Got LDPC sum-product decoder working

decode_ft8.cpp

@@ -88,11 +88,10 @@ void find_sync(const uint8_t * power, int num_blocks, int num_bins, int num_cand
             for (int freq_offset = 0; freq_offset < num_bins - 8; ++freq_offset) {
                 int score = 0;
 
-                // Compute score over bins 0-7, 36-43, 72-79
+                // Compute score over Costas symbols (0-7, 36-43, 72-79)
                 for (int m = 0; m <= 72; m += 36) {
                     for (int k = 0; k < 7; ++k) {
                         int offset = ((time_offset + k + m) * 4 + alt) * num_bins + freq_offset;
-                        // score += 8 * (int)power[time_offset + k + m][alt][freq_offset + ICOS7[k]] -
                         score += 8 * (int)power[offset + ICOS7[k]] -
                                     power[offset + 0] - power[offset + 1] - 
                                     power[offset + 2] - power[offset + 3] - 
@@ -101,17 +100,17 @@ void find_sync(const uint8_t * power, int num_blocks, int num_bins, int num_cand
                     }
                 }
 
-                // update the candidate list
+                // If the heap is full AND the current candidate is better than 
+                // the worst of the heap, we remove the worst and make space
                 if (heap_size == num_candidates && score > heap[0].score) {
-                    // extract the least promising candidate
                     heap[0] = heap[heap_size - 1];
                     --heap_size;
 
                     heapify_down(heap, heap_size);
                 }
 
+                // If there's free space in the heap, we add the current candidate
                 if (heap_size < num_candidates) {
-                    // add the current candidate
                     heap[heap_size].score = score;
                     heap[heap_size].time_offset = time_offset;
                     heap[heap_size].freq_offset = freq_offset;
@@ -176,7 +175,7 @@ void extract_power(const float * signal, int num_blocks, int num_bins, uint8_t *
                     float db = (db1 + db2) / 2;
 
                     // Scale decibels to unsigned 8-bit range
-                    int scaled = (int)(0.5f + 2 * (db + 100));
+                    int scaled = (int)(2 * (db + 100));
                     power[offset] = (scaled < 0) ? 0 : ((scaled > 255) ? 255 : scaled);
                     ++offset;
                 }
@@ -188,6 +187,60 @@ void extract_power(const float * signal, int num_blocks, int num_bins, uint8_t *
 }
 
 
+uint8_t max2(uint8_t a, uint8_t b) {
+    return (a >= b) ? a : b;
+}
+
+
+uint8_t max4(uint8_t a, uint8_t b, uint8_t c, uint8_t d) {
+    return max2(max2(a, b), max2(c, d));
+}
+
+
+// Compute log likelihood log(p(0) / p(1)) of 174 message bits 
+// for later use in soft-decision LDPC decoding
+void extract_likelihood(const uint8_t * power, int num_bins, const Candidate & c, float * log174) {
+    int offset = (c.time_offset * 4 + c.time_sub * 2 + c.freq_sub) * num_bins + c.freq_offset;
+
+    int k = 0;
+    // Go over FSK tones and skip Costas sync symbols
+    for (int i = 7; i < NN - 7; ++i) {
+        if (i == 36) i += 7;
+
+        // Pointer to 8 bins of the current symbol
+        const uint8_t * ps = power + (offset + i * 4 * num_bins);
+
+        // Extract bit significance (and convert them to float)
+        // 8 FSK tones = 3 bits
+        log174[k + 0] = -(int)max4(ps[4], ps[5], ps[6], ps[7]) + (int)max4(ps[0], ps[1], ps[2], ps[3]);
+        log174[k + 1] = -(int)max4(ps[2], ps[3], ps[6], ps[7]) + (int)max4(ps[0], ps[1], ps[4], ps[5]);
+        log174[k + 2] = -(int)max4(ps[1], ps[3], ps[5], ps[7]) + (int)max4(ps[0], ps[2], ps[4], ps[6]);
+        // printf("%d %d %d %d %d %d %d %d : %.0f %.0f %.0f\n", 
+        //         ps[0], ps[1], ps[2], ps[3], ps[4], ps[5], ps[6], ps[7], 
+        //         log174[k + 0], log174[k + 1], log174[k + 2]);
+        k += 3;
+    }
+
+    // Compute the variance of log174
+    float sum   = 0;
+    float sum2  = 0;
+    float inv_n = 1.0f / (3 * ND);
+    for (int i = 0; i < 3 * ND; ++i) {
+        sum  += log174[i];
+        sum2 += log174[i] * log174[i];
+    }
+    float var = (sum2 - sum * sum * inv_n) * inv_n;
+
+    // Normalize log174 such that sigma = 2.83 (Why? It's in WSJT-X)
+    float norm_factor = 2.83f / sqrtf(var);
+
+    for (int i = 0; i < 3 * ND; ++i) {
+        log174[i] *= norm_factor;
+        //printf("%.1f ", log174[i]);
+    }
+    //printf("\n");
+}
+
 int main(int argc, char ** argv) {
     // Expect one command-line argument
     if (argc < 2) {
@@ -223,37 +276,25 @@ int main(int argc, char ** argv) {
     find_sync(power, num_blocks, num_bins, num_candidates, heap);
 
     for (int i = 0; i < num_candidates; ++i) {
-        float freq_offset = (heap[i].freq_offset + heap[i].freq_sub / 2.0f) * fsk_dev;
+        float freq_hz = (heap[i].freq_offset + heap[i].freq_sub / 2.0f) * fsk_dev;
-        float time_offset = (heap[i].time_offset + heap[i].time_sub / 2.0f) / fsk_dev;
+        float time_sec = (heap[i].time_offset + heap[i].time_sub / 2.0f) / fsk_dev;
-        // int offset = (heap[i].time_offset * 4 + heap[i].time_sub * 2 + heap[i].freq_sub) * num_bins + heap[i].freq_offset;
+        // printf("%03d: score = %d freq = %.1f time = %.2f\n", i, 
-        printf("%03d: score = %.1f freq = %.1f time = %.2f\n", i, heap[i].score / 7.0f / 2, freq_offset, time_offset);
+        //         heap[i].score, freq_hz, time_sec);
+
+        float log174[3 * ND];
+        extract_likelihood(power, num_bins, heap[i], log174);
+
+        const int num_iters = 10;
+        int plain[3 * ND];
+        int ok;
+
+        ldpc_decode(log174, num_iters, plain, &ok);
+        //printf("ldpc_decode() = %d\n", ok);
+        if (ok == 87) {
+            printf("%03d: score = %d freq = %.1f time = %.2f\n", i, 
+                    heap[i].score, freq_hz, time_sec);
+        }
     }
-
-    /*
-
-     // take absolute magnitude
-     s2(0:7,k)=abs(csymb(1:8))/1e3
-     
-     // skip Costas sync symbols
-     s1(0:7,j)=s2(0:7,k)
-     
-     // Normalize by median magnitude
-     s1=s1/xmeds1
-
-     // Extract bit significance
-     ps=s1(0:7,j)
-     bmeta(i4)=max(ps(4),ps(5),ps(6),ps(7))-max(ps(0),ps(1),ps(2),ps(3))
-     bmeta(i2)=max(ps(2),ps(3),ps(6),ps(7))-max(ps(0),ps(1),ps(4),ps(5))
-     bmeta(i1)=max(ps(1),ps(3),ps(5),ps(7))-max(ps(0),ps(2),ps(4),ps(6))
-
-     // Normalize by std. deviation
-     call normalizebmet(bmeta,3*ND)
-     
-     // Magical fudge/scale factor
-     scalefac=2.83
-     llr0=scalefac*bmeta
-
-     */
 
     return 0;
 }

ft8/ldpc.cpp

@@ -8,8 +8,6 @@
 // from Sarah Johnson's Iterative Error Correction book.
 // codeword[i] = log ( P(x=0) / P(x=1) )
 //
-// cc -O3 libldpc.c -shared -fPIC -o libldpc.so
-//
 
 #include <stdio.h>
 #include <math.h>
@@ -21,14 +19,11 @@ int ldpc_check(int codeword[]);
 
 // thank you Douglas Bagnall
 // https://math.stackexchange.com/a/446411
-float fast_tanh(float x)
+float fast_tanh(float x) {
-{
+    if (x < -4.97f) {
-    if (x < -4.97f)
-    {
         return -1.0f;
     }
-    if (x > 4.97f)
+    if (x > 4.97f) {
-    {
         return 1.0f;
     }
     float x2 = x * x;
@@ -42,86 +37,64 @@ float fast_tanh(float x)
 // plain is a return value, 174 ints, to be 0 or 1.
 // iters is how hard to try.
 // ok == 87 means success.
-void ldpc_decode(float codeword[], int iters, int plain[], int *ok)
+void ldpc_decode(float codeword[], int iters, int plain[], int *ok) {
-{
     float m[87][174];       // ~60 kB
     float e[87][174];       // ~60 kB
     int best_score = -1;
     int best_cw[174];
 
-    for (int i = 0; i < 174; i++)
+    for (int i = 0; i < 174; i++) {
-        for (int j = 0; j < 87; j++)
+        for (int j = 0; j < 87; j++) {
             m[j][i] = codeword[i];
-
-    for (int i = 0; i < 174; i++)
-        for (int j = 0; j < 87; j++)
             e[j][i] = 0.0f;
+        }
+    }
 
-    for (int iter = 0; iter < iters; iter++)
+    for (int iter = 0; iter < iters; iter++) {
-    {
+        for (int j = 0; j < 87; j++) {
-        for (int j = 0; j < 87; j++)
+            for (int ii1 = 0; ii1 < 7; ii1++) {
-        {
-            for (int ii1 = 0; ii1 < 7; ii1++)
-            {
                 int i1 = Nm[j][ii1] - 1;
-                if (i1 < 0)
+                if (i1 < 0) {
                     continue;
+                }
                 float a = 1.0f;
-                for (int ii2 = 0; ii2 < 7; ii2++)
+                for (int ii2 = 0; ii2 < 7; ii2++) {
-                {
                     int i2 = Nm[j][ii2] - 1;
-                    if (i2 >= 0 && i2 != i1)
+                    if (i2 >= 0 && i2 != i1) {
-                    {
                         a *= fast_tanh(m[j][i2] / 2.0f);
                     }
                 }
-                e[j][i1] = log((1 + a) / (1 - a));
+                e[j][i1] = logf((1 + a) / (1 - a));
             }
         }
 
         int cw[174];
-        for (int i = 0; i < 174; i++)
+        for (int i = 0; i < 174; i++) {
-        {
             float l = codeword[i];
             for (int j = 0; j < 3; j++)
                 l += e[Mn[i][j] - 1][i];
             cw[i] = (l <= 0.0f);
         }
         int score = ldpc_check(cw);
-        if (score == 87)
-        {
-            // Found a perfect answer
-#if 0
-      int cw1[174];
-      for(int i = 0; i < 174; i++)
-        cw1[i] = cw[colorder[i]];
-      for(int i = 0; i < 87; i++)
-        plain[i] = cw1[174-87+i];
-#else
-            for (int i = 0; i < 174; i++)
-                plain[i] = cw[colorder[i]];
-#endif
-            *ok = 87;
-            return;
-        }
 
-        if (score > best_score)
+        if (score > best_score) {
-        {
+            for (int i = 0; i < 174; i++) {
-            for (int i = 0; i < 174; i++)
                 best_cw[i] = cw[i];
+            }
             best_score = score;
         }
 
-        for (int i = 0; i < 174; i++)
+        if (score == 87) {
-        {
+            // Found a perfect answer
-            for (int ji1 = 0; ji1 < 3; ji1++)
+            break;
-            {
+        }
+
+        for (int i = 0; i < 174; i++) {
+            for (int ji1 = 0; ji1 < 3; ji1++) {
                 int j1 = Mn[i][ji1] - 1;
                 float l = codeword[i];
-                for (int ji2 = 0; ji2 < 3; ji2++)
+                for (int ji2 = 0; ji2 < 3; ji2++) {
-                {
+                    if (ji1 != ji2) {
-                    if (ji1 != ji2)
-                    {
                         int j2 = Mn[i][ji2] - 1;
                         l += e[j2][i];
                     }
@@ -131,16 +104,15 @@ void ldpc_decode(float codeword[], int iters, int plain[], int *ok)
         }
     }
 
-    // decode didn't work, return something anyway.
+    // decode complete (perhaps partially)
 #if 0
-  int cw1[174];
+    for(int i = 0; i < 87; i++) {
-  for(int i = 0; i < 174; i++)
+        plain[i] = best_cw[colorder[174-87+i]];
-    cw1[i] = best_cw[colorder[i]];
+    }
-  for(int i = 0; i < 87; i++)
-    plain[i] = cw1[174-87+i];
 #else
-    for (int i = 0; i < 174; i++)
+    for (int i = 0; i < 174; i++) {
         plain[i] = best_cw[colorder[i]];
+    }
 #endif
 
     *ok = best_score;
@@ -152,25 +124,22 @@ void ldpc_decode(float codeword[], int iters, int plain[], int *ok)
 // returns the number of parity checks that passed.
 // 87 means total success.
 //
-int ldpc_check(int codeword[])
+int ldpc_check(int codeword[]) {
-{
     int score = 0;
 
     // Nm[87][7]
-    for (int j = 0; j < 87; j++)
+    for (int j = 0; j < 87; j++) {
-    {
         int x = 0;
-        for (int ii1 = 0; ii1 < 7; ii1++)
+        for (int ii1 = 0; ii1 < 7; ii1++) {
-        {
             int i1 = Nm[j][ii1] - 1;
-            if (i1 >= 0)
+            if (i1 >= 0) {
-            {
                 x ^= codeword[i1];
             }
         }
-        if (x == 0)
+        if (x == 0) {
             score++;
         }
+    }
     return score;
 }