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

[Dave Chapman <dave@dchapman.com>]

On Tuesday 24 April 2001 10:45 am, Ralph Metzler wrote:
>  > If I tune to an active transponder, then it seems to work  But then, if
>  > I tune to another, inactive frequency, then quite often, the tuner will
>  > stay locked at the current transponder.  Is this now the desired
>  > behaviour when tuning fails?
>
> No.
> But the driver now does a zigzag-scan around the frequency. So, it
> might lock onto the old transponder if it was very close.
> Maybe I should make the range of the scan smaller.
>

The channels on my test satellite (31W) can be as close as 7 (MHz?) (e.g. 
10959v and 10966v), and I rarely get a signal lock +- 1 from the actual 
frequency (with older CVS drivers).  This is probably the exception and 
"mainstream" satellites would work fine with your current code.

> Does this also happen (jumping back to old transponder) if it was
> further away, or does it then jump to a close by one?

Yes, it seems to.  When I was testing it this morning, only one transponder 
was active on the satellite, and any frequency (e.g. +-200) seemed to leave 
the tuner locked on the old transponder.  What is your current "zig-zag" 
range?

>
> I expected that this kinds of problems would show up.
> Some read the API in such a way that the driver should handle the
> zigzag scan and fine tuning (which I now implemented), others want
> direct control because it is much better for scans.
> In the old API I had a flag for that (AFC flag). The new API provides
> a call which should return the recommended step size for a scan. The
> frequency range around the step size should then be searched by the
> driver or the hardware. Since every DVB demodulator has its own way of
> frequency correction and search I think this should be handled in
> the driver. It is just not so easy to figure out and control the right
> step size since it can depend on symbol rate, frequency and used hardware.

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 ?

On a related topic, do you have any suggestions about searching for 
transponders with unknown symbol rates?  Are some tuner chips (e.g. old/new 
Hauppauge) better than others when it comes to locking onto a signal with the 
wrong symbol rate?  Can you suggest a (hopefully small) list of symbol rates 
that I could try to identify if there is a signal on a particular frequency?

Thanks,

Dave.


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