[linux-dvb] Re: Problems using mplex

[ma.hoff@bigfoot.com]

> I only determine the difference at the start, because I want both
> streams to start at the same time. Of course the difference can be
> different at some other point in the stream, but since the stream is
> demultiplexed that really shouldn't matter.
Yes but the audio and video seem to have different length or so to say a different timebase.
So at the start, where you check for A/V offset it's corrected, but in the file the A/V offset changes (don't ask me why) and this isn't corrected.

> The remuxing part of the software is not mine, so I don't know if it
> is always correct.
I think as far as we do not get around the 2GB barrier it works - I've always done my muxing with bbmepg under Windows...
> Maybe it is better to split the file into many
> relatively small parts and then remux those separately.
Of course this works, but just because of the fact, that this method determines the audio video offsets for each clip of the file and not just one at the beginning. So if you have an overall drift of 200 ms and do 4 parts, each has only 50 ms - but my clip is only 20 Minutes and the drifting offset is quite noticeble !

So don't get me wrong, but I've had the same discussion about the drifting sync about 4 months ago in an Windows DVBs discussion board, and at first nobody seemed to believe...:-) And until today I can't understand, how the streams can drift - but they definitively do - not in every recording, but in some (mainly from SAT1 ?) - sometimes stronger sometimes not so strong...
Because for my understanding one could check for the samplerate of the audio (eg 48kHz) and the framerate of the video (25 fps) and could easily calculate how they should be aligned - but that's wrong for some streams ! Believe me 4 months ago I did a lot of testing...

So for Windows DVB work PVAstrumento (www.offeryn.com) solved the problem by checking the whole stream for Audio and Video PTS and properly align them - On the other hand with just demux and mux it was still out of sync - and if you did a fix correction at the start of the stream - it was just in sync at the start and drifted towards the end :-( Just like it is now using mplex.

So what we need is either an implementation of pvastrumento for linux, but unfortunately wiljo (the author) doesn't seem to have any time at the moment, or some additions to mplex, which calculates and corrects the VPTS-APTS difference more often per stream...

If you don't believe or if you want to do a little testing, I can cut the 001.vdr file a little and burn it on a CD-R and send it to you by mail, if this helps to solve the problem... ?

Regards, Martin.




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