>>> 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? 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? 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. -- Bryan Henderson San Jose, California _______________________________________________ ivtv-users mailing list [email protected] http://ivtvdriver.org/mailman/listinfo/ivtv-users
