[linux-dvb] Re: CX22702 conexant/leadtek support

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

[snip highly useful stuff!!]

> The notes I acquired from LG on their tuner indicated that the IF saw
> filter was to be activated only on the UHF band, and only in Germany.  If
> Germany only uses UHF for DVB-T then this change makes sense and I'll
> update this to activate it on 8MHz bandwidth and UHF.  Is there a logical
> reason why one would only want to activate said filter on a 8MHz
> transponder but only on UHF (and not VHF) and only in one particular
> country?  Is there some kind of co-channel interference that it helps with
> in this particular environment?

Lots of (non-linux) tuner code I've seen is hardcoded so that if the frequency 
is < 300000000 (i.e. VHF), it automatically assumes a bandwidth of 7MHz. 
Personally I don't like that - I prefer such things to be explicitly set - 
but it perhaps explains why they say only UHF+8MHz.

As for Germany explicitly... umm, anyone?

> Andrew, where did you find the product notes on the Thomson 7579 to
> determine that it has no SAW filter?  The recently released document was
> for the 759x family; I havent' seen anything on the 757x.

It was a guess based on my analysis of the conexant windows driver. It does 
not explicitly test the bandwidth and change settings accordingly. As Robert 
points out, it may just be that this was hard coded, and it could indeed have 
a SAW filter (me being too hasty as usual). In fact, if its usable in AU and 
EU then it probably has to?

It would be good to know for sure... if I had one here, I'd open it up and 
beep it out with my multimeter :)

To summarise what I found (omitting charge pump settings): 
        if (freq <= 506000000) {
                buf[3] = 0x02; // i.e. LOW BAND
        } else if (freq <= 896000000) {
                buf[3] = 0x08; // i.e. HIGH BAND + P3
        }

I wonder what P3 could be.
(see cx22702.c in CVS for more details)