[FFmpeg-devel] [PATCH] ac3dec: fix center_mix_level handling.

[Michael Niedermayer]

Previously the 3bit center mix level for eac3 was
used to index in a 4 entry table leading to out of array reads.
this change removes the table and offsets the ac3 variable by 4
so it matches the meanings for eac3 except the reserved case.
The reserved case is then explicitly handled.

Signed-off-by: Michael Niedermayer <michaelni at gmx.at>
---
 libavcodec/ac3_parser.c |   12 +++++++++---
 libavcodec/ac3dec.c     |   10 ++--------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/libavcodec/ac3_parser.c b/libavcodec/ac3_parser.c
index 17f205b..d2ec6a0 100644
--- a/libavcodec/ac3_parser.c
+++ b/libavcodec/ac3_parser.c
@@ -53,7 +53,7 @@ int avpriv_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
     hdr->num_blocks = 6;
 
     /* set default mix levels */
-    hdr->center_mix_level   = 1;  // -4.5dB
+    hdr->center_mix_level   = 5;  // -4.5dB
     hdr->surround_mix_level = 1;  // -6.0dB
 
     if(hdr->bitstream_id <= 10) {
@@ -75,8 +75,14 @@ int avpriv_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
         if(hdr->channel_mode == AC3_CHMODE_STEREO) {
             skip_bits(gbc, 2); // skip dsurmod
         } else {
-            if((hdr->channel_mode & 1) && hdr->channel_mode != AC3_CHMODE_MONO)
-                hdr->center_mix_level = get_bits(gbc, 2);
+            if ((hdr->channel_mode & 1) && hdr->channel_mode != AC3_CHMODE_MONO) {
+                hdr->center_mix_level = get_bits(gbc, 2) + 4;
+                if (hdr->center_mix_level == 3 + 4) {
+                    hdr->center_mix_level = 5;
+                    av_log(NULL, AV_LOG_DEBUG,
+                           "Reserved center_mix_level, using -4.5dB as mandated by spec\n");
+                }
+            }
             if(hdr->channel_mode & 4)
                 hdr->surround_mix_level = get_bits(gbc, 2);
         }
diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 325d23b..ac1ae90 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -77,12 +77,6 @@ static const float gain_levels[9] = {
 };
 
 /**
- * Table for center mix levels
- * reference: Section 5.4.2.4 cmixlev
- */
-static const uint8_t center_levels[4] = { 4, 5, 6, 5 };
-
-/**
  * Table for surround mix levels
  * reference: Section 5.4.2.5 surmixlev
  */
@@ -320,7 +314,7 @@ static int parse_frame_header(AC3DecodeContext *s)
 static void set_downmix_coeffs(AC3DecodeContext *s)
 {
     int i;
-    float cmix = gain_levels[center_levels[s->center_mix_level]];
+    float cmix = gain_levels[                s->  center_mix_level ];
     float smix = gain_levels[surround_levels[s->surround_mix_level]];
     float norm0, norm1;
 
@@ -1400,7 +1394,7 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
         avctx->channels       = s->out_channels;
         avctx->channel_layout = s->channel_layout;
 
-        s->loro_center_mix_level   = gain_levels[  center_levels[s->  center_mix_level]];
+        s->loro_center_mix_level   = gain_levels[                s->  center_mix_level ];
         s->loro_surround_mix_level = gain_levels[surround_levels[s->surround_mix_level]];
         s->ltrt_center_mix_level   = LEVEL_MINUS_3DB;
         s->ltrt_surround_mix_level = LEVEL_MINUS_3DB;
-- 
1.7.5.4