Oversampling, round two.

This now seems to be at the point where it will decode more messages
than when using 2MHz with --phase-enhance.

dump1090.c

@@ -508,7 +508,12 @@ static void display_stats(void) {
     printf("%d sample blocks dropped\n",                      Modes.stat_blocks_dropped);
 
     printf("%d ModeA/C detected\n",                           Modes.stat_ModeAC);
+    printf("%d Mode-S preambles with poor correlation\n",     Modes.stat_preamble_no_correlation);
+    printf("%d Mode-S preambles with noise in the quiet period\n", Modes.stat_preamble_not_quiet);
     printf("%d valid Mode-S preambles\n",                     Modes.stat_valid_preamble);
+    for (j = 0; j < MODES_MAX_PHASE_STATS; ++j)
+        if (Modes.stat_preamble_phase[j] > 0)
+            printf("   %d with phase offset %d\n",                Modes.stat_preamble_phase[j], j);
     printf("%d DF-?? fields corrected for length\n",          Modes.stat_DF_Len_Corrected);
     printf("%d DF-?? fields corrected for type\n",            Modes.stat_DF_Type_Corrected);
     printf("%d demodulated with 0 errors\n",                  Modes.stat_demodulated0);
@@ -516,6 +521,9 @@ static void display_stats(void) {
     printf("%d demodulated with 2 errors\n",                  Modes.stat_demodulated2);
     printf("%d demodulated with > 2 errors\n",                Modes.stat_demodulated3);
     printf("%d with good crc\n",                              Modes.stat_goodcrc);
+    for (j = 0; j < MODES_MAX_PHASE_STATS; ++j)
+        if (Modes.stat_goodcrc_phase[j] > 0)
+            printf("   %d with phase offset %d\n",                Modes.stat_goodcrc_phase[j], j);
     printf("%d with bad crc\n",                               Modes.stat_badcrc);
     printf("%d errors corrected\n",                           Modes.stat_fixed);
 
@@ -545,6 +553,8 @@ static void display_stats(void) {
         Modes.stat_blocks_dropped = 0;
 
     Modes.stat_ModeAC =
+        Modes.stat_preamble_no_correlation =
+        Modes.stat_preamble_not_quiet =
         Modes.stat_valid_preamble =
         Modes.stat_DF_Len_Corrected =
         Modes.stat_DF_Type_Corrected =
@@ -569,6 +579,11 @@ static void display_stats(void) {
         Modes.stat_ph_bit_fix[j] = 0;
         Modes.stat_bit_fix[j] = 0;
     }
+
+    for (j = 0;  j < MODES_MAX_PHASE_STATS;  j++) {
+        Modes.stat_preamble_phase[j] = 0;
+        Modes.stat_goodcrc_phase[j] = 0;
+    }
 }
 
 

dump1090.h

@@ -347,12 +347,17 @@ struct {                             // Internal state
     struct stDF    *pDF;              // Pointer to DF list
 
     // Statistics
+#define MODES_MAX_PHASE_STATS 12
+    unsigned int stat_preamble_no_correlation;
+    unsigned int stat_preamble_not_quiet;
     unsigned int stat_valid_preamble;
+    unsigned int stat_preamble_phase[MODES_MAX_PHASE_STATS];
     unsigned int stat_demodulated0;
     unsigned int stat_demodulated1;
     unsigned int stat_demodulated2;
     unsigned int stat_demodulated3;
     unsigned int stat_goodcrc;
+    unsigned int stat_goodcrc_phase[MODES_MAX_PHASE_STATS];
     unsigned int stat_badcrc;
     unsigned int stat_fixed;
 

mode_s.c

@@ -1947,20 +1947,25 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
 // starting at the given sample, and assuming that the symbol starts at a fixed 0-5 phase offset within
 // m[0]. They return a correlation value, generally interpreted as >0 = 1 bit, <0 = 0 bit
 
+// TODO check if there are better (or more balanced) correlation functions to use here
+
+// nb: the correlation functions sum to zero, so we do not need to adjust for the DC offset in the input signal
+// (adding any constant value to all of m[0..3] does not change the result)
+
 static inline int correlate_phase0(uint16_t *m) {
-    return 5 * m[0] - 3 * m[1] - 2 * m[2];
+    return (5 * m[0] - 3 * m[1] - 2 * m[2]) * 10 / 19;
 }
 static inline int correlate_phase1(uint16_t *m) {
-    return 4 * m[0] - 1 * m[1] - 3 * m[2];
+    return (4 * m[0] - 1 * m[1] - 3 * m[2]) * 10 / 13;
 }
 static inline int correlate_phase2(uint16_t *m) {
-    return 3 * m[0] + 1 * m[1] - 4 * m[2];
+    return (3 * m[0] + 1 * m[1] - 4 * m[2]) * 10 / 13;
 }
 static inline int correlate_phase3(uint16_t *m) {
-    return 2 * m[0] + 3 * m[1] - 5 * m[2];
+    return (2 * m[0] + 3 * m[1] - 5 * m[2]) * 10 / 14;
 }
 static inline int correlate_phase4(uint16_t *m) {
-    return 1 * m[0] + 5 * m[1] - 5 * m[2] + 1 * m[3];
+    return (1 * m[0] + 5 * m[1] - 5 * m[2] - 1 * m[3]) * 10 / 28;
 }
 
