[FFmpeg-devel] [PATCH] MPEG-TS demuxer: Report 4cc in codec_tag field instead of SMTPE RID

[Baptiste Coudurier]

On 8/31/2009 10:42 AM, M?ns Rullg?rd wrote:
> Reimar D?ffinger<Reimar.Doeffinger at gmx.de>  writes:
>
>> On Mon, Aug 31, 2009 at 09:18:29AM -0700, Baptiste Coudurier wrote:
>>> On 8/31/2009 1:27 AM, Reimar D?ffinger wrote:
>>>> On Mon, Aug 31, 2009 at 10:56:18AM +0300, Christian P. Schmidt wrote:
>>>>> I know that mpeg-ts streams to not support 4cc tags for the codecs.
>>>>> However, the mpeg-ts demuxer currently copies the registration ID into
>>>>> the codec's tag field, which in my opinion is worse than leaving the
>>>>> field empty.
>>>>>
>>>>> As an easy workaround the current tables for codec detection could be
>>>>> expanded to hold the 4cc codes as used in riff.c and return those in the
>>>>> codec_tag field.
>>>>>
>>>>> There are two codecs that do not have an official 4cc tag, namely
>>>>> bluray-pcm and E-AC-3. I'd go with mplayer's definitions for those, BPCM
>>>>> and EAC3.
>>>> The codec_tag is not really supposed to be a fourcc, if there is no
>>>> stream-specific tag that (more or less uniquely) indicates the codec used
>>>> in the file it should be 0. Obviously it shouldn't be "random" nonsense
>>>> like it seems to be currently either though.
>>> Huh, it's not "random" nonsense, it's the registration descriptor if
>>> present.
>> Well, with DVB-T receptions I know I got something else about each try
>> (sorry, haven't properly debugged it, nor recorded enough samples).
>> And HDMV for LPCM audio _and_ and some video codecs is not exactly a
>> "tag that (more or less uniquely) indicates the codec".
>> I don't mind much, but I am having some doubts that the thing that
>> currently ends up in codec_tag belongs there.
>
> If anything, codec_tag should be set to stream_type from the PMT.  The
> problem is that the meaning of stream_type values in the private range
> (0x80 -- 0xff) depends on the registration descriptor and possibly
> other private descriptors.  There is simply no way to uniquely
> identify, in the general case, an MPEG-TS stream in 32 bits.

Actually, I just double checked, codec_tag is set to stream_type then it 
is overwritten to registration descriptor if present.
ISO specifies registration of private data through registration descriptor:
"The registration_descriptor provides a method to uniquely and 
unambiguously identify formats of private data"

This was discussed and Michael agreed to set codec_tag.

It is clear that only one field, codec_tag, can be limiting in the 
Bluray situation. ATSC and DVB uses registration descriptors correctly I 
think.

> The correct solution is to make the MPEG-TS demuxer figure out the correct
> codec_id value based on all available information.  Then apps will not
> need to care about codec_tag.

Exactly, and that's what is done. If codec is not determined or not 
known by libavformat, codec_tag can help application determine format.

Assuming codec based on fourcc has been proven deficient many times.

-- 
Baptiste COUDURIER                              GnuPG Key Id: 0x5C1ABAAA
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