[linux-dvb] A/V sync issues with DVB-S

hermann pitton hermann-pitton at arcor.de
Tue Aug 19 04:31:46 CEST 2008


Am Montag, den 18.08.2008, 07:13 +0200 schrieb Thomas Börkel: 
> HI!
> Is it normal, that every now and then (1-2 of 10 recordings), I get one
> serious (suddenly 30 frames off) A/V sync problem with DVB-S?
> I am seeing this with TT 1500 (budget_ci) and Nova-S Plus (cx88). I
> tested kernel 2.6.23 and 2.6.25.
> OK, when playing this recording with mplayer, I see no glitch and no
> warnings.
> But when I play it with MythTV, it stutters for a few seconds when it
> tries to softly match the sync.
> I haven't seen this with DVB-C, so I'd like to know if something is
> wrong with my SAT system.
> Any help would be greatly appreciated.
> Thanks!
> Thomas

DVB-C is in general much better protected against troubles generated by
changing environmental conditions, but in case of weather conditions
which suddenly create ice on any surface, it is better to know the one
in charge even there.

Means, watch your dish. Maybe birds or a cat like to stay there
sometimes or wind might move branches of a tree in the near of it.

Next, for my still very limited DVB-S experiences, get an idea about
which transponders are critical for you. Kaffeine for example annotates
the SNR to them during a scan and you always can check if you have some
needed minimum when you start to record.

This is card/frontend specific, but for the very few I have in critical
reception conditions, SNR below 50% is a clear indicator that I won't
have to blame any apps later. A thunderstorm comes up.

After that, on normal mpeg2 DVB-S all applications should still do
fairly well, but if you go out for x264 HDTV, it is another and
completely different additional story, especially for what I see on
x86-64 quad CPUs.

Mostly OT, but since I did spent time on it recently, maybe have it too.

Compiling with CPU optimizations can make it even worse and concerning
multithreading the default one thread some have is for sure not enough,
but in best case eight at once might come through, with 32 all is in
vain and so on. 

However, kaffeine for recording and xine for playback from that file do
work. Every 15 to 20 minutes I have to hit break/play once to restore
suddenly lost A/V sync, when it becomes too much.

But it looks even worse on files recorded with full GPU hardware
acceleration on vista trying to play them at GNU/Linux. OK, both play
without visible issues on vista.

With that I can live already very comfortable now with 1080i non S2
HDTV ;) and don't have to reboot to the other stuff anymore.


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