On Monday 21 April 2008 04:07:22 Bryan Henderson wrote:
> >>> I'm working to get Mplayer to do closed captions with IVTV
> >>> embedded VBI (MPEG) input, and have encountered a problem with
> >>> the PTS timestamps in the VBI packets. The timestamps are
> >>> referenced to some arbitrary time rather than the beginning of
> >>> the program, while the timestamps in the audio and video stream
> >>> do appear to be referenced to the beginning of the program (open
> >>> time, I guess). The VBI timescale apparently spans open/close.
> >>>
> >>> What would cause that? Where does the time come from?
> >
> >I have noticed the same thing. The PTS comes from the firmware, but
> > the VBI PTS and the audio/video PTS have an offset. I am working on
> > the whole VBI capturing part of the driver and I'll take a look at
> > this as well. I might be able to fairly simply adjust the VBI PTS
> > so that it is in sync with the A/V PTS.
>
> Any news to report on this?
Sadly, no. I haven't had the time to continue with the VBI code. I hope
I'll have some more time in the future.
> FYI, I have succeeded in getting Mplayer to understand IVTV embedded
> VBI input, in particular the EIA608 closed caption data.
>
> For the time scale, I compute an offset by comparing the first VBI
> packet's time stamp with the timestamp from the most recently
> received video packet. I then use that offset to adjust the
> timestamp in future VBI packets. It is clumsy and error prone to
> make the VBI stream depend upon the video stream in that way, but I'd
> probably have to even if we could get IVTV to generate synchronized
> timestamps because I'd want it to work with old IVTV.
>
>
> Along the way, I discovered a related protocol. When people put TV
> programs on a DVD, they put the closed caption information with it.
> The DVD player does not generate the captions, but rather encodes
> them into the VBI of its output so that the monitor can generate
> them. Ergo, the information on the DVD is in the same format as you
> receive off the air -- EIA608. While IVTV embeds this (along with
> other VBI data) in the MPEG stream as a separate Private Stream 1
> stream, in the DVD format, it is integrated into the video stream as
> "picture user data," whatever that is.
>
> So I wonder: Does the TV capture device make it possible for IVTV to
> generate a video stream that contains closed caption data as picture
> user data?
In theory, yes. It's been on my TODO list for a long time, but I suspect
it is a lot of work so it's low prio for me. Especially since I cannot
test it properly here in PAL country.
> Incidentally, there is one other way of putting television closed
> caption data on a DVD: EIA708. That's the same way in comes in a
> digital television broadcast. I'm not talking about that one.
Regards,
Hans
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