Add syncword hot re-sync fix DC removal

DC removal filter had a bug that created weird positive spikes on
Module17, add a syncword hot re-sync to be able to correct small phase
errors that appear every five seconds more or less.

TG-81

openrtx/include/protocols/M17/M17Demodulator.h

@@ -122,7 +122,8 @@ private:
     static constexpr size_t M17_FRAME_BYTES        = M17_FRAME_SYMBOLS / 4;
     static constexpr size_t M17_FRAME_SAMPLES      = M17_FRAME_SYMBOLS * M17_SAMPLES_PER_SYMBOL;
     static constexpr size_t M17_SAMPLE_BUF_SIZE    = M17_FRAME_SAMPLES / 2;
-    static constexpr size_t M17_BRIDGE_SIZE        = M17_SAMPLES_PER_SYMBOL * M17_SYNCWORD_SYMBOLS;
+    static constexpr size_t M17_SYNCWORD_SAMPLES   = M17_SAMPLES_PER_SYMBOL * M17_SYNCWORD_SYMBOLS;
+    static constexpr size_t M17_BRIDGE_SIZE        = M17_SYNCWORD_SAMPLES;
 
     static constexpr float  CONV_STATS_ALPHA       = 0.005f;
     static constexpr float  CONV_THRESHOLD_FACTOR  = 3.40;
@@ -239,9 +240,19 @@ private:
 
     /**
      * Compute Hamming Distance between two bytes
+     *
+     * @param x: first byte
+     * @param y: second byte
      */
     uint8_t hammingDistance(uint8_t x, uint8_t y);
 
+    /**
+     * Perform a limited search for a syncword using correlation
+     *
+     * @param offset: sample index right after a syncword
+     * @return int32_t sample of the beginning of a syncword
+     */
+    int32_t syncwordSweep(int32_t offset);
 };
 
 } /* M17 */

openrtx/src/core/dsp.cpp

@@ -81,24 +81,26 @@ void dsp_dcRemoval(filter_state_t *state, audio_sample_t *buffer, size_t length)
 
     if(length < 2) return;
 
+    static constexpr float alpha = 0.999f;
+    size_t pos = 0;
+
     if(state->initialised == false)
     {
         state->u[1] = static_cast< float >(buffer[0]);
         state->initialised = true;
+        pos = 1;
     }
 
-    static constexpr float alpha = 0.999f;
-
-    for(size_t i = 1; i < length; i++)
+    for(; pos < length; pos++)
     {
-        state->u[0] = static_cast< float >(buffer[i]);
+        state->u[0] = static_cast< float >(buffer[pos]);
         state->y[0] = (state->u[0])
                     - (state->u[1])
                     + alpha * (state->y[1]);
 
         state->u[1] = state->u[0];
         state->y[1] = state->y[0];
-        buffer[i] = static_cast< audio_sample_t >(state->y[0] + 0.5f);
+        buffer[pos] = static_cast< audio_sample_t >(state->y[0] + 0.5f);
     }
 }
 

openrtx/src/protocols/M17/M17Demodulator.cpp

@@ -309,6 +309,25 @@ uint8_t M17Demodulator::hammingDistance(uint8_t x, uint8_t y)
     return __builtin_popcount(x ^ y);
 }
 
+int32_t M17Demodulator::syncwordSweep(int32_t offset)
+{
+    int32_t max_conv = 0, max_index = 0;
+    // Start from 5 samples behind, end 5 samples after
+    for(int i = -5; i <= 5; i++)
+    {
+        // TODO: Extend for LSF and BER syncwords
+        int32_t conv = convolution(offset + i,
+                                   stream_syncword,
+                                   M17_SYNCWORD_SYMBOLS);
+        if (conv > max_conv)
+        {
+            max_conv = conv;
+            max_index = i;
+        }
+    }
+    return max_index;
+}
+
 bool M17Demodulator::update()
 {
     M17::sync_t syncword = { 0, false };
@@ -392,11 +411,16 @@ bool M17Demodulator::update()
                     std::swap(activeFrame, idleFrame);
                     frame_index = 0;
                     newFrame   = true;
+
+                    // Locate syncword to correct clock skew between Tx and Rx
+                    int32_t expected_sync =
+                        offset + phase + M17_SAMPLES_PER_SYMBOL * decoded_syms;
+                    int32_t new_sync = syncwordSweep(expected_sync);
+                    phase += new_sync;
                 }
 
                 if (frame_index == M17_SYNCWORD_SYMBOLS)
                 {
-
                     // If syncword is not valid, lock is lost, accept 2 bit errors
                     uint8_t hammingSync = hammingDistance((*activeFrame)[0],
                                                           stream_syncword_bytes[0])
@@ -409,7 +433,7 @@ bool M17Demodulator::update()
                                                          lsf_syncword_bytes[1]);
 
                     // Too many errors in the syncword, lock is lost
-                    if ((hammingSync > 4) && (hammingLsf > 4))
+                    if ((hammingSync > 2) && (hammingLsf > 2))
                     {
                         syncDetected = false;
                         locked = false;

scripts/plot_m17_demod_csv.py

@@ -14,6 +14,6 @@ plt.plot(df.index, df.Threshold * -1 / 10)
 plt.plot(df.index, df.Index)
 plt.plot(df.index, df.Max)
 plt.plot(df.index, df.Min)
-plt.plot(df.index, df.Symbol * 4000)
+plt.plot(df.index, df.Symbol * 10)
 plt.plot(df.index, df.I)
 plt.show()