[SCM] libav/experimental: 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.

[siretart at users.alioth.debian.org]

The following commit has been merged in the experimental branch:
commit e96dc767e41afcef429149ec87c078b2f8b8a289
Author: Justin Ruggles <justin.ruggles at gmail.com>
Date:   Thu Dec 23 18:01:26 2010 +0000

    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

diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c
index ed86e61..3908447 100644
--- a/libavcodec/ac3enc.c
+++ b/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);
 }
 

-- 
Libav/FFmpeg packaging