[FFmpeg-devel] MPEG Audio elementary streams and layers

[Michael Niedermayer]

On Mon, Dec 15, 2008 at 12:08:06PM +0100, Marc Mason wrote:
> Hello,
> 
> CODEC_ID_MP2 and CODEC_ID_MP3 are defined in avcodec.h
> 
> As far as I understand,
> CODEC_ID_MP2 = MPEG Audio Layer II
> CODEC_ID_MP3 = MPEG Audio Layer III
> 
> CODEC_ID_MP3 appeared in rev 2231 with the following comment.
> /* preferred ID for MPEG Audio layer 1, 2 or 3 decoding */
> http://svn.ffmpeg.org/ffmpeg/trunk/libavcodec/avcodec.h?r1=2217&r2=2231
> 
> What does the comment mean ?
> 
> 
> ff_mpegaudio_decode_header() determines the layer of the ES.
> http://cekirdek.pardus.org.tr/~ismail/ffmpeg-docs/mpegaudiodecheader_8c-source.html#l00033
> s->layer = 4 - ((header >> 17) & 3);
> 
> ff_mpa_decode_header() then copies the info to avctx->sub_id
> http://cekirdek.pardus.org.tr/~ismail/ffmpeg-docs/mpegaudio__parser_8c-source.html#l00047
> avctx->sub_id = s->layer;
> 
> 
> When av_find_stream_info() is given an MPEG Audio Layer II elementary
> stream, it returns CODEC_ID_MP3 with sub_id=2
> 
> This seems ambiguous to me.
> 
> What was the original intent ? How is one supposed to determine the
> layer of an MPEG Audio elementary stream ?
> 
> I can see two possibilities.
> 
> 1.) Use codec_id
> 
> i.e. libavformat returns codec_id = one of CODEC_ID_MP1, CODEC_ID_MP2,
> CODEC_ID_MP3 according to the layer.
> 
> Then ff_mpa_decode_header() should set the field to CODEC_ID_MP2 when
> appropriate.
> 
> Something along the lines of
> 
> $ svn diff
> Index: mpegaudio_parser.c
> ===================================================================
> --- mpegaudio_parser.c  (revision 16054)
> +++ mpegaudio_parser.c  (working copy)
> @@ -62,6 +62,7 @@
>           break;
>       case 2:
>           avctx->frame_size = 1152;
> +        avctx->codec_id = CODEC_ID_MP2;
>           break;
>       default:
>       case 3:
> 

I suspect this patch is ok given it is tested and the indention isnt off
and codec_id is also set for the MP3  case similarly


> However, what happens with layer I ES ?
> CODEC_ID_MP1 might have to be defined ?

yes



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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

There will always be a question for which you do not know the correct awnser.
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