[Ffmpeg-devel] possible bug - ac3 transcoding

Michael Niedermayer michaelni
Mon Dec 18 21:25:57 CET 2006


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 or MediaPlayerClassic 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?

> I've tried several ffmpeg builds from that repository (celtic_druid's) as well
> as from other repositories and I always get this problem.
> FYI I am quite sure that this used to work with an older ffmpeg build from
> 19/04/2006, which already had the "ac3 low volume bug" fixed.
> Unfortunately in one of my latest HDD housekeeping I accidentally erased that
> build and I can't run a test with it just to make sure.

you can check out any past version with svn, svn up --help and the svn manual
will tell you how, you will have to complie it yourself then, also note
binaries build by others then the user himself are NOT supported HERE
send bugtreports to whoever build the binaries


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates

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