[linux-dvb] Re: Budget DVB-C tuning problems

[Johannes Stezenbach <js@convergence.de>]

Robert Schlabbach wrote:
> From: "Johannes Stezenbach" <js@convergence.de>
> > > and AFC is interesting even if there is no sync.
> >
> > (sync & 2) tests if the carrier has been recovered.
> > If we don't have a carrier, then AFC is undefined
> > (AFC := difference between given frequency and
> >  recovered carrier frequency).
> 
> True, the docs say "if the carrier has been recovered". However, the value
> of the AFC register seems to reflect where the VES1820 is currently
> "looking" for the carrier if it has no sync. And you can see when it gets
> "stuck", because it stops changing. Thus, I stand by my statement: The
> value of the VAFC register is _interesting_ even if there is no sync.

OK, I print AFC now always, surrounded by [] if the carrier indicator
is unset. But the AFC value is only reported by FE_GET_FRONTEND
if it is valid according to the data sheet.

> > >   ((s32)(p->u.qam.symbol_rate * afc) >> 10));
> >
> > The calcuation above avoids overflow errors at the expense of some
> > loss in precision.
> 
> Unnecessarily so. The maximum symbol_rate supported by the VES1820 is 8.75
> Mbaud. AFC can be -128 maximum. 8.75M * -128 = -1120M -> that'll fit into a
> signed 32-bit integer.

OK, you're right.

Thanks,
Johannes


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