On 11/24/24 13:17, Marton Balint wrote:
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....@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?)
I hadn't noticed that and that is very confusing. I think it should say
VBI_teletext_descriptor there.
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.
Making the codec AVMEDIA_TYPE_SUBTITLE is fine with me. Even though the
VBI data is backwards compatible with the EBU teletext data, I think it
should be a separate codec since AV_CODEC_ID_DVB_TELETEXT is known to
contain *only* EBU teletext data. (This information would be lost if
they were combined into one codec.)
It is not clear to me if "Inverted Teletext" has any difference from EBU
teletext data other than the inverted framing_code.
libzvbi-teletextdec should work unmodified with the new codec type since
it already checks for the correct data_identifier and data_unit_id.
The automatic probing (libavformat/dvbtxt.c) would return 0 if any other
data_identifier or data_unit_id values appeared, which I would assume
would be true for any stream with a VBI_data_descriptor because
otherwise the stream would only have EBU teletext data and should have a
teletext_descriptor.
Regards,
Scott Theisen
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