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

[Matthias Weingart <matthias@pentax.boerde.de>]

On Tue, 24 Apr 2001, Ralph Metzler wrote:

>  > I agree with that - the driver should try to make all the hardware look the 
>  > same.  I'll help all I can on various satellites.  For now, would it cause 
>  > any problems if I set the AFC flag to 0 in dvb.c ?
> 
> No, but it does not do anything right now ...
> 
> You can change the "(i==20)" to "(i==1)" in mon_zigzag() in dvb.c to get the 
> old behavior.

Hi Ralph,

is there a way to integrate a AFC-OFF in the new API too? I think
sometimes (at least in Dave's case) a switch off of auto tuning (zig
zag scanning) would be nice and useful. (another example: fast channel
zapping: fast with no AFC, but after a while with AFC) 

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! :-) 

        Matthias



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