[FFmpeg-devel] [PATCH 2/2] libavformat/mpegts: demux DVB VBI data

[Marton Balint]

On Fri, 22 Nov 2024, Scott Theisen wrote:

> On 11/22/24 16:12, Marton Balint wrote:
>> 
>>
>>  On Fri, 15 Nov 2024, Scott Theisen wrote:
>>
>>>  From: ulmus-scott <scott.the.elm at gmail.com>
>>>
>>>  DVB VBI data is defined in ETSI EN 301 775 and can include EBU teletext
>>>  data
>>>  as defined in ETSI EN 300 472.
>>>
>>>  ETSI EN 300 468 defines teletext_descriptor, VBI_data_descriptor, and
>>>  VBI_teletext_descriptor, which has the same definition as, but different
>>>  use
>>>  from, teletext_descriptor.
>>
>>  Hmm, adding a new codec for VBI data might be OK, but I am not sure if
>>  there is a better way to automagically let the framework know if there are
>>  teletext compatible VBI packets, so the teletext decoder can be used. 
>
> Would another AVCodecID AV_CODEC_ID_DVB_VBI_WITH_TELETEXT that would use the 
> teletext decoder work?  Would this be AVMEDIA_TYPE_SUBTITLE even though it 
> also includes data?  (Also, what teletext decoder?)

I am not that big fan of adding both DVB_VBI and DVB_VBI_WITH_TELETEXT, 
considering these use the same essence format.

And VBI_teletext_descriptor is an additional descriptor to 
VBI_data_descriptor as far as I understand the standard. (Although ETSI EN 
300 468 6.2.47 seems to be referring to teletext_descriptor as an 
additional descriptor, but that makes little sense to me, they meant 
VBI_teletext_descriptor here?)

>
>>  Or maybe we should simply set the codec id to DVB_TELETEXT if there is a
>>  VBI_TELETEXT_DESCRIPTOR, and make the teletext decoder parse all data
>>  units in search for a comptaible data unit id?
>> 
>
> That would work only if you don't want to use any of the other VBI data, 
> which is what MythTV currently does, ignoring packets where data_identifier 
> is not EBU data and data_field()s where data_unit_id is not EBU teletext data 
> as defined in ETSI EN 300 472.
>
> My main concern is if someone wants to decode the other types of VBI data how 
> would they do that, or know to try, if it claims it is just EBU teletext 
> data?

FFmpeg does not have data decoders, so the user has to parse the packets 
anyway if he is interested in VBI.

>>  Because if we set the codec to VBI for VBI_TELETEXT cases, won't this
>>  break the automatic probing of unknown codecs? (which otherwise might
>>  recognize the stream as teletext)? So maybe for VBI_TELETEXT_DESCRIPTOR
>>  case we should keep the coded id NONE and rely entirely on probing?
>
> I have no idea how this would interact with the automatic probing.

Me neither to be honest, and relying entirely on probing is not very nice 
in the first place, so let's just forget this.

Maybe we should stick to your original idea, use a single codec for the 
VBI streams, but make it AVMEDIA_TYPE_SUBTITLE, considering it can contain 
teletext (even if there is no VBI teletext descriptor in case of inverted 
teletext) and even closed captions, which our decoders might be able to 
decode to something meaninful.

Then anybody interested can add a new decoder for this new codec ID 
(which obviously can share code with libzvbi-teletextdec), so it will be 
able to decode teletext as well.

Regards,
Marton