[linux-dvb] Re: statmux / sync / PTS demux handling in new head?

[Gregor Lawatscheck <gpel@mpex.net>]

At 03:26 14/05/2003, Í wrote:
> What is known about the channels (namely BBC2, BBC3, BBC News24) is that 
> they use statistical multiplexing i.e. a variable bitrate. Remuxing the 
> stream with pvastrumento fixes the problem - but that's not really a 
> solution for watching these channels live.
I found some more important stuff on this (Murphey's law after I filed the 
initial report)!

Andy Carter's hint about statistical multiplexing / VBR was the correct 
root cause and wise indeed. Using pvastrumento and *only* using the option 
"flag stream as variable bitrates stream" on remuxing suffices to get the 
stream played back perfectly with both vdr and mplayer -vo/-ao mpegpes.

Comparing the files with a hexeditor shows that each PES header is changed 
at three bytes. mpegtools "analyze" then also manages to show the bitrates 
correctly. I've tried to find some documentation on the MPEG-2 header and 
how such a VBR tag might be set but could find anything useful after quite 
some googling.  Wiljo Heinen, the author of pvastrumento will no doubt know 
more about this :).

The questions emerging now are:
- Why did 0.9.4 / oldstruct not have a problem with VBR / statmux streams
What has changed and was there a reason for the change?
- What can we make of the fact that this flag is only crucial on the first 
couple of seconds but doesn't introduce problems once the decoder is in the 
middle of the stream. Should it matter whether this flag is set correctly?
- Is it the responsibility of the player software (i.e. mplayer, vdr, 
dvbstream -ps, ts2pes, ts2ps) to set the right header or should the driver 
deal with it?
- and/or would it be possible to have a fall back mode (i.e. sanity check 
on starting playback) in the driver in case some client software didn't 
provide the correct flag to stop the decoder from locking up?

I'm glad this "blank screen" stuff finally looks like it could be close to 
resolution :)

- Gregor 


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