up eventual pending threads
+    if(dmaWaiting == 0) return;                 // Wake up eventual pending threads
     dmaWaiting->IRQwakeup();
     if(dmaWaiting->IRQgetPriority()>Thread::IRQgetCurrentThread()->IRQgetPriority())
         Scheduler::IRQfindNextThread();
@@ -251,7 +255,7 @@ void toneGen_encodeAFSK1200(const uint8_t* buf, const size_t len)
 }
 
 void toneGen_playAudioStream(const uint16_t* buf, const size_t len,
-                             const uint32_t sampleRate)
+                             const uint32_t sampleRate, const bool circMode)
 {
     if((buf == NULL) || (len == 0) || (sampleRate == 0)) return;
 
@@ -270,7 +274,7 @@ void toneGen_playAudioStream(const uint16_t* buf, const size_t len,
      * Timebase for triggering of DMA transfers.
      * Bus frequency is 84MHz.
      */
-    uint32_t ratio = (84000000/sampleRate) - 1;
+    uint32_t ratio = (84000000/sampleRate);
 
     TIM7->CNT  = 0;
     TIM7->PSC  = 0;
@@ -291,9 +295,18 @@ void toneGen_playAudioStream(const uint16_t* buf, const size_t len,
                      | DMA_SxCR_MINC          // Increment source pointer
                      | DMA_SxCR_DIR_0         // Memory to peripheral
                      | DMA_SxCR_TCIE          // Transfer complete interrupt
-                     | DMA_SxCR_TEIE          // Transfer error interrupt
+                     | DMA_SxCR_TEIE;         // Transfer error interrupt
-                     | DMA_SxCR_EN;           // Enable transfer
 
+    if(circMode)
+    {
+        DMA1_Stream2->CR |= DMA_SxCR_CIRC     // Circular buffer mode
+                         |  DMA_SxCR_HTIE;    // Half transfer interrupt
+        circularMode = true;
+    }
+
+    DMA1_Stream2->CR |= DMA_SxCR_EN;           // Enable transfer
+
+    // Enable DMA interrupts
     NVIC_ClearPendingIRQ(DMA1_Stream2_IRQn);
     NVIC_SetPriority(DMA1_Stream2_IRQn, 10);
     NVIC_EnableIRQ(DMA1_Stream2_IRQn);
@@ -346,13 +359,27 @@ void toneGen_stopAudioStream()
     TIM7->CR1   = 0;
     TIM3->CCER &= ~TIM_CCER_CC3E;
 
-    // Shut down TIM7 and DMA
+    // Stop DMA transfer and clear pending interrupt flags
-    RCC->AHB1ENR &= ~RCC_AHB1ENR_DMA1EN;
+    DMA1_Stream2->CR   = 0;
+    DMA1_Stream2->M0AR = 0;
+    DMA1->LIFCR       |= DMA_LIFCR_CTCIF2
+                      |  DMA_LIFCR_CHTIF2
+                      |  DMA_LIFCR_CTEIF2;
+
+    // Shut down TIM7 clock
     RCC->APB1ENR &= ~RCC_APB1ENR_TIM7EN;
+    __DSB();
 
     // Unlock tones and wake up the thread waiting for completion
     tonesLocked = false;
-    if(dmaWaiting) dmaWaiting->IRQwakeup();
+    if(dmaWaiting)
+    {
+        dmaWaiting->IRQwakeup();
+        dmaWaiting = 0;
+    }
+
+    // Clear flag for circular double buffered mode
+    circularMode = false;
 }
 
 bool toneGen_toneBusy()

platform/drivers/tones/toneGenerator_MDx.h

@@ -113,9 +113,11 @@ void toneGen_encodeAFSK1200(const uint8_t *buf, const size_t len);
  * @param buf: pointer to a buffer containing the audio samples.
  * @param len: length of the data buffer.
  * @param sampleRate: sample rate of the audio stream in samples per second.
+ * @param circMode: treat buf as a double circular buffer, continuously
+ * reproducing its content.
  */
 void toneGen_playAudioStream(const uint16_t *buf, const size_t len,
-                             const uint32_t sampleRate);
+                             const uint32_t sampleRate, const bool circMode);
 
 /**
  * When called, this function blocks the execution flow until the reproduction
@@ -128,7 +130,7 @@ bool toneGen_waitForStreamEnd();
 
 /**
  * Interrupt the ongoing reproduction of an audio stream, also making the
- * toneGen_playAudioStream return to the caller.
+ * toneGen_waitForStreamEnd return to the caller.
  */
 void toneGen_stopAudioStream();