[vlc-devel] Private substream for closed captioning data from video capture cards

RATH, CHRISTOPHER A (CHRISTOPHER A) car at research.att.com
Thu Sep 10 15:47:49 CEST 2009

A few days ago I posted a message concerning a problem we were having
playing video generated by a video capture card with closed caption
extraction enabled.  I think I have found evidence that suggests VLC may
need to detect this stream and ignore the timestamp found in the PES header
for these packets.

The MPEG generated by the video capture card using ivtv contains a Private
Stream 1 substream ID: 0x69 for the closed captioning data.  The PES header
for the packets in this stream have a timestamp that is not relative to the
beginning of the program.  The VLC ps.c demuxer, however, is using this
timestamp to update the clock and thus we get frequent clock gap errors and
the video is never able to be played.

I found a description of this problem at
http://videoman.giraffe-data.com/~branh/d/note/closedcap and it seems to
match with the behavior I see in the mpeg generated by our capture cards.

There is already a special case in the ps demuxer for handling VCD and SVCD
data tracks that causes VLC to ignore the timestamp in the PES header for
that packet.  I believe we need to add another case to detect the closed
captioning data in substream 0x69.  I have already tested a fix in my local
branch of 1.0-bugfixes to do this and now the video is playable.  The fix
would have no other affect as VLC does not have a way to process the 0x69
substream and thus ignores the data in those packets anyway.


Chris Rath
AT&T Labs Research
car at research.att.com

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