[Ffmpeg-devel] possible bug - ac3 transcoding

[Michael Niedermayer]

Hi

On Tue, Dec 19, 2006 at 09:10:16AM +0100, V?ctor Paesa wrote:
> Hi,
> > On Sat, Dec 16, 2006 at 08:39:45PM +0000, Rui Correia wrote:
> >> Hello there,
> >> I have XP Pro SP2 and I'm using a ffmpeg for windows binary that I
> >> downloaded
> >> here http://tirnanog.fate.jp/mirror/ffmpeg/.
> >> It is not the latest SVN but since I don't know how to build the latest
> >> SVN on
> >> cygwin/mingw I have to post my results with SVN-7260.
> >> This is the CLI that I'm using.
> >> It was taken from BeHappy a windows frontend for many encoders, namely
> >> FFmpeg.
> >>
> >> ffmpeg.exe -i gotk448.ac3 -y -acodec ac3 -ab 384 gotk384.ac3
> >>
> >> And here is the log
> >>
> >> FFmpeg version SVN-r7260, Copyright (c) 2000-2006 Fabrice Bellard, et
> >> al.
> >> configuration:  --enable-mp3lame --enable-libnut --enable-libogg
> >> --enable-vorbis --enable-faad --enable-faac --enable-libgsm
> >> --enable-xvid
> >> --enable-x264 --enable-mingw32 --enable-a52 --enable-dts --enable-pp
> >> --enable-amr_nb --enable-amr_wb --enable-swscaler --enable-avisynth
> >> --enable-gpl --enable-memalign-hack --cpu=i686 --extra-ldflags=-static
> >>   libavutil version: 49.1.0
> >>   libavcodec version: 51.26.0
> >>   libavformat version: 51.6.0
> >>   built on Dec  8 2006 17:40:35, gcc: 4.0.3
> >> Input #0, ac3, from 'gotk448.ac3':
> >>   Duration: 01:34:06.9, start: 0.000000, bitrate: 448 kb/s
> >>   Stream #0.0: Audio: ac3, 48000 Hz, 5 channels, 448 kb/s
> >> Output #0, ac3, to 'gotk384.ac3':
> >>   Stream #0.0: Audio: ac3, 48000 Hz, 5 channels, 384 kb/s
> >> Stream mapping:
> >>   Stream #0.0 -> #0.0
> >> Press [q] to stop encoding
> >> size=  317640kB time=6776.3 bitrate= 384.0kbits/s
> >> video:0kB audio:317640kB global headers:0kB muxing overhead 0.000000%
> >>
> >> Note: I'm only using ffmpeg for audio transcoding.
> >> I use a different encoder for the video part.
> >> In the end I'll just mux them back with DVDLab Pro or Muxman Std..
> >> In the above test the source movie is a original DVD PAL 25fps and it
> >> has a
> >> total of 141175 frames.
> >> I extracted the ac3 stream using DGMPGDec/DGIndex 1.4.8. which reports
> >> the ac3
> >> stream as a 5.1 448kbps stream and it uses 308819KB of disc space.
> >> After transcoding to 384kbit ac3 using the above CLI I get a file with
> >> 317640KB
> >> thus making it _even_bigger_than_the_original_.
> >> For me this doesn't make sense because if my math is right I should be
> >> getting
> >> a file from ffmpeg with ~264703KB.
> >> My math for the transcoded ac3:
> >> ((141175/25)*(384/8))/1024*1000=264703,125
> >> And for the source ac3:
> >> ((141175/25)*(448/8))/1024*1000=308820,3125
> >> Plus if I try to play the ffmpeg-transcoded file with either
> >> foobar2000 0.9.4.1 or MediaPlayerClassic 6.4.9.0 my PC crashes and
> >> reboots.
> >
> > well, get a real OS, sorry but a system crash is _always_ your OSs fault
> > unless your hardware is broken but then again that too is off topic here
> >
> > does ffmpeg/ffplay play the resulting file? do the regression tests pass?
> 
> The problem seems easy to reproduce, I downloaded
> >from http://samples.mplayerhq.hu/A-codecs/AC3-samples/
> the file Broadway-5.1-48khz-448kbit.ac3
> 
> The original is 33.9 seconds long, but trancoding produces a 40.7 seconds
> duration:
> 
> $ ffmpeg.exe -i Broadway-5.1-48khz-448kbit.ac3 -ab 384 broad384.ac3
> FFmpeg version SVN-r7317, Copyright (c) 2000-2006 Fabrice Bellard, et al.
>   configuration:  --enable-mp3lame --enable-a52 --disable-static
> --enable-shared --enable-pthreads --enable-swscaler --enable-avisynth
> --enable-gpl --extra-cflags=-DX_DISPLAY_MISSING -march=pentium4
>   libavutil version: 49.1.0
>   libavcodec version: 51.26.0
>   libavformat version: 51.6.0
>   built on Dec 14 2006 12:58:18, gcc: 3.4.4 (cygming special) (gdc 0.12,
> using dmd 0.125)
> Input #0, ac3, from 'Broadway-5.1-48khz-448kbit.ac3':
>   Duration: 00:00:33.9, start: 0.000000, bitrate: 448 kb/s
>   Stream #0.0: Audio: ac3, 48000 Hz, 5 channels, 448 kb/s
> Output #0, ac3, to 'broad384.ac3':
>   Stream #0.0: Audio: ac3, 48000 Hz, 5 channels, 384 kb/s
> Stream mapping:
>   Stream #0.0 -> #0.0
> Press [q] to stop encoding
> size=    1910kB time=40.7 bitrate= 384.0kbits/s
> video:0kB audio:1910kB global headers:0kB muxing overhead 0.000000%
> 
> $ ffmpeg -i broad384.ac3
> FFmpeg version SVN-r7317, Copyright (c) 2000-2006 Fabrice Bellard, et al.
>   configuration:  --enable-mp3lame --enable-a52 --disable-static
> --enable-shared --enable-pthreads --enable-swscaler --enable-avisynth
> --enable-gpl --extra-cflags=-DX_DISPLAY_MISSING -march=pentium4
>   libavutil version: 49.1.0
>   libavcodec version: 51.26.0
>   libavformat version: 51.6.0
>   built on Dec 14 2006 12:58:18, gcc: 3.4.4 (cygming special) (gdc 0.12,
> using dmd 0.125)
> Input #0, ac3, from 'broad384.ac3':
>   Duration: 00:00:40.7, start: 0.000000, bitrate: 384 kb/s
>   Stream #0.0: Audio: ac3, 48000 Hz, 5 channels, 384 kb/s
> Must supply at least one output file
> 
> 
> And the file encoded at 384k is larger than the one at 448k:
> $ ls -l
> total 3772
> -rwx------+ 1 me Users 1903104 Dec 19 08:47 Broadway-5.1-48khz-448kbit.ac3
> -rw-r--r--  1 me Users 1955328 Dec 19 08:55 broad384.ac3
> 
> MPlayer 1.0pre8-3.4.4 reproduced broad384.ac3 without crashes, but sounds
> garbled (the original sounded OK).
> 
> The AC3 regression tests for my SVN-r7317 passed correctly.

hmm, and did any past versions of ffmpeg convert (try 1 or 2 old checkouts)
Broadway-5.1-48khz-448kbit.ac3 correctly, if so binary search ...

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When the tyrant has disposed of foreign enemies by conquest or treaty, and
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