[FFmpeg-devel] Supporting DirecTV captioning.

Joseph Van Riper vanriper.trey at gmail.com
Tue Sep 26 00:27:22 EEST 2017

On Sat, Sep 23, 2017 at 5:44 PM Helmut K. C. Tessarek <tessarek at evermeet.cx>

> On 2017-09-22 16:34, Joseph Van Riper wrote:
> > I would prefer not to sprinkle changes throughout something
> > if I can avoid it.
> Have you thought about a third party library then?
> This library can be maintained by the DirecTV folks and you can work on
> the glue between ffmpeg and the lib.
> The library can still use calls to existing functions...

That might be nice, but I doubt DirecTV cares to make a library for anyone,
especially something is simple as this.  In any event, I don't have any
pull with DirecTV whatsoever.  I've been asked to help by people impacted
by their receiver and this library.  The required changes probably won't be
that difficult to perform, and I am hopeful that I can make very light
changes that folks won't mind... at the very least, something that makes is
relatively easy to extend the code.

For example, it might be a good idea to make two functions instead of just
the one, and let that function call one of the other two functions based on
parameters.  This would make the library a tad more extensible, in case
some other receivers have a need like this (heavens forbid).

This is sort of a strange area of codecs, closed captioning.  This is
strongly influenced by the use of lines that appeared in line 21 of the
incoming video signal in standard definition days.... four bytes of data
per frame, 2 for two channels of captioning, and 2 for another two
channels, with the 608 standard detailing which channel based on the codes
flowing through the video in this way, back in the 1970s, I think, before
anyone imagined this stuff going digital.  To fit into digital streams, new
standards had to be made... and I guess DirecTV needed something sooner
than the standards bodies could manage at the time... or they didn't know
there was a standard until later, heh.

Still, one could use this same area of the stream to put other interesting
bits of information, potentially, although there's already a lot of
meta-data available through the 608 captioning standard if you wanted to
use it (for example, encoding relevant URLs one could go to while watching
the video stream, which is part of the non-viewable captioning available in
the standard).

But, I digress, significantly, from the point.  I took a closer look at how
this stuff gets configured, and while it's a clever system, to make use of
it for this purpose, I think a little code rework needs to be done... just
a bit.  Hopefully, when the folks return from their convention, I might be
able to chat with the right person to make sure I do something that helps
make anyone else who needs to do something like this have a straightforward

I might also want to accommodate decoding.  I don't think it'd be right to
do this without supporting decoding as well.

- Trey

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