> -----Original Message-----
> From: ffmpeg-devel <ffmpeg-devel-boun...@ffmpeg.org> On Behalf Of
> Devin Heitmueller
> Sent: Monday, January 27, 2025 9:16 PM
> To: FFmpeg development discussions and patches <ffmpeg-
> de...@ffmpeg.org>
> Cc: Marth64 <mart...@proxyid.net>; Kieran Kunhya
> <kieran...@googlemail.com>
> Subject: Re: [FFmpeg-devel] [PATCH v2 00/11] fix broken CC detection
> and ffprobe fields (cover letter)
>
> > Do you have an example stream recording?
> > And who are those who wouldn't be following? Broadcasters?
>
> For what it's worth, I have seen this and it can definitely happen.
> It's not common though since it both technically violates the spec
> and
> also not best practice (for example, some televisions won't show you
> the option to enable captions if the caption stream isn't present at
> all). Generally it occurs when the broadcaster is doing splicing of
> TS streams entirely in the compressed domain (e.g. for ad insertion),
> or cases where ads or programs don't contain captions and the
> encoder/transcoder isn't smart enough to generate empty caption
> packets and include them in the output.
>
> That said, I think it's very defensible to say, "We don't set the
> flag
> saying captions are present if not detected within the probing
> window".
>
> Devin
Hi Devin,
thanks a lot for your insights. Sure that those kinds of cases exist. Another
example would be shared program slots where one broadcaster sends CC and the
other doesn't.
But that's different from what Kieran said (= "sparse"), which would mean that
there's no continuous stream and caption data comes around only when there's
some caption content to display. I was about to say that this isn't even
possible due to the way TVs are working, but I didn't feel to have sufficient
evidence, so thanks for confirming this.
In case of "sparse" data, it would have meant that there could be captions
which can only be detected by scanning frames for a certain time range (like
20s).
But as we agree - the result from the initial parsing is sufficient to reliably
know whether there are captions at the start of the stream - same like we
determine the frame size. Both can change at a later time, but both are
definitive for the probed section of the stream.
Besides: one doesn't exclude the other, we should anyway keep the
-analyze_frames feature because it's very useful for other side data, and that
means you'd also be able to get an answer to the question whether there's CC
data _anywhere_ in the stream - if you want it.
Thanks
sw
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