[linux-dvb] Re: Another issue: frontend timeout value and low symbol rates

[Andrew de Quincey <adq_dvb@lidskialf.net>]

On Saturday 21 February 2004 02:44, Robert Schlabbach wrote:
> From: "Andrew de Quincey" <adq_dvb@lidskialf.net>
>
> > Now, I'm able to lock on to low symbol rate signals...
> > I've just managed it with a 4340000 symbol rate...
> > getting a REALLY good signal from it, but it took about 10 seconds
> > to tune (the current timeout is hardcoded to 2 seconds). The STV0299B
> > docs say it needs a certain number of symbols before being able to
> > get a lock...
>
> That can't explain the long lock time. My card is locking to the 22000 and
> 27500 MBaud ASTRA transponders in less than 5ms. Since 4340 MBaud is about
> 1/5th to 1/6th of that, the demodulator would get the same amount of
> symbols in 5-6 times that, i.e. in 25-30ms. That's nowhere near 10 seconds.
>
> But have you checked the AFC reading after the demodulator was locked on?
> Low symbol rate transponders are using a much narrower frequency band than
> high symbol rate ones, and thus exact tuning may be much more important. A
> 1MHz offset may not be much in a 30MHz band, but it makes much more of a
> difference in a 5MHz band...
>
> If possible, try compensating the offset by adjusting the tuner PLL
> frequency as much as possible and see if/how that affects the lock times...

Hi, I've finally found the problem (after much fiddling)

The linux core drivers already do the seeking-about mechanism you suggest if 
it can't lock on to the supplied base frequency. And that works fine (seeks 
about in steps of symbolrate/16000).

The problem is that with the frontend driver as it is/was, it retunes the 
TSA5959 PLL every time. Retuning the PLL every few milliseconds doesn't seem 
to agree with this module. 

Also, the PLL resolution is quite low (500kHz). I initially tried increasing 
the resolution of the PLL but that didn't seem to help. If anything, it 
seemed to worsen matters.

Instead, if the frequency change is quite small (i.e. its obviously seeking 
about), I program the derotator frequency registers with the offset. I can 
now reliably lock on to any frequency/symbolrate within a reasonable time 
(i.e. sub-second).


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