[linux-dvb] Re: Problems using mplex

[Hans-Guenter Luetke Uphues <hg_lu@t-online.de>]

Hi Martin!

ma.hoff@bigfoot.com writes:

> > It's supposed to be like this
> > 
> >    bad mux        demux        good mux
> > AVVAAVVAAVVAA ->  VVVVVV -> AVAVAVAVAVAVA
> >                  AAAAAAA
> > 
> 
> Okay, I alway do hard on explaining, so let's try:
> 
> I think we have got somethinh like this
> (the thumber stands somehow for the PTS Info)
> 
> A0 A1 V1 V2 A2 A3 V3 V4 A4 A5 V5 ...
> 
> by demuxing we get
> 
> A0 A1 A2 A3 A4 ...
> V1 V2 V3 V4 V5 ...
> 
> Which tells us that we have a VPTS-APTS diff of 1 (or whatever)
> 
> This is corrected by cutting off one stream (here audio)
> So we have 2 separate streams
> 
> A1 A2 A3 A4 A5 ...
> V1 V2 V3 V4 V5 ...
> 
> By Muxing we get
> A1 V1 A2 V2 A3 V3 A4 V4 ...
> 
> which is in theory is fine :-)
> 
> 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.
> 
> 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...
> 
> I hope this helps to clrarify the problem...
> 
> Regards, Martin.

I think you are right:
Two years ago I bought a Grundig DVB standalone receiver. This box hasn't a S/P-DIF, so
I build it by myself. But I had/have one problem:
When recording audio via S/P-DIF from a DVB tv station, sometimes the interface is for a
short moment out of sync. My DATs go to standby for this moment. Recording DVB radio
stations, I never had this effect. Listening the sound via audio analog outputs: there
were never any problems.

Greetings
Hans-Günter


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