Make sure that the 2nd CRC value does not match the AC-3 sync word.

This is optional for encoders, but it's a good idea and has minimal impact
on performance.
This will change the output for some files, but it happens not to affect the
regression tests.

Originally committed as revision 26083 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/ac3enc.c

@@ -1572,7 +1572,7 @@ static unsigned int pow_poly(unsigned int a, unsigned int n, unsigned int poly)
 static void output_frame_end(AC3EncodeContext *s)
 {
     const AVCRC *crc_ctx = av_crc_get_table(AV_CRC_16_ANSI);
-    int frame_size_58, pad_bytes, crc1, crc2, crc_inv;
+    int frame_size_58, pad_bytes, crc1, crc2_partial, crc2, crc_inv;
     uint8_t *frame;
 
     frame_size_58 = ((s->frame_size >> 2) + (s->frame_size >> 4)) << 1;
@@ -1594,9 +1594,15 @@ static void output_frame_end(AC3EncodeContext *s)
     AV_WB16(frame + 2, crc1);
 
     /* compute crc2 */
-    crc2 = av_bswap16(av_crc(crc_ctx, 0,
-                             frame + frame_size_58,
-                             s->frame_size - frame_size_58 - 2));
+    crc2_partial = av_crc(crc_ctx, 0, frame + frame_size_58,
+                          s->frame_size - frame_size_58 - 3);
+    crc2 = av_crc(crc_ctx, crc2_partial, frame + s->frame_size - 3, 1);
+    /* ensure crc2 does not match sync word by flipping crcrsv bit if needed */
+    if (crc2 == 0x770B) {
+        frame[s->frame_size - 3] ^= 0x1;
+        crc2 = av_crc(crc_ctx, crc2_partial, frame + s->frame_size - 3, 1);
+    }
+    crc2 = av_bswap16(crc2);
     AV_WB16(frame + s->frame_size - 2, crc2);
 }