From fa33f15417bcb5837c32fe6e0a62a3d9b314ec0e Mon Sep 17 00:00:00 2001
From: Jacob McSwain <jacob@mcswain.dev>
Date: Sat, 16 Jul 2022 04:53:43 -0500
Subject: [PATCH] Implemented M17 EOT marker

---
 .../include/protocols/M17/M17Constants.hpp    |  1 +
 .../include/protocols/M17/M17Modulator.hpp    |  6 +-
 openrtx/src/protocols/M17/M17Modulator.cpp    | 56 ++++++++++++-------
 3 files changed, 41 insertions(+), 22 deletions(-)

diff --git a/openrtx/include/protocols/M17/M17Constants.hpp b/openrtx/include/protocols/M17/M17Constants.hpp
index f631fffe..c75952ab 100644
--- a/openrtx/include/protocols/M17/M17Constants.hpp
+++ b/openrtx/include/protocols/M17/M17Constants.hpp
@@ -41,6 +41,7 @@ static constexpr syncw_t LSF_SYNC_WORD    = {0x55, 0xF7};  // LSF sync word
 static constexpr syncw_t BERT_SYNC_WORD   = {0xDF, 0x55};  // BERT data sync word
 static constexpr syncw_t STREAM_SYNC_WORD = {0xFF, 0x5D};  // Stream data sync word
 static constexpr syncw_t PACKET_SYNC_WORD = {0x75, 0xFF};  // Packet data sync word
+static constexpr syncw_t EOT_SYNC_WORD    = {0x55, 0x5D};  // End of transmission sync word
 
 }      // namespace M17
 
diff --git a/openrtx/include/protocols/M17/M17Modulator.hpp b/openrtx/include/protocols/M17/M17Modulator.hpp
index 209d217e..8d2233c7 100644
--- a/openrtx/include/protocols/M17/M17Modulator.hpp
+++ b/openrtx/include/protocols/M17/M17Modulator.hpp
@@ -58,7 +58,7 @@ public:
     void init();
 
     /**
-     * Shutdown modulator and deallocate data buffers.
+     * Forcefully shutdown modulator and deallocate data buffers.
      */
     void terminate();
 
@@ -89,6 +89,9 @@ private:
      */
     void sendBaseband();
 
+    /** Gracefully end the transmission **/
+    void stop();
+
     static constexpr size_t M17_TX_SAMPLE_RATE     = 48000;
     static constexpr size_t M17_SAMPLES_PER_SYMBOL = M17_TX_SAMPLE_RATE / M17_SYMBOL_RATE;
     static constexpr size_t M17_FRAME_SAMPLES      = M17_FRAME_SYMBOLS * M17_SAMPLES_PER_SYMBOL;
@@ -106,7 +109,6 @@ private:
     stream_sample_t              *idleBuffer;      ///< Half baseband buffer, free for processing.
     streamId                     outStream;        ///< Baseband output stream ID.
     bool                         txRunning;        ///< Transmission running.
-    bool                         stopTx;           ///< Stop transmission request.
 
     #if defined(PLATFORM_MD3x0) || defined(PLATFORM_MDUV3x0)
     filter_state_t   pwmFilterState;
diff --git a/openrtx/src/protocols/M17/M17Modulator.cpp b/openrtx/src/protocols/M17/M17Modulator.cpp
index 80190965..c5b41e86 100644
--- a/openrtx/src/protocols/M17/M17Modulator.cpp
+++ b/openrtx/src/protocols/M17/M17Modulator.cpp
@@ -76,7 +76,6 @@ void M17::M17Modulator::start()
     if(txRunning) return;
 
     txRunning = true;
-    stopTx    = false;
 
     // Fill symbol buffer with preamble, made of alternated +3 and -3 symbols
     for(size_t i = 0; i < symbols.size(); i += 2)
@@ -112,11 +111,45 @@ void M17Modulator::send(const frame_t& frame, const bool isLast)
         it       = std::copy(sym.begin(), sym.end(), it);
     }
 
+    symbolsToBaseband();
+    sendBaseband();
+
     // If last frame, signal stop of transmission
-    if(isLast) stopTx = true;
+    if(isLast) stop();
+}
+
+void M17Modulator::stop()
+{
+    if(txRunning == false)
+        return;
+
+    frame_t eotFrame;
+    // Fill EOT frame with 0x55, 0x5D as per M17 spec.
+    for(size_t i = 0; i < eotFrame.size(); i += 2)
+    {
+        eotFrame[i]     = 0x55;
+        eotFrame[i + 1] = 0x5D;
+    }
+
+    // Convert to symbols
+    auto it = symbols.begin();
+    for(size_t i = 0; i < eotFrame.size(); i++)
+    {
+        auto sym = byteToSymbols(eotFrame[i]);
+        it       = std::copy(sym.begin(), sym.end(), it);
+    }
 
     symbolsToBaseband();
     sendBaseband();
+
+    outputStream_stop(outStream);
+    outputStream_sync(outStream, false);
+    txRunning  = false;
+    idleBuffer = baseband_buffer.get();
+
+    #if defined(PLATFORM_MD3x0) || defined(PLATFORM_MDUV3x0)
+    dsp_resetFilterState(&pwmFilterState);
+    #endif
 }
 
 void M17Modulator::symbolsToBaseband()
@@ -148,24 +181,7 @@ void M17Modulator::sendBaseband()
 
     // Transmission is ongoing, syncronise with stream end before proceeding
     outputStream_sync(outStream, true);
-
-    // Check if transmission stop is requested, if so stop the output stream
-    // and wait until its effective termination.
-    if(stopTx == true)
-    {
-        outputStream_stop(outStream);
-        outputStream_sync(outStream, false);
-        stopTx     = false;
-        txRunning  = false;
-        idleBuffer = baseband_buffer.get();
-        #if defined(PLATFORM_MD3x0) || defined(PLATFORM_MDUV3x0)
-        dsp_resetFilterState(&pwmFilterState);
-        #endif
-    }
-    else
-    {
-        idleBuffer = outputStream_getIdleBuffer(outStream);
-    }
+    idleBuffer = outputStream_getIdleBuffer(outStream);
 }
 #else
 void M17Modulator::sendBaseband()