 //
@@ -2014,25 +2019,28 @@ static inline int correlate_check_4(uint16_t *m) {
 }
 
 // Work out the best phase offset to use for the given message.
-// Note that the message may start at anywhere between
-// m[19] offset 1 and m[20] offset 0
 static int best_phase(uint16_t *m) {
     int test;
     int best = -1;
     int bestval = 50; // minimum correlation quality we will accept
 
-    // look at the first 5 bits with each possible phase
+    // empirical testing suggests that 3..8 is the best range to test for here
-    test = correlate_check_1(&m[19]);
+    // (testing a wider range runs the danger of picking the wrong phase for
-    if (test > bestval) { bestval = test; best = 1; }
+    // a message that would otherwise be successfully decoded - the correlation
-    test = correlate_check_2(&m[19]);
+    // functions can match well with a one symbol / half bit offset)
-    if (test > bestval) { bestval = test; best = 2; }
-    test = correlate_check_3(&m[19]);
-    if (test > bestval) { bestval = test; best = 3; }
-    test = correlate_check_4(&m[19]);
-    if (test > bestval) { bestval = test; best = 4; }
-    test = correlate_check_0(&m[20]);
-    if (test > bestval) { best = 5; }
 
+    test = correlate_check_3(&m[0]);
+    if (test > bestval) { bestval = test; best = 3; }
+    test = correlate_check_4(&m[0]);
+    if (test > bestval) { bestval = test; best = 4; }
+    test = correlate_check_0(&m[1]);
+    if (test > bestval) { bestval = test; best = 5; }
+    test = correlate_check_1(&m[1]);
+    if (test > bestval) { bestval = test; best = 6; }
+    test = correlate_check_2(&m[1]);
+    if (test > bestval) { bestval = test; best = 7; }
+    test = correlate_check_3(&m[1]);
+    if (test > bestval) { bestval = test; best = 8; }
     return best;
 }
 
@@ -2052,69 +2060,98 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
 
     for (j = 0; j < mlen; j++) {
         uint16_t *preamble = &m[j];
-        int high, i, phase, errors, errors56, errorsTy; 
+        int high, i, initial_phase, phase, errors, errors56, errorsTy; 
         int msglen, scanlen;
         uint16_t *pPtr;
         uint8_t theByte, theErrs;
 
-        // Rather than clear the whole mm structure, just clear the parts which are required. The clear
+        // Look for a message starting at around sample 0 with phase offset 3..7
-        // is required for every bit of the input stream, and we don't want to be memset-ing the whole
-        // modesMessage structure two million times per second if we don't have to..
-        mm.bFlags          =
-            mm.crcok           = 
-            mm.correctedbits   = 0;
 
-        // Look for a message starting at sample 1 with phase offset 0-4
+        // Ideal sample values for preambles with different phase
+        // Xn is the first data symbol with phase offset N
+        //
+        // sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
+        // phase 3: 2/4\0/5\1 0 0 0 0/5\1/3 3\0 0 0 0 0 0 X4
+        // phase 4: 1/5\0/4\2 0 0 0 0/4\2 2/4\0 0 0 0 0 0 0 X0
+        // phase 5: 0/5\1/3 3\0 0 0 0/3 3\1/5\0 0 0 0 0 0 0 X1
+        // phase 6: 0/4\2 2/4\0 0 0 0 2/4\0/5\1 0 0 0 0 0 0 X2
+        // phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3
+        //
         
