[mythtv-users] DVB-T playback thru PVR (FIXED)

Oliver Gorwits oliver.gorwits at computing-services.oxford.ac.uk
Wed Aug 18 17:34:21 EDT 2004

Hi there,

A few weeks back I posted[1] asking about difficulties fast forwarding, rewinding and using bookmarks with DVB-T recordings when playing back through a PVR-350 card.

I have now resolved these issues by patching one of the source files in mythtv. I have for the time being decided not to post a patch to mythtv-dev, for reasons outlined below, but if this is requested then I will consider it. The (lengthy) explanation below is for mythtv-users readers; I understand the mythtv-dev list is aware of this issue.

Essentially, it appears to be a problem in the <ivtvdecoder.cpp> file that manages seeking around in the MPEG stream whilst playing back. There is a database "position map" for keyframes that is used to seek, but the keyframes appear differently in DVB-T streams and the current code cannot handle it.

As mentioned on this thread, the problem does not surface when playing back on non-PVR hardware. This is because in that situation a different part of mythtv code handles the seeking; a patch was applied to CVS around eight months ago[2] to fix the issues in <avformatdecoder.cpp>, courtesy of an HDTV user (HDTV perhaps shares the keyframe differences that DVB-T does, or at least they take advantage of the same fix).

In essence all I did to fix the problem was port chunks of <avformatdecoder.cpp> over to <ivtvdecoder.cpp>, plus some tweaking up here and there.

Now, from my brief fishing around in the mythtv-dev and mythtv-commit archives it looks to me like this problem is known about, and also that the general issue of unnecessary decoder code differences is known about and will be addressed in the future[3]. For this reason I'm not posting a patch - it would only dissuade a better programmer than myself from writing something that fixes the problem in a more complete way.

Of course please correct me on the above - I have only learnt about MPEG and Mythtv internals from reading the sources. I hope this proves helpful to the list or archive searchers.

[1] http://www.gossamer-threads.com/lists/mythtv/users/77943?do=post_view_threaded
[2] http://www.gossamer-threads.com/lists/mythtv/dev/15609?do=post_view_threaded and http://www.gossamer-threads.com/lists/mythtv/dev/16064?do=post_view_threaded and http://www.gossamer-threads.com/lists/mythtv/dev/16652?do=post_view_threaded
[3] http://www.gossamer-threads.com/lists/mythtv/commits/78701?do=post_view_threaded#78701

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