[mythtv] [experimental patch] Teletext decoder for IVTV cards

Martin Barnasconi mythtv at barnasconi.net
Tue May 3 20:09:04 UTC 2005


On Tuesday 03 May 2005 07:06, Isaac Richards wrote:

> What's the status of this (readyness 
> for CVS, etc)?  

Some of the TODOs should be solved first before we can commit to CVS:
I've no idea yet how to detect during playback from what source the stream 
comes from. This is necessary to select the 'old' CC/Subtitle decoding for 
saa7134/bttv cards or any new one based for IVTV cards.
Perhaps even better is to migrate the existing CC/Subtitle stuff from 
NuppelVideoRecorder and -Player into the new vbidecoder class. Will take some 
time as the CC/Subtitle (not vbi) embedding done in NuppelVideoRecorder is 
again different from IVTV and DVB vbi embedding.

> What's needed for NTSC/CC support?  
Here in 'PAL land' I cannot test/develop this. PAL subtitles are stored in 
teletext pages (eg 888) and captured and stored in some sort of page buffer. 
For CC, page detection is not possible. First we have to check if all the CC 
data is embedded, next question is how to extract/store/present it. Some 
extraction code is already part of the vbidecoder class.
Note that for PAL subtitles, the problem related to missed subtitles pages 
still exists (they simply do not exist in the recorded mpeg file); probably 
related to myth, as a directly stored ivtv streams (cat /dev/video >test.mpg) 
contains all embedded data. Just guessing: timing problem related to DMA 
usage or memcpy? 

> And wasn't there an additional patch to handle dvb better, 
> as well?

DVB uses same framework, slightly different detection of vbi packets. Approach 
is to come with combined vbi patch. But for dvb also some issues, see other 
posts.

Conclusion: my feeling is we should decouple teletext from CC/subtitles in 
myth functionality and UI: different CC decoder/overlay class and in UI 
menu/key bindings item 'Teletext' (T) (if available) and 'CC/Subtitles' (S).

Martin


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