[linux-dvb] Re: Couple of frontend questions

[Andrew de Quincey <adq_dvb@lidskialf.net>]

> Well, I think every FE has a limited AFC range, and if it doesn't
> get a lock one should try zigzag. 

Yes

> OTOH, if someone unplugs the 
> cable, and the FE can resync by itself, a zigzag scan would
> only do harm. So:

> - if initial tuning fails -> try zigzag
> - if we have a signal, read AFC
> - if signal drops and AFC was close to zero -> no zigzag
> - if AFC was large -> maybe signal was lost due to LNB drift -> zigzag
>   (or better try to correct freq using AFC first)
>
> IIRC there are some frontends that cause TS dropouts or even lose signal
> if one tries to correct the freq using the AFC value read back
> from the frontend, so I think one should not touch the PLL unless
> the signal was lost.

The problem with that suggestion is that the AFC could be large for a number 
of reasons, one of which might be the LNB drifting. Others are: the frequency 
might have been slightly misspecified initially, the tuner might just not 
have "wanted" to tune to the frequency at a low AFC that time.. (yuck, 
analogue things), momentary noise. I'm sure there are others.

If only frontends had a bit you could read which said when the cable was 
plugged in. But as they don't, we have to assume the worst case, and that the 
LNB has drifted, and start zigzagging. 

We could always stop zigzagging after a few seconds if we don't get a lock, 
and assume its a cable unplug. But again, you might have the combination of 
LNB drift AND cable unplugging. Perhaps we should just keep zigzagging.

If the frontend is plugged back in later, then the zigzag will quickly come 
round and lock on correctly again.

Whatever happens, you're going to lose some data.


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