[Ffmpeg-devel] 4XM audio codec_tag

[Baptiste Coudurier]

Michael Niedermayer wrote:
> Hi
> 
> On Mon, Nov 06, 2006 at 12:17:20PM +0100, Baptiste Coudurier wrote:
>> Hi
>>
>> Michael Niedermayer wrote:
>>>>> [...]
>>>>> if you wish to prevent the user from doing things (s)he wants to do
>>>>> then you shouldnt write open source software
>>>> That doesn't mean we should make it easier for users to do stupid
>>>> things, like creating files in violation of the standards.  Of course
>>>> we can't completely stop them from doing it, but they should at least
>>>> have to put some effort into it.
>>> could you point me to the spec which disallows a new codec to be stored in
>>> it? iam not even sure if mpeg-ps/ts do that and if they do, are really all
>>> the things various _different_ standard comittees put in it valid?
>>>
>> Depends on container, but I know for sure that ISO Media and derivates
>> does not support it by defining really constrained specs. (mp4 only
>> allows 'mp4v' and 'avc1' as fourcc and defines codec type in object type
>> id, all values are defined or reserved, same for 3gp, I don't really
>> know for mov, I did not check deeply). I think that overriding a
>> reserved value is non standard and therefore forbidden.
> 
> so to summarize, iso mp4 supports mpeg4video and h264, 3gp which is
> not mp4 is copy and pasted from mp4 with the table of alowed codec
> ids edited to support another set of codecs? (no iam not sure, the
> mp4 systems spec is so fragmented over various documents that i simply
> cant make sense of it,i dont even know which documents i should look
> at)

mp4 also supports jpeg2000, png, mpeg2video, they all have a specific
object type id in esds (mp4ra.org), non defined values for object type
id are reserved, therefore forbidden, and I saw vorbis in mp4 using a
reserved object type id which after was defined for qclp, ouch. That's
why doing that is really really not a good idea.

> and mpeg-ps ... there is no mpeg-ps spec which lists
> all allowed codecs and how they should be stored or? its rather that every
> standard comitee hacks mpeg-ps as they see fit to squeeze their codec
> into it (again iam not sure, the draft i have definitly has no hint
> about aac ac3 mpeg4 or anything like that)
>
> and last, you can store anything in mpeg-ps by using a private stream
> that is not violating the standard, and you can do the same with mp4
> and some codecs like ac3 are stored as private stream (a mess worse
> then avi)
> 

Dunno for PS, but for TS it's mentionned that every other streams must
use registration id mechanism, for example S302M specifies which id to
use to identify defined streams, and in particular how to map pcm in ts
through aes3.

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
SMARTJOG S.A.                                    http://www.smartjog.com
Key fingerprint                 8D77134D20CC9220201FC5DB0AC9325C5C1ABAAA
Phone: +33 1 49966312