[mythtv] Ticket #1049: DVBSignalMonitor needs to be able to monitor NIT/SDT

[Allan Stirling]

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 - But not really significantly, which is what I 
hoped (And Daniel predicted) would happen.


I seem to remember that the 'actual' frequency tuned-to used 
to be stored on a scan. I'm not too sure if that is present 
any more from Daniel / Stuarta's -commits list comments. If 
people are indeed seeing extreme drift, it would possibly 
tend to be only in one direction. Maybe this is throwing 
some people's ex-working setups off?

Cheers,

Allan.