From 184953ff921fccc0b84c0d57443880b25419e953 Mon Sep 17 00:00:00 2001
From: antirez <antirez@gmail.com>
Date: Sat, 26 Jan 2013 12:50:23 +0100
Subject: [PATCH] Phase correction fixed, code reshaped to apply more
 corrections.

The phase correction was applied only to a subset of bits! Because of an
offset error.

The detection code layout was modified a bit to make it simpler to
implement more corrections in the future. However only phase correction
is performed currently. Slope correction, or to compensate for the cycloid
effect are two possible improvements.
---
 dump1090.c | 34 +++++++++++++++++++---------------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/dump1090.c b/dump1090.c
index 4f446f8..9990aef 100644
--- a/dump1090.c
+++ b/dump1090.c
@@ -1246,11 +1246,15 @@ void computeMagnitudeVector(void) {
 
 /* Return -1 if the message is out of fase left-side
  * Return  1 if the message is out of fase right-size
- * Return  0 if the message is not particularly out of phase. */
+ * Return  0 if the message is not particularly out of phase.
+ *
+ * Note: this function will access m[-1], so the caller should make sure to
+ * call it only if we are not at the start of the current buffer. */
 int detectOutOfPhase(uint16_t *m) {
-    m += 16; /* Skip preamble. */
-    if (m[3] > m[2]/2) return 1;
-    if (m[6] > m[7]/2) return -1;
+    if (m[3] > m[2]/3) return 1;
+    if (m[10] > m[9]/3) return 1;
+    if (m[6] > m[7]/3) return -1;
+    if (m[-1] > m[1]/3) return -1;
     return 0;
 }
 
@@ -1285,6 +1289,7 @@ int detectOutOfPhase(uint16_t *m) {
 void applyPhaseCorrection(uint16_t *m) {
     int j;
 
+    m += 16; /* Skip preamble. */
     for (j = 0; j < (MODES_LONG_MSG_BITS-1)*2; j += 2) {
         if (m[j] > m[j+1]) {
             /* One */
@@ -1331,7 +1336,7 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
      */
     for (j = 0; j < mlen - MODES_FULL_LEN*2; j++) {
         int low, high, delta, i, errors;
-        int good_message = 0, out_of_phase;
+        int good_message = 0;
 
         if (use_correction) goto good_preamble; /* We already checked it. */
 
@@ -1392,13 +1397,15 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
         Modes.stat_valid_preamble++;
 
 good_preamble:
-        out_of_phase = detectOutOfPhase(m+j);
-
         /* If the previous attempt with this message failed, retry using
          * magnitude correction. */
         if (use_correction) {
             memcpy(aux,m+j+MODES_PREAMBLE_US*2,sizeof(aux));
-            applyPhaseCorrection(m+j);
+            if (j && detectOutOfPhase(m+j)) {
+                applyPhaseCorrection(m+j);
+                Modes.stat_out_of_phase++;
+            }
+            /* TODO ... apply other kind of corrections. */
         }
 
         /* Decode all the next 112 bits, regardless of the actual message
@@ -1473,7 +1480,7 @@ good_preamble:
             decodeModesMessage(&mm,msg);
 
             /* Update statistics. */
-            if (mm.crcok || (!out_of_phase || use_correction)) {
+            if (mm.crcok || use_correction) {
                 if (errors == 0) Modes.stat_demodulated++;
                 if (mm.errorbit == -1) {
                     if (mm.crcok)
@@ -1491,7 +1498,7 @@ good_preamble:
             }
 
             /* Output debug mode info if needed. */
-            if (!out_of_phase || use_correction) {
+            if (use_correction) {
                 if (Modes.debug & MODES_DEBUG_DEMOD)
                     dumpRawMessage("Demodulated with 0 errors", msg, m, j);
                 else if (Modes.debug & MODES_DEBUG_BADCRC &&
@@ -1514,19 +1521,16 @@ good_preamble:
             /* Pass data to the next layer */
             useModesMessage(&mm);
         } else {
-            if (Modes.debug & MODES_DEBUG_DEMODERR &&
-                (!out_of_phase || use_correction))
-            {
+            if (Modes.debug & MODES_DEBUG_DEMODERR && use_correction) {
                 printf("The following message has %d demod errors\n", errors);
                 dumpRawMessage("Demodulated with errors", msg, m, j);
             }
         }
 
         /* Retry with phase correction if possible. */
-        if (!good_message && !use_correction && out_of_phase) {
+        if (!good_message && !use_correction) {
             j--;
             use_correction = 1;
-            Modes.stat_out_of_phase++;
         } else {
             use_correction = 0;
         }