[mythtv] mythcommflag gets mpeg2video errors not seen in playback

Ken Mandelberg km at mathcs.emory.edu
Sun Sep 9 00:31:59 UTC 2007

Previously I posted:

 > mythcommflag gives a large number of
 > [mpeg2video @ 0xb73ddb68]current_picture not initalized
 > 2007-09-03 11:09:32.693 AFD Error: Unknown decoding error
 > on firewire streams captured on a comcast cable box. The commerical
 > marking on these streams are very unreliable, clearly the time offsets
 > generated are out of sync with the video playback. These particular
 > streams are from extended basic sd digital channels (fx,usa, comedy
 > central, ..) which are encrypted. However, the firewire captured mpeg
 > transport streams play perfectly well in myth.

At that time I thought the problem was that mythcommflag may have been 
using a different codec then mythfrontend. Thats not the case, they are 
using exactly the same mpeg_decode_frame in mpeg12.c.

After some debugging the real problem boils down to this. The errors 
don't show when playing the streams because they are played 
sequentially. mythcommflag seeks forward skipping frames incorrectly for 
these streams and then tries to decode garbage. Eventually it recovers 
but the indexing is then off.

I need someone more expert than me to help look at a sample stream and 
help pin down the error. I have a 1MB stream that illustrates the 
problem. mythcommflag tries to skip from frame 30 to frame 58 and thats 
where the problem occurs. mpeg_decode_frame is called 5 times 
successfully, the 6th time follows a AvFormatDecoder::SeekReset which 
attempts to skip 28 frames, and this one is given garbage.

I submitted this as ticket number 3925. I would like to have attached my 
  1MB sample but that exceeds the 256K limit, but I can provide it to 
anyone who wants to take a look. Alternately if you can suggest a 
diagnostic tool to analyze the 58 frames to define the issue that would 
be helpful.

I don't think this is an isolated error or anything to do with 
corruption. I see this consistently on streams from certain digital 

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