[vlc-devel] [PATCH] Add support for MPEG-4 ALS to the MP4 demuxer and FFmpeg wrapper.

[Alex Converse]

On Wed, Mar 24, 2010 at 4:48 AM, Laurent Aimar <fenrir at via.ecp.fr> wrote:
>
> Hi,
>
> On Wed, Mar 24, 2010, Alex Converse wrote:
> >
> > MPEG-4 ALS is a pure losses audio codec from MPEG. Like other MPEG-4
> > Audio variants it's muxed into the MP4 container with Object Type
> > Indication 0x40 followed by its Audio Object Type (36).
> >
> > Sample:
> > http://streams.videolan.org/Mpeg_Conformance/ftp.iis.fhg.de/mpeg4audio-conformance/compressedMp4/als_00_2ch48k16b.mp4
> > ---
> >  include/vlc_fourcc.h           |    1 +
> >  modules/codec/avcodec/fourcc.c |    1 +
> >  modules/demux/mp4/mp4.c        |    6 ++++++
> >  src/misc/fourcc.c              |    4 ++++
> >  4 files changed, 12 insertions(+), 0 deletions(-)
> >
>
> > diff --git a/include/vlc_fourcc.h b/include/vlc_fourcc.h
> > index f766933..8d86254 100644
> > --- a/include/vlc_fourcc.h
> > +++ b/include/vlc_fourcc.h
> > @@ -194,6 +194,7 @@
> >  /* Audio codec */
> >  #define VLC_CODEC_MPGA      VLC_FOURCC('m','p','g','a')
> >  #define VLC_CODEC_MP4A      VLC_FOURCC('m','p','4','a')
> > +#define VLC_CODEC_ALS       VLC_FOURCC('a','l','s',0x00)
>  If possible (ie if the fourcc is arbitray), I would prefer pure
> ASCII value, for example using ' ' (a space) instead of 0x00 as it
> is nicer when printing.
>

I meant to match the signature on from the ALS file header but that
would be "ALS\0" (capitalized)
http://wiki.multimedia.cx/index.php?title=ALS#File_Header

If you prefer, I can change it to "als " or "als0" or similar.

>
> > diff --git a/modules/demux/mp4/mp4.c b/modules/demux/mp4/mp4.c
> > index 68b105d..3b85938 100644
> > --- a/modules/demux/mp4/mp4.c
> > +++ b/modules/demux/mp4/mp4.c
> > @@ -1727,6 +1727,12 @@ static int TrackCreateES( demux_t *p_demux, mp4_track_t *p_track,
> >                  break;
> >              case( 0x40):
> >                  p_track->fmt.i_codec = VLC_FOURCC( 'm','p','4','a' );
> > +                if( p_decconfig->i_decoder_specific_info_len >= 2 &&
> > +                     p_decconfig->p_decoder_specific_info[0]       == 0xF8 &&
> > +                    (p_decconfig->p_decoder_specific_info[1]&0xE0) == 0x80 )
>  Is checking the extra data the only way ? (if yes, then it is ok).
> > +                {
> > +                    p_track->fmt.i_codec = VLC_FOURCC( 'a','l','s',0x00 );
>  As said by jb, here using the VLC_CODEC_ALS is better (the rest of the files doesn't
> due to historical reasons...)

ok

--Alex