[linux-dvb] AGC settings for dtt761x, removal of lgh06xf driver

Trent Piepho xyzzy at speakeasy.org
Mon Feb 26 04:52:28 CET 2007


Some tuners benefit from changing the AGC take over point from its default
value.  For my HD-3000 with a Thomson DTT7612, it appears to increase SNR
by 0.19 dB.  I tested it against my weakest (20.0 dB) and strongest (28.2
dB) OTA stations.  Both SNR readings went up by an average 0.19 dB from
when the agc-top was set to 103 dBuV (which was the best) from the default
value of 112 dBuV.  There was no significant difference in the effect of
agc-top between the two stations.  I observed this same effect earlier when
testing with digital cable.

http://linuxtv.org/hg/~tap/v4l-dvb

These patches add support in dvb-pll for sending an initialization sequence
to the pll when the tuner is opened.  A sequence to adjust the AGC TOP to
103 dBuV by setting the aux byte to 0x50 is added for the Thomson DTT761x
and LG H06xF tuners.  Since the lgh06xf driver's only significant
difference from dvb-pll is that it sets this value, it is no longer
necessary and can be removed.

I would appreciate it if anyone with a card with a DTT761x or LG-H06xF
tuner could test this patch and see if it improves their SNR (DTT761x) or
continues to work as it did before (lgh06xf).

Specifically, I'm worried about this possible problem, from my patch
comments:

There is one other difference.  dvb-pll will probe for the presence of an
I2C pll chip by doing a one byte read, the lgh06xf driver did not do
this.  In some devices the PLL is not reachable over I2C at the time the
tuner is attