soft decoding: info update

demod/mod/README.md

@@ -40,3 +40,23 @@ alternative decoders using cross-correlation for better header-synchronization
   &nbsp;&nbsp;&nbsp;&nbsp; `<sr>`: sample rate <br />
   &nbsp;&nbsp;&nbsp;&nbsp; `<bs>=8,16,32`: bits per (real) sample (u8, s16 or f32)
 
+#### Remarks
+  FM-demodulation is sensitive to noise at higher frequencies. A narrow low-pass filter is needed before demodulation.
+  For weak signals and higher modulation indices IQ-decoding is usually better.
+  <br />
+
+  DFM:<br />
+  The high modulation index has advantages in IQ-decoding. <br />
+  `--ecc2` uses soft decision for 2-error words. If weak signals frequently produce errors, it is likely that
+  more than 2 errors occur in a received word. Since there is no additional frame protection (e.g. CRC), the
+  frames will not be decoded reliably in weak conditions. The `--dist` option has a thredshold for the number
+  of errors per packet.
+  <br />
+
+  LMS6-403:<br />
+  `lms6Xmod_soft.c` (testing) provides a soft viterbi decoding option `--vit2`;
+  IQ-decoding is recommended for soft decoding (noisy/spikey FM-signals don't always help soft decision).
+  The difference between hard and soft viterbi becomes only apparent at lower SNR. The inner convolutional
+  code does most of the error correction. The concatenated outer Reed-Solomon code kicks in only at low SNR.
+
+

demod/mod/dfm09mod.c

@@ -1242,11 +1242,15 @@ int main(int argc, char **argv) {
                         if (option_iq >= 2) {
                             float bl = -1;
                             if (option_iq > 2) bl = 4.0;
-                            bitQ = read_softbit(&dsp, &hsbit, 0/*gpx.option.inv*/, bitofs, bitpos, bl, 0);
+                            bitQ = read_softbit(&dsp, &hsbit, 0, bitofs, bitpos, bl, 0);
                         }
                         else {
-                            bitQ = read_softbit(&dsp, &hsbit, 0/*gpx.option.inv*/, bitofs, bitpos, -1, spike);
+                            bitQ = read_softbit(&dsp, &hsbit, 0, bitofs, bitpos, -1, spike);
                         }
+                        // optional:
+                        // normalize soft bit s_j by
+                        //   rhsbit.sb /= dsp._spb+1; // all samples in [-1,+1]
+
                     }
                     if ( bitQ == EOF ) { frm = nfrms; break; } // liest 2x EOF
 

demod/mod/lms6Xmod_soft.c

@@ -1246,6 +1246,12 @@ int main(int argc, char **argv) {
                 bitQ = read_softbit(&dsp, &rhsbit, 0, bitofs, bitpos, -1, 0); // symlen=1
                 if (bitQ == EOF) { break; }
 
+                // optional:
+                // normalize soft bit s_j by
+                //   rhsbit.sb /= dsp._spb+1; // all samples in [-1,+1]
+                // or at the end by max|s_j| over all bits in rawframe
+                // (only if |sj| >> 1 by factor 100)
+
                 hsbit.hb = rhsbit.hb ^ (bc%2);  // (c0,inv(c1))
                 int sgn = -2*(((unsigned int)bc)%2)+1;
                 hsbit.sb = sgn * rhsbit.sb;