Fix bug where field was not recoverable due to noise

If CRT_DO_VSYNC was disabled and the noise was high, the field was being incorrectly determined since it was still scanning the noisy signal to find out which field it is. This patch makes it so if CRT_DO_VSYNC is disabled, the field is determined before noise is added to the signal.

crt_core.c

@@ -320,6 +320,26 @@ crt_demodulate(struct CRT *v, int noise)
     huecs >>= 11;
 
     rn = v->rn;
+#if !CRT_DO_VSYNC
+    /* determine field before we add noise,
+     * otherwise it's not reliably recoverable
+     */
+    for (i = -CRT_VSYNC_WINDOW; i < CRT_VSYNC_WINDOW; i++) {
+        line = POSMOD(v->vsync + i, CRT_VRES);
+        sig = v->analog + line * CRT_HRES;
+        s = 0;
+        for (j = 0; j < CRT_HRES; j++) {
+            s += sig[j];
+            if (s <= (CRT_VSYNC_THRESH * SYNC_LEVEL)) {
+                goto found_field;
+            }
+        }
+    }
+found_field:
+    /* if vsync signal was in second half of line, odd field */
+    field = (j > (CRT_HRES / 2));
+    v->vsync = -3;
+#endif
 #if ((CRT_SYSTEM == CRT_SYSTEM_NTSCVHS) && CRT_VHS_NOISE)
     line = ((rand() % 8) - 4) + 14;
 #endif
@@ -346,6 +366,7 @@ crt_demodulate(struct CRT *v, int noise)
     }
     v->rn = rn;
 
+#if CRT_DO_VSYNC
     /* Look for vertical sync.
      * 
      * This is done by integrating the signal and
@@ -370,13 +391,11 @@ crt_demodulate(struct CRT *v, int noise)
         }
     }
 vsync_found:
-#if CRT_DO_VSYNC
     v->vsync = line; /* vsync found (or gave up) at this line */
-#else
-    v->vsync = -3;
-#endif
     /* if vsync signal was in second half of line, odd field */
     field = (j > (CRT_HRES / 2));
+#endif
+
 #if CRT_DO_BLOOM
     max_e = (128 + (noise / 2)) * AV_LEN;
     prev_e = (16384 / 8);