[linux-dvb] Re: Couple of frontend questions

[Johannes Stezenbach <js@convergence.de>]

Andrew de Quincey wrote:
> >
> > FE_CAN_COMPENSATE_DRIFT is a bit misleading. I think FE_CAN_RECOVER
> > means that if you unplug the antenna cable and replug it, the
> > frontend will resynchronize itself without manual intervention.
> > You'd think that every frontend should be able to do this, but...
> 
> Oh right. At the moment, the new code doesn't start a zigzag scan if 
> FE_CAN_RECOVER is set. But is this the right behaviour? If FE_CAN_RECOVER 
> means it can recover from the cable being unplugged, that doesn't necessarily 
> mean it doesn't need to have the zigzag scan for LNB drift, because we can't 
> tell the difference between the two.
> 
> So in that case, is there any point in having FE_CAN_RECOVER? To ensure it 
> works, we have to do a zigzag scan anyway. 
> 
> I _suppose_ you could just do one zigzag scan if FE_CAN_RECOVER is set. If 
> that fails, then you assume the cable has been unplugged, and stop 
> zigzagging. But I don't like that; that could easily fail if for some reason 
> the first zigzag scan failed to lock on (e.g. the old noisy signal lock time 
> problem) and there really WAS LNB drift.

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

Maybe some more experienced frontend programmers can comment on this.

Johannes


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