Re: LGDT3305 configuration questions

[Devin Heitmueller <dheitmueller@kernellabs.com>]

Hi Frank,

I might be able to help out here a bit.

On Sun, Dec 2, 2012 at 6:53 AM, Frank Schäfer
<fschaefer.oss@googlemail.com> wrote:
> 1) When should deny_i2c_rptr in struct lgdt3305_config be set ?
> I can see what the code does, but I'm unsure which value to use.
> What's the i2c repeater / i2c gate ctrl and how does it work ?

Basically an i2c gate is a construct where i2c traffic from the bridge
to the tuner goes *through* the demodulator, and the demodulator has
the ability to open/close the gate to block traffic.  This is very
typical in tuners since in many cases servicing i2c traffic can
reducing tuning quality (hence you want it to only receive traffic if
it's actually intended for the tuner).  It's sometimes referred to as
an I2C repeater, but it's the same thing.

I discuss some more of the details here:

How Tuners Work
http://www.kernellabs.com/blog/?p=1045

> 2) User defined IF frequencies (fields vsb_if_khz and qam_if_khz in
> struct lgdt3305_config):
> What happens if no user defined values are selected ? The corresponding
> registers seem to be 0x00000000 in this case (for the LGDT3305).

You won't get a signal lock.  This value must be set, and it must
match the value configured in the tuner.

> Which IF is used in this case ?
> The USB log of the stick shows that the registers are set to 4a 3d 70 a3
> (for QAM; don't have a log for VSB).
> According to the code in lgdt3305_set_if() this corresponds to a value
> of .qam_if_khz = 4000.
> In the em28xx driver we have another device with the LGDT3304 which sets
> this value to 4000, too.
> OTOH, these fields claim to hold the value in kHz, so this would be 4MHz
> only. But AFAIK intermediate frequencies are usually about 10 times
> higher !? ATSC seems to use 44MHz.

4000 MHz is a very common intermediate frequency for Both QAM and
ATSC.  The value is user definable but in reality the value chosen
will directly impact the quality of tuning reception (the manufacturer
typically determines the ideal IF by putting a spectrum analyzer on
the output and evaluating performance at different IF levels).  Hence
if you got 4000 from the em28xx trace, that is probably the correct
value (make sure the same value is specified in both the demodulator
and tuner configuration blocks - in this case the tda18271_cfg block
passed at dvb_attach).

It's also not impossible (although uncommon) to have different IF
settings for ATSC versus ClearQAM, so you should indeed get a second
trace to see if it's 4MHz in both cases.

Cheers,

Devin

-- 
Devin J. Heitmueller - Kernel Labs
http://www.kernellabs.com