[Ffmpeg-devel] [RFC] mp4 object types comments

Måns Rullgård mans
Sat Mar 24 18:59:53 CET 2007


Michael Niedermayer <michaelni at gmx.at> writes:

> Hi
>
> On Sat, Mar 24, 2007 at 02:44:41PM +0000, M?ns Rullg?rd wrote:
>> Baptiste Coudurier <baptiste.coudurier at smartjog.com> writes:
>> 
>> > Hi
>> >
>> > M?ns Rullg?rd wrote:
>> >> Baptiste Coudurier <baptiste.coudurier at smartjog.com> writes:
>> >> 
>> >>> Hi
>> >>>
>> >>> Here are some modifications to mp4 object types Im not sure,
>> >>> can someone confirm that 13818-3 can be decoded with MP2 id ?
>> >> 
>> >> 13818-3 extends 11172-3 (MPEG1 audio) with lower sampling rates and
>> >> more channels.  An 11172-3 decoder will correctly decode basic stereo
>> >> from a 13818-3 stream.  There are 3 defined layers corresponding to
>> >> those in 11172-3.

[...]

>> >> This mixes addition of comments with actual changes.  The added
>> >> comments are fine.  The changed codec IDs are not.  All MPEG audio
>> >> should use CODEC_ID_MP3.  Please make it so, but add the other two
>> >> comments separately.
>> >
>> > Yes. About codec_id, are you sure?
>> 
>> See the comment in avcodec.h, line 210.
>> 
>> > Different tags are needed if codec_id is MP2 or MP3, and when stream
>> > copying codec_id is not changed/altered.
>> 
>> Both 11172-3 and 13818-3 define layers 1, 2 and 3.  The tag does not
>> specify which layer is used, only which standard, so CODEC_ID_MP3 is
>> the only safe choice.  As 13818-3 is an extension to 11172-3, it
>> should really be assigned its own codec_id.  Until we have a decoder
>> for these extensions, the best choice is to use CODEC_ID_MP3.
>
> our mp3 decoder should support these extensions, do you suggest that we
> add a dummy codec_id for it? if so iam not opposed ...

I didn't know we supported the multichannel extensions.  Have a
separate codec_id for 13818-3 seems right to me, even if our
implementation uses largely the same code to decode both.  It's not
high on my list of priorities though.

-- 
M?ns Rullg?rd
mans at mansr.com




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