[linux-dvb] Re: Driver problems

["Robert Schlabbach" <robert_s@gmx.net>]

From: "Holger Waechtler" <holger@convergence.de>
> On the Technotrend DVB-C boards this signal is used to adjust one of the
> parameters that vary with fabrication tolerance (don't remember how it
> was connected exactly, investigate your DVB-C board or check the mailing
> list archives to be sure).

Umm, I think this only applies to the _Siemens_ DVB-C board, not to the
TechnoTrend DVB-C board. There is an extra chip between the tuner and the
demodulator on the Siemens DVB-C, the TDA9819, which is an analog/digital
IF processor, obviously required for the analog reception capability of the
Siemens card.

The VES1820 PWM output is connected to the DVB VCO control pin of the
TDA9819. From the 9819 datasheet:

"DVB mixer

The VCO can be controlled by an external DC signal to
provide a carrier for the down-conversion of the DVB-IF
signal. The VCO frequency as a function of the voltage at
pin 16 is given in Figs 10 and 11."

In the end, this makes the PWM value an _AFC_ value. You can find the
optimum value by checking the VES1820 VAFC register - if it reads a
negative value, increase the PWM value, if it reads a positive value,
decrease the PWM value - until you get VAFC as close to 0 as possible.

> Then there was a second problem - some time ago people reported that
> some of the cards needed other values than the ones stored in the
> EEPROM. Since we did not found a easy automagic solution that worked for
> all cards and it's even possible that the optimal required PWM value
> changes with the card's age the discussion came up to either reprogram
> the EEPROM or to override the EEPROM parameter with an optional module
> parameter.

FWIW, you can write to the EEPROM with a simple I2C write operation. I.e.
you write two bytes to I2C address 0xA0: 0xFF (offset), PWM value. My
Siemens DVB-C is two years old and I adjusted the value only very slightly
from 0x6B to 0x6D.

BTW, with this VCO adjustment capability, I get the VAFC to a range of 0 to
1, whereas my DVB-C budget card, which cannot adjust the carrier, the VAFC
is typically -3 to -4, i.e. the Siemens DVB-C can slightly better tune to a
channel, probably making it the better choice in cases with marginal
reception.

Regards,
--
Robert Schlabbach
e-mail: robert_s@gmx.net
Berlin, Germany



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