[FFmpeg-devel] [RFC] DVB teletext and PTS

[Reimar Döffinger]

On 23 Sep 2012, at 04:16, Michael Niedermayer <michaelni at gmx.at> wrote:
> On Sat, Sep 15, 2012 at 04:29:02PM +0200, Reimar Döffinger wrote:
>> Hello,
>> when specifying PTS handling for DVB teletext they made quite a mess it
>> seems (see Annex A of ETSI EN 300 472), which resulted in some stations
>> not using sensible PTS values (for example for ORF the teletext PTS
>> values seem to indicate a time multiple hours after the video time,
>> I have a ORF1HD.ts sample here but I don't remember where exactly it
>> is from).
>> For ordinary teletext, processing them straight away works fine, but
>> will completely mess up teletext based subtitles.
>> This will work for hardware decoders, because at the same time a
>> "maximum retention time" of 40 ms was specified.
>> Now the question is: how should we handle this? Would a patch/hack that
>> limits DVB teletext time stamps to last stream ts + 40 ms be acceptable for
>> example? It's not really accurate since it should be next ts + 40 ms
>> though, and quite hacky. But I can't really think of anything better.
> 
> sounds reasonable, but i dont understand the prev/next ts issue
> what ts is this ? pcr ? dts ? something more complex ?

Well what I guessed from the specification is that they mean real time received + 40 ms.
But we can't specify such a time stamp.
Since this is just a "make sure things do not break too badly" mechanism and also since I believe the VBI info usually is stored before the frame it belonged to, making it relative to the following frame dts seemed most reasonable to me.
In practice, I guess just using the last and maybe increasing the margin to a few more frames, like 80 ms, would probably work perfectly fine as well, and more along the lines of "avoid breaking good files for the benefit of broken files".
But I don't really know, which is why I am asking for comments :-)