[FFmpeg-devel] [PATCH] lavc: Replace 181 magic number with ITU_T_T35_COUNTRY_CODE_US

[Devin Heitmueller]

On Tue, Mar 4, 2025 at 4:02 AM Tomas Härdin <git at haerdin.se> wrote:
> So it's "US" even if something is broadcast in Sweden? T.35 has a whole
> list of countries, some of which don't exist any more, and some new
> ones are missing

Correct.  So the T.35 country code doesn't actually have anything to
do with where the stream is broadcast.  It's intended to define which
standards body (via a country code and provider code) is responsible
for defining the structure of the payload that follows (in the case of
CEA-708 captions case the country code is US and the provider code is
ATSC).

> > Even if the captions are intended to
> > serve other countries, the country code will stay the same (defining
> > the country code for individual languages goes in the PMT).
>
> How is one supposed to mux multiple subtitle languages in MPEGTS +
> H.264 anyway? This isn't clear at all from what I've read. Country
> codes, as opposed to language codes, being used in the 708 SEI makes it
> worse. And what of other container formats that have no PMT?

So unlike with DVB/teletext, the caption data streams are not provided
on separate ES streams.  They are embedded in the video stream, and
the CEA-708 standard supports multiple languages within that payload
(depending on specifically which form is used, it can be up to six or
up to 64).  The actual languages assigned to the different streams
within the 708 payload are specified either in the PMT via the "ATSC
Caption" descriptor, or in the ATSC PSIP data.

Muxing together captions from different sources is pretty painful,
since you have to parse/decompose the 708 stream and recombine streams
from different sources (and then update the PMT).  I have code which
does it but haven't made any effort to open source it, and I'm not
confident it could easily be done within ffmpeg due to limitations in
the ffmpeg framework.

It's also worth noting that the Caption descriptor as defined in the
standard does not let you specify the language of individual CTA-608
channels within a 708 stream (which is what most people care about).
The only way to specify the language for the 608 channels (e.g.
CC1-CC4) is via XDS bytes within the 608 stream, which almost nobody
does nowadays.  I ran a scan across my network of thousands of
channels from different commercial hardware encoders, and couldn't
find a single one that specified the 608 language in XDS (if I found
cases where it was, I was prepared to submit patches to VLC to show
the language in the subtitle dropdown menu).

Devin

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Devin Heitmueller, Senior Software Engineer
LTN Global Communications
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