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

hermann pitton hermann-pitton at arcor.de
Thu Aug 21 01:40:30 CEST 2008


Hi,

Am Mittwoch, den 20.08.2008, 10:07 -0400 schrieb Steven Toth:
> Antti Palosaari wrote:
> > Thomas Börkel wrote:
> >> hermann pitton wrote:
> >> Measuring the strength and SNR is not so easy now. My SAT receiver
> >> always says strength 70% and SNR 80%. My new Nova S Plus cards always
> >> report strength 97% and SNR 99%. Only my TT 1500s reported reasonable
> >> numbers like strength 63-80% and SNR 75-90%.
> > 
> > What is the recommended way to report signal strength or SNR by the 
> > driver? Both of those are 16bit unsigned value. Is it better to scale 
> > 0-0xffff or return as decibels? If SNR is reported as decibels it will 
> > look like very small value...
> > 
> > Antti
> 
> May of the drivers I've added recently report their values in db, but I 
> typically also have one precision point. So I report 20.4 as 204 decimal.
> 
> - Steve
> 

please allow me to abuse the thread a little for some status I seem to
have on saa7134 and DVB-S so far, not going into x264 issues.

Can only report for the saa7134 driver and DVB-S and what we have for
the dual triple Medion Quad Creatix/944 and the new single triple
Creatix/CTX948 with tda10086/tda8283 and isl6405 dual.

If SNR goes down 50% caused by external influences on some few
transponders (1 or 2), artifacts will become visible from time to time
in the preview. This has effects on the apps trying to keep A/V sync on
the recorded immediately too and can be seen with verbosity enabled for
them in more details.

The apps do differ a lot, how well they deal with the mess, but in
general without such mess for mpeg2 they have no problems.

My external DVB-S receiver is at the very same point with loopthrough to
second DVB-S connector and to the external receiver of the Medion Quad
from its first, which really can actively control voltage and tone.

It was exactly the other way round, before we had support for the
isl6405. Worked fine too. (A new PC had the mobo then upside down on the
left, helped to become aware of the loopthrough ;)

Since the external box runs almost without much buffering, one can see
major glitches there at first and they will happen in the buffers too a
little later on our drivers. This is fully in sync, just that delay.

This works very reliable here.

However, signal strength and SNR reported differ for the external box
too, vista is even worse on BDA drivers and always reports 100% for both
until you are really off and then seems to display some fake.

Also the late 2006 driver seems to behave exactly like what we have now
on this dual triple Medion Quad. Others did say it is all fine
previously.

On the second DVB-S device without i2c-access to the isl6405, powering
it up from the first to 18 Volts and on close disable it works fine.
But the attempted voltage/tone switching in between does nothing on that
one.

On the late 2007 driver they seem to have given up on the second DVB-S
and nothing at all happens any more. That recent .inf is also only
listing the first 16be:0007 as DVB-S capable anymore.

Since we still have loopthrough by default enabled on the tda8263, it is
still usable so far.

For the record, the internal DVB-S loopthrough has some loss for what I
can see, but this does not count for what goes through to an external
receiver. It was the other way round previously the same.

Still no high symbol rates here for testing, but the Oliver/Manu/Hartmut
patch for those is confirmed by others to be well done.

Also for the record, it might take years before anyone comes back to
it :), the manual DVB-T/TV analog switch distributes the RF signal
evenly to both tuners, means works much better than a passive splitter,
don't know why. No external LNA found. Likely the assumed slightly
better splitters are still crap.

All in all, much fun on the Quads now.

Thanks to everybody involved, especially to Hartmut and of course Andrew
previously and to all others who paved this road.

Cheers,
Hermann













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