[FFmpeg-devel] [PATCH v3 1/1] avformat/mpegtsenc: Write necessary descriptors into PMT for arib_caption muxing

[Jan Ekström]

On Tue, Apr 20, 2021 at 3:03 PM zheng qian <xqq at xqq.im> wrote:
>
> On Tue, Apr 20, 2021 at 6:46 AM Jan Ekström <jeebjp at gmail.com> wrote:
> >
> > On Tue, Apr 20, 2021 at 12:11 AM Marton Balint <cus at passwd.hu> wrote:
> > >
> > >
> > >
> > > On Mon, 19 Apr 2021, zheng qian wrote:
> > >
> > > > Is there anyone who could review this patch?
> > >
> > > Jan was interested in this, so preferably he should also comment, but it
> > > looks fine to me.
> > >
> >
> > OK, this explains why I didn't see my response on patchwork.
> > Apparently he had CC'd me and thus the "reply" button in gmail sent an
> > e-mail directly to him and I was hurrying due to being on a lunch
> > break -_- (and thus didn't notice).
> >
> > In any case, I did some comments and am now waiting for a second
> > opinion regarding the usage of stream_identifiers in the ARIB context.
> > After all, the specifications do let one utilize 0x30-0x37 for profile
> > A/full-seg ARIB captions, so there must be a reason for them to not be
> > as limited as profile C/1seg to a single identifier :)
> >
>
> I've found related definitions in ARIB TR-B14, Fascicle 1, 4.2.8.1
> and you can find it in
> http://web.archive.org/web/20160319090430/http://arib.or.jp/english/html/overview/doc/8-TR-B14v2_8-1p3-2-E2.pdf
>
> 4.2.8.1 section says:
> "However, for component tag values of default ES of caption,
> set 0x30 or 0x87, for component tag value of default ES of
> superimpose, set 0x38 or 0x88."
>
> That means 0x30 is considered as the default value for
> Profile A caption ES. The section didn't describe how to
> utilize other values rather than 0x30 in the 0x30~0x37 range,
> and due to the second language caption is designed to be
> multiplexed in the same ES, seems that it's assumed that
> there will be usually only one ARIB caption within a program.
>

Alright, so I took another look as well throughout these Technical
Recommendations:

TR B14 fascicle 4
- http://www.arib.or.jp/english/html/overview/doc/8-TR-B14v6_5-4p5-E1.pdf

See:
14  Use of “component_tag”

> Table 14-1   Assignment of “component_tag” Values
> Others
> 0x30 to 0x7F
> Please note that 0x40 is assigned to the default ES for data
> broadcasting. 0x30, 0x31 to 0x37, 0x38 and 0x39 to 0x3F are assigned to subtitle
> main, subtitle sub, teletext main and teletext sub respectively.

Then in TR B14 fascicle 2:
- http://www.arib.or.jp/english/html/overview/doc/8-TR-B14v6_5-2p5-E1.pdf

> 4.2.1  Specification for composition and transmission
> (3) Number of ES
> The maximum number of ESs that can be transmitted simultaneously for the same service is
> 1  for  captions  and  1  for  superimpositions  when  the  component  group  descriptor  is  not
> transmitted. When the component group descriptor is transmitted, the maximum number of ESs
> for captions is 1 and the maximum number of ESs for superimpositions is 1 for each component
> group.

So to recap my understanding:
1. STD side (specs): It seems like these values have to be unique at
least at the level of a PMT (thus, per-program) - discussed this with
a person working on ARIB area of operations :) .
2. STD side: We can indeed have multiple caption/subtitle ES in a
single program (PMT) for Profile A. Profile C is always limited to one
ES per program since only one content_tag is possible.
3. TR side (tech. rec.): 0x30/0x87 should be the default for Profile A/C
4. TR side: If we do not code Component Group Descriptor (0xD9 in the
EIT), we should stick to a single ES (the default one) on both Profile
A and C.
5. TR side: If we code a Component Group Descriptor in the EIT, we can
have one ES for each composition group.

So basically:
1. STD side lets us go for it
2. TR side says "nope, you stick to one - unless you're defining
composition groups". Which is why you do not see multi-subtitle
streams in the wild, since almost nobody utilizes Content Group
Descriptors.

We don't and not sure if we will ever support component groups, so the
alternatives are:
1. Follow TR side, and be more strict than the specification. Limit in
`mpegts_init` each program to a single ARIB caption stream (be it
Profile A or Profile C). Set component_tag in descriptor to the
default one (0x30 or 0x87 respectively).
2. Follow STD side, and utilize what the specification enables. Limit
in `mpegts_init` each program to either 8 Profile A ARIB caption
streams, or 1 Profile C ARIB caption stream. Set component_tag
iteratively in order.

In both cases during counting the profile should be checked, since we
require the profile info to be there for writing of the descriptor.

Jan