Add fuzzy syncword validation

Now syncword match is not exact but can tolerate up to two erroneous
bits.

TG-81

openrtx/include/protocols/M17/M17Demodulator.h

@@ -252,6 +252,11 @@ private:
      */
     int8_t quantize(int32_t offset);
 
+    /**
+     * Compute Hamming Distance between two bytes
+     */
+    uint8_t hammingDistance(uint8_t x, uint8_t y);
+
 };
 
 } /* M17 */

openrtx/src/protocols/M17/M17Demodulator.cpp

@@ -272,6 +272,11 @@ bool M17Demodulator::isFrameLSF()
     return isLSF;
 }
 
+uint8_t M17Demodulator::hammingDistance(uint8_t x, uint8_t y)
+{
+    return __builtin_popcount(x ^ y);
+}
+
 void M17Demodulator::update()
 {
     M17::sync_t syncword = { 0, false };
@@ -283,6 +288,7 @@ void M17Demodulator::update()
     FILE *csv_log = fopen("demod_log_2.csv", "a");
 #endif
 
+
     if(baseband.data != NULL)
     {
         // Apply RRC on the baseband buffer
@@ -342,12 +348,12 @@ void M17Demodulator::update()
                     //    printf(" %02X%02X", (*idleFrame)[i], (*idleFrame)[i+1]);
                     //}
                 }
-                // Check if the decoded syncword matches with frame or lsf sync
+                // If syncword is not valid, lock is lost, accept 2 bit errors
                 if (frameIndex == M17_SYNCWORD_SYMBOLS &&
-                    ((*activeFrame)[0] != stream_syncword_bytes[0] ||
-                     (*activeFrame)[1] != stream_syncword_bytes[1]) &&
-                    ((*activeFrame)[0] != lsf_syncword_bytes[0] ||
-                     (*activeFrame)[1] != lsf_syncword_bytes[1]))
+                    (hammingDistance((*activeFrame)[0], stream_syncword_bytes[0]) +
+                     hammingDistance((*activeFrame)[1], stream_syncword_bytes[1]) > 2) &&
+                    (hammingDistance((*activeFrame)[0], lsf_syncword_bytes[0]) +
+                     hammingDistance((*activeFrame)[1], lsf_syncword_bytes[1]) > 2))
                 {
                     locked = false;
                     std::swap(activeFrame, idleFrame);