[linux-dvb] Re: Problems using mplex

[Marcus O.C. Metzler <mocm@convergence.de>]

 > But practically it doesn't look like this but instead :
 > 
 > A1 V1 A2 V2 ...... A100 V100 A102 V101 A103 V102 A104 V103 ...
 > 
 > So, one step back this does mean:
 > 
 > A1 A2 .... A100 A102 A103 A104 ...
 > V1 V2 .... V100 V101 V102 V103 ...
 > 
 > So an analysis gives us the following result:
 > 
 > At the start we had VPTS-APTS = 1 / This was corrected by cutting off audio by mplex - so we have VPTS-APTS = 0 !
 > But take a look at the VPTS-APTS diff at packet 101 - there is again VPTS-APTS diff. ! Which should be corrected ! Otherwise it's accumulating with others coming later !
 > 
 > I still cannot understand why this happens, and how the DVBs Card or any standalone Receiver corrects this, but I guess they just align the packets according to their PTS Info.

The synching part of the firmware takes the audio and video PTS and if
the gap is too large it skips or delays a frame. So the above wouldn't
matter.

 > 
 > So in the upper example the playback would play
 > A100 and V100 together
 > then just V101 (no audio, because we don't have A101!)
 > A102 and V102
 > A103 and V103
 > 
 > so it's in sync !!! (for the DVBs Card or a standalone receiver)
 > But the stream length of Audio and Video differ ! Thats why we get problems when simply demuxing streams ! 
 > On the other hand a small gap in audio (A101) isn't noticeble ! But if we have enough of these the streams differ a few 100 ms and we will hear this !
 > 
 > As said before not every recording has this stuff...

Are there specific transponders that have the problem, or certain
types of programs? 

 > 
 > I hope this helps to clrarify the problem...
 > 

I already knew what you were talking about, I just didn't believe that
anyone would broadcast that way.

 > Regards, Martin.
 > 
 > BTW: Marcus, is the PTS info for the output file of mplex completely rebuild, or is it the same as in the original streams ?  
 >  

Yes, all the PTS information is thrown away in demuxing, but now I see
why my attempts to write a remuxer didn't work. At first I was using
the PTS provided by the original stream and tried to calculate
additional PTS, but they never matched. Only at the beginnig, but than
the audio usually began to drift. So I took mplex and thought it would
make it better. Well, if I had time I would start from scratch and try
to do it differently, with all the information I have now.


Marcus

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Dr. Marcus Metzler                             
mocm@netcologne.de                     http://www.metzlerbros.de
mocm@convergence.de                    http://www.convergence.de

Convergence Integrated Media GmbH          
Rosenthaler Str. 51                   
D-10178 Berlin                             
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