[mythtv] Ticket #1049: DVBSignalMonitor needs to be able to monitor NIT/SDT
abraham.manu at gmail.com
Thu Jun 8 17:57:45 UTC 2006
Allan Stirling wrote:
> Yeasah Pell wrote:
>> Allan Stirling wrote:
>>> Daniel Kristjansson wrote:
>>>> I doubt this is as much of a problem as the DVB devs seem to think.
>>>> I actually asked Kenneth Aafloy to chart the LNB drift and get back
>>>> to me, he never did. My EE background is in radio engineering, the
>>>> job of the downconverter portion of the LNB is just to frequency
>>> I had a spare tuner, so here you go:
>>> Total swing for this test is 45kHz (0.003%) which is
>>> probably not significant. Obviously, longer cable runs,
>>> worse power supplies, cheaper LNBs may give worse figures.
>> Interesting figures, thanks for the experiment!
>> For the acquisition search, the rate of change of the LOF is pretty much
>> irrelevant though, since acquisition happens over a brief period of time
>> -- it's the total error from the LOF's "correct" frequency that
>> acquisition has to deal with, since that determines how far you have to
>> search just to find the signal that you are looking for.
> These aren't showing deltas at all - It's just a count of
> *all* of the frequencies that occurred over a fairly hot day
> / cool night period.
>> 12.226GHz. If you were on that tp:
>> 1.626GHz - 1626758GHz = 758kHz
> Ahem. Apologies. For some reason, I didn't check the actual
> frequency I was zapping to, preferring to look it up on
> lyngsat. Indeed, you are correct.
>> Which definitely sounds more like it.
>> The long-term LOF drift that you show in the chart is something that
>> most cards deal with automatically by tracking, but swzigzag isn't in
>> the picture for that (it only kicks in after acquisition if signal lock
>> is lost for some period), and the tracking capabilities of cards
>> presumably are designed only to deal with slow incremental changes in
>> frequency, so I think it'd be unlikely that tracking would cause
>> significant detuning.
> Sure - But it shows the absolute frequency that the card
> believes at the time is the center frequency and also that
> it does move
Are you saying that you loose a FE_LOCK due to LNB LO drift ?
AFAIK, swzigzag is used only once for acquiring a LOCK, once acquisition
is complete, it does the tracking by itself. Newer devices, don't do
To understand how it works, a brief ground up description on PLL's is
These days, new devices don't even have separate PLL chips , they are
synthesized on the FPGA core itself (Well, there is no separate tuner
itself these days, everything is on a single FPGA based DSP core, gone
are those good old days where we had discrete chips). doing this in
Software doesn't bring in any advantage at all.
If you look at the dvb drivers, there is indeed support for this, you
can look at the pll lib etc
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