Added sync score threshold to speed up decoding with few or no signals

decode_ft8.cpp

@@ -15,6 +15,7 @@
 
 #define LOG_LEVEL   LOG_INFO
 
+const int kMin_score = 40;		// Minimum sync score threshold for candidates
 const int kMax_candidates = 120;
 const int kLDPC_iterations = 25;
 
@@ -277,4 +278,4 @@ int main(int argc, char **argv) {
     LOG(LOG_INFO, "Decoded %d messages\n", num_decoded);
 
     return 0;
-}
+}

ft8/decode.cpp

@@ -19,7 +19,7 @@ static int get_index(const MagArray *power, int block, int time_sub, int freq_su
 
 // Localize top N candidates in frequency and time according to their sync strength (looking at Costas symbols)
 // We treat and organize the candidate list as a min-heap (empty initially).
-int find_sync(const MagArray *power, const uint8_t *sync_map, int num_candidates, Candidate *heap) {
+int find_sync(const MagArray *power, const uint8_t *sync_map, int num_candidates, Candidate *heap, int min_score) {
     int heap_size = 0;
     int num_alt = power->time_osr * power->freq_osr;
 
@@ -74,6 +74,8 @@ int find_sync(const MagArray *power, const uint8_t *sync_map, int num_candidates
                     }
                     score /= num_symbols;
 
+                    if (score < min_score) continue;
+
                     // If the heap is full AND the current candidate is better than 
                     // the worst in the heap, we remove the worst and make space
                     if (heap_size == num_candidates && score > heap[0].score) {
@@ -268,4 +270,4 @@ static void decode_multi_symbols(const uint8_t *power, int num_bins, int n_syms,
     }    
 }
 
-} // namespace
+} // namespace

ft8/decode.h

@@ -23,11 +23,11 @@ struct Candidate {
 
 // Localize top N candidates in frequency and time according to their sync strength (looking at Costas symbols)
 // We treat and organize the candidate list as a min-heap (empty initially).
-int find_sync(const MagArray * power, const uint8_t *sync_map, int num_candidates, Candidate *heap);
+int find_sync(const MagArray * power, const uint8_t *sync_map, int num_candidates, Candidate *heap, int min_score = 0);
 
 
 // Compute log likelihood log(p(1) / p(0)) of 174 message bits 
 // for later use in soft-decision LDPC decoding
 void extract_likelihood(const MagArray *power, const Candidate & cand, const uint8_t *code_map, float *log174);
 
-}
+}