[Ffmpeg-devel] 4XM audio codec_tag

[Måns Rullgård]

Michael Niedermayer <michaelni at gmx.at> writes:

> Hi
>
> On Mon, Nov 06, 2006 at 01:33:54PM -0000, M?ns Rullg?rd wrote:
>> 
>> Michael Niedermayer said:
>> > Hi
>> >
>> > On Mon, Nov 06, 2006 at 02:55:14AM +0000, M?ns Rullg?rd wrote:
>> >> Diego Biurrun <diego at biurrun.de> writes:
>> >> > So please remind me: What is the problem with storing arbitrary
>> >> > audio and video formats in multimedia containers, especially generic
>> >> > ones?
>> >>
>> >> The format needs to be able to store global headers (aka extradata),
>> >> and it needs some way of identifying the codec.  The extradata part is
>> >> easy: either there is a place for it or there isn't.  If the format
>> >> can't accommodate an extradata chunk, codecs that need it can't be
>> >> used.  To identify the codec, format specifications typically include
>> >> a list of tag/codec pairs.  Any codec in this list can be stored using
>> >> the format in question.  If the codec is not in the list, it is of
>> >> course possible to invent a tag.  The problem with this approach is
>> >> that nobody else will know what this tag means, and thus nobody else
>> >> will be able to play the file.  Is this really so difficult to
>> >> understand?
>> >
>> > yes, a codec has a name, and often a fourcc which is used in many
>> 
>> Wrong.  A codec has a name.  A fourcc is not an inherent property of a
>> codec.  It is something that might be used to identify it in one particular
>> file format.  If a file format does not specify a fourcc/tag for some
>> codec, that codec can't be portably used in that file format.  Period.
>
> then why do so many codecs work in avi? they arent in the standard table
> from ms, and still many different applications can mux and demux them

Well, it sort of works most of the time, but by accident, not by design.

-- 
M?ns Rullg?rd
mru at inprovide.com