lavc/aac_ac3_parser: improve the raw AAC file bit rate calculation

Now we just use one ADTS raw frame to calculate the bit rate, it's
lead to a larger error when get the duration from bit rate, the
improvement cumulate Nth ADTS frames to get the average bit rate.

e,g used the command get the duration like:
ffprobe -show_entries format=duration -i fate-suite/aac/foo.aac

before this improvement dump the duration=2.173935
after this improvement  dump the duration=1.979267

in fact, the real duration can be get by command like:
ffmpeg -i fate-suite/aac/foo.aac -f null /dev/null with time=00:00:01.97

Also update the fate-adtstoasc_ticket3715.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/libavcodec/aac_ac3_parser.c b/libavcodec/aac_ac3_parser.c
index 54e459844f..0746798dab 100644
--- a/libavcodec/aac_ac3_parser.c
+++ b/libavcodec/aac_ac3_parser.c
@@ -97,8 +97,13 @@ get_next:
             avctx->audio_service_type = s->service_type;
         }
 
-        if (avctx->codec_id != AV_CODEC_ID_EAC3)
-            avctx->bit_rate = s->bit_rate;
+        /* Calculate the average bit rate */
+        s->frame_number++;
+        if (avctx->codec_id != AV_CODEC_ID_EAC3) {
+            avctx->bit_rate =
+                (s->last_bit_rate * (s->frame_number -1) + s->bit_rate)/s->frame_number;
+            s->last_bit_rate = avctx->bit_rate;
+        }
     }
 
     return i;