-        // Check for peaks at (1,12) or (3,9)
+        // quick check: we must have a rising edge 0->1 and a falling edge 12->13
-        if (preamble[1] > preamble[0] &&
+        if (! (preamble[0] < preamble[1] && preamble[12] > preamble[13]) )
-            preamble[1] > preamble[2] &&
+           continue;
-            preamble[12] > preamble[11] &&
+
-            preamble[12] > preamble[13]) {
+        if (preamble[1] > preamble[2] &&                                       // 1
-            high = (preamble[1] + preamble[12]) / 2;
+            preamble[2] < preamble[3] && preamble[3] > preamble[4] &&          // 3
-        } else if (preamble[3] > preamble[2] &&
+            preamble[8] < preamble[9] && preamble[9] > preamble[10] &&         // 9
-                   preamble[3] > preamble[4] &&
+            preamble[10] < preamble[11]) {                                     // 11-12
-                   preamble[9] > preamble[8] &&
+            // peaks at 1,3,9,11-12: phase 3
-                   preamble[9] > preamble[10]) {
+            high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4;
-            high = (preamble[3] + preamble[9]) / 2;
+        } else if (preamble[1] > preamble[2] &&                                // 1
+                   preamble[2] < preamble[3] && preamble[3] > preamble[4] &&   // 3
+                   preamble[8] < preamble[9] && preamble[9] > preamble[10] &&  // 9
+                   preamble[11] < preamble[12]) {                              // 12
+            // peaks at 1,3,9,12: phase 4
+            high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4;
+        } else if (preamble[1] > preamble[2] &&                                // 1
+                   preamble[2] < preamble[3] && preamble[4] > preamble[5] &&   // 3-4
+                   preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10
+                   preamble[11] < preamble[12]) {                              // 12
+            // peaks at 1,3-4,9-10,12: phase 5
+            high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4;
+        } else if (preamble[1] > preamble[2] &&                                 // 1
+                   preamble[3] < preamble[4] && preamble[4] > preamble[5] &&    // 4
+                   preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
+                   preamble[11] < preamble[12]) {                               // 12
+            // peaks at 1,4,10,12: phase 6
+            high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4;
+        } else if (preamble[2] > preamble[3] &&                                 // 1-2
+                   preamble[3] < preamble[4] && preamble[4] > preamble[5] &&    // 4
+                   preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
+                   preamble[11] < preamble[12]) {                               // 12
+            // peaks at 1-2,4,10,12: phase 7
+            high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4;
         } else {
-            // No peaks
+            // no suitable peaks
             continue;
         }
 
-        // The samples between the two spikes must be < than the average
+        // Check that the "quiet" bits 6,7,15,16,17 are actually quiet
-        // of the high spikes level. We don't test bits too near to
-        // the high levels as signals can be out of phase so part of the
-        // energy can be in the near samples
 
-        if (preamble[5] >= high ||
+        if (preamble[6] >= high ||
-            preamble[6] >= high ||
             preamble[7] >= high ||
             preamble[14] >= high ||
             preamble[15] >= high ||
             preamble[16] >= high ||
-            preamble[17] >= high ||
+            preamble[17] >= high) {
-            preamble[18] >= high)
+            ++Modes.stat_preamble_not_quiet;
             continue;
+        }
         
         // Crosscorrelate against the first few bits to find a likely phase offset
-        phase = best_phase(preamble);
+        initial_phase = best_phase(&preamble[19]);
-        if (phase < 0) {
+        if (initial_phase < 0) {
+            ++Modes.stat_preamble_no_correlation;
             continue; // nothing satisfactory
         }
 
         Modes.stat_valid_preamble++;
+        Modes.stat_preamble_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
+
+        // Rather than clear the whole mm structure, just clear the parts which are required. The clear
+        // is required for every possible preamble, and we don't want to be memset-ing the whole
+        // modesMessage structure if we don't have to..
+        mm.bFlags          =
+            mm.crcok           = 
+            mm.correctedbits   = 0;
 
         // Decode all the next 112 bits, regardless of the actual message
         // size. We'll check the actual message type later
 
         pMsg = &msg[0];
-        pPtr = &m[j+19];
+        pPtr = &m[j+19] + (initial_phase/5);
-        if (phase == 5) {
+        phase = initial_phase % 5;
-            ++pPtr;
-            phase = 0;
-        }
-
         theByte = 0;
         theErrs = 0; errorsTy = 0;
         errors  = 0; errors56 = 0;
@@ -2158,12 +2195,10 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
                 break;
             }
 
-            if (test > 10)
-                theByte |= 1;
-            else if (test >= -10) {
             if (test > 0)
-                    theByte |= 1; // best guess
+                theByte |= 1;
-
+            /* else if (test < 0) theByte |= 0; */
+            else if (test == 0) {
                 if (i >= MODES_SHORT_MSG_BITS) { // poor correlation, and we're in the long part of a frame
                     errors++;
                 } else if (i >= 5) {             // poor correlation, and we're in the short part of a frame                    
@@ -2261,7 +2296,6 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
             
             // Update statistics
             if (Modes.stats) {
-                if (mm.crcok || mm.correctedbits) {                
                 switch (errors) {
                 case 0: {Modes.stat_demodulated0++; break;}
                 case 1: {Modes.stat_demodulated1++; break;}
@@ -2270,8 +2304,10 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
                 }
                 
                 if (mm.correctedbits == 0) {
-                        if (mm.crcok) {Modes.stat_goodcrc++;}
+                    if (mm.crcok) {
-                        else          {Modes.stat_badcrc++;}
+                        Modes.stat_goodcrc++;
+                        Modes.stat_goodcrc_phase[initial_phase%MODES_MAX_PHASE_STATS]++;
+                    } else {Modes.stat_badcrc++;}
                 } else {
                     Modes.stat_badcrc++;
                     Modes.stat_fixed++;
@@ -2281,7 +2317,6 @@ void detectModeS_oversample(uint16_t *m, uint32_t mlen) {
                     }
                 }
             }
-            }
             
             // Skip this message if we are sure it's fine
             if (mm.crcok) {