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

[Daniel Kristjansson]

On Tue, 2006-06-06 at 23:55 -0400, Yeasah Pell wrote:
> Actually, many cards do much more than a little PLL, and there's a good 
> reason -- LOF error on LNBs can be significant (easily greater than your 
> tp spacing -- remember, it's some consumer quality ~10GHz oscillator 
> powered by some dodgy PCI card's power supply connected by a long piece 
> of coax in conditions which are alternately hot and cold, dry and wet), 
> so a zigzag frequency walk sometimes needs to happen.

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
shift and you can do this cheaply (<$5) with a quartz crystal, a
clock multiplier and some analog electronics. The voltage doesn't
matter much as long as you get something like +5 Volts and the quartz
is stable +/- 60 degrees Centigrade. If you are really cheap (<$2)
the crystal will have a constant error which could be easily compensated
for. With the $5 electronics the shift will be < 100 kHz, with the $2
electronics you will need to compensate in the card or software, but
we're talking <1 Mhz, not 3+ Mhz... I realize some of the LNB designers
might be uneducated cut-n-paste hacks, but still I can't see anyone
trying to get this circuit to work in an open loop; the only way you
could really screw it up enough would be to cut-n-paste a 50K
terrestrial transmitter circuit diagram into your LNB design.

> drifting RF oscillators), but when you combine the search with a moving 
> dish, that's when things go wrong. I think the answer is just to tune 
> after the dish has stopped.
I can see that, if you think it is wise I'll reapply the hack until
your stuff is done...

> Basically swzigzag doesn't start until a timer has expired. This timer 
> is painfully short by default, but can be set by giving the dvb-core 
> module a dvb_override_tune_delay parameter (which is in milliseconds)
I didn't realize this was implemented yet, this was the solution talked
about the last time I talked with a DVB dev about this.
> For example, I have in my modprobe.conf:
> options dvb-core dvb_override_tune_delay=30000
Ugh, I was hoping for an override in the API.

> While I was patching the heck out of the cx24123 frontend driver to get 
> tuning and diseqc to work, I asked some questions on the dvb list about 
> swzigzag (it was *really* shafting the cx24123, because it would retune 
> faster than the card could lock, thus preventing any lock from happening 
> until much later when swzigzag kicked into slow mode), but nobody had 
> anything to say about it. The cx24123 demod does do its own search, so I 
> set the cx24123 registers to search in a conservative range, and then 
> disabled swzigzag with the module parameters.
Maybe you can convince them to put the dvb_override_tuning_delay in the
API when you submit your cx24123 patches? I'd rather integrate this into
the scanning and tuning than have the driver do bad things we don't know
about...

-- Daniel