[linux-dvb] Re: Latest CVS - tuning changes/problems

[Johannes Stezenbach <js@convergence.de>]

Matthias Weingart wrote:
> 
> In my experiences (I wrote some firmwares for analog sat receivers in the
> past) it was always useful to let the AFC switchable (by the user). With
> most tuners the signal quality (visible as 'fishes' in the analog TV
> picture) was'nt the best at the maximal RSSI level (received signal
> strength delivered by the tuner). With that RSSI used for AFC the receiver
> did not deliver the best pictures. Switching AFC off and some finetuning
> solved that problem (well, at least for that temperature of the LNB ;-)
>  I can imagine that it is similiar with the digital receivers. But I have
> no idea how to detect or see what finetuning value is the best (there are
> no longer "fishes" :-). Maybe the number of CRC-errors can be a hint.
>  Another issue is that a permanent zig-zag scanning can produce additional
> distortions to the signal - in the analog receiver visible as "wobbling"
> of the picture. I think this can cause some errors for the digital
> receiption too. -> The AFC should tune very slow. Permanent tuning will at
> least produce some additional noise caused by the values send over the I2C
> bus to the tuner and in the synthesis chip itself. And it add some kind of
> a low frequency FM (frequenz modulation) to the signal. So be careful with
> auto tuning! :-)

a) The QPSK demodulator chips do their own AFC in hardware, which cannot
   be switched off. You can only set the covered frequency range. My
   understanding is that AFC is necessary for the QPSK demodulator to work,
   as you cannot tune exact enough without it.
b) The AFC thread in the driver does a slow adjustment of the frequency
   to ensure that thermal drift of the LNB or tuner does not exceed the
   locking range of the hardware AFC.
c) The zig-zag scan is performed by the driver only once after a tuning
   operation, and only if no signal could acuired at the given frequency.
   This is mostly to compensate for cheap LNBs where the frequency
   offset is so big that the signal falls outside the hardware AFCs range.


Regards,
Johannes


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