[FFmpeg-devel] AC3 incomplete frame/invalid frame size

[Phil Rutschman]

I uploaded to ftp.mplayerhq.com/MPlayer/incoming/ a 19k file called
"invalidframe-then-buzz.ac3" which decodes poorly in ffmpeg SVN-r15735.
The file is stereo at 192 kbps and consists of 25 AC3 syncframes.

$ ffmpeg -i invalidframe-then-buzz.ac3 ffmpeg-out.wav
FFmpeg version SVN-r15735, Copyright (c) 2000-2008 Fabrice Bellard, et
al.
  configuration: --enable-postproc --enable-libfaac --enable-libx264
--enable-pthreads --enable-pthreads --enable-gpl --enable-libfaad
--extra-libs=-L/usr/local/lib --enable-libmp3lame
  libavutil     49.12. 0 / 49.12. 0
  libavcodec    52. 1. 0 / 52. 1. 0
  libavformat   52.23. 1 / 52.23. 1
  libavdevice   52. 1. 0 / 52. 1. 0
  libpostproc   51. 2. 0 / 51. 2. 0
  built on Oct 28 2008 15:04:35, gcc: 4.1.2 20071124 (Red Hat 4.1.2-42)
Input #0, ac3, from 'invalidframe-then-buzz.ac3':
  Duration: 00:00:00.80, bitrate: 192 kb/s
    Stream #0.0: Audio: ac3, 48000 Hz, stereo, s16, 192 kb/s
Output #0, wav, to 'ffmpeg-out.wav':
    Stream #0.0: Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
Press [q] to stop encoding
[ac3 @ 0x9b44420]incomplete frame
[ac3 @ 0x9b44420]invalid frame size
[ac3 @ 0x9b44420]incomplete frame
[ac3 @ 0x9b44420]invalid frame size
size=     156kB time=0.83 bitrate=1536.4kbits/s
video:0kB audio:156kB global headers:0kB muxing overhead 0.027544%

It decodes without errors in a52dec: (which doesn't prove anything by
itself)

$ a52dec invalidframe-then-buzz.ac3 -o wav > a52dec-out.wav
a52dec-0.7.4 - by Michel Lespinasse <walken at zoy.org> and Aaron Holtzman
No accelerated IMDCT transform found

25 frames decoded in 0.00 seconds (0.00 fps)

Comparing the output files:

$ ls -l *.wav
-rw-rw-r-- 1 philr philr 153644 Oct 28 15:06 a52dec-out.wav
-rw-rw-r-- 1 philr philr 159788 Oct 28 15:05 ffmpeg-out.wav

a52dec-out.wav has the expected size of 25*1536*4+44 bytes, but
ffmpeg-out.wav is 1536*4 bytes larger.

Listening to the two files, there is a brief buzzing at the very
beginning of ffmpeg-out.wav which is not present in a52dec-out.wav.
There are no glitches audible in a52dec-out.wav, and looking at the
waveform in audacity leads me to believe that there isn't a blanked
frame either. Although I certainly can't rule out the possibility that
the file is corrupt in some way, a52dec handles it more gracefully if
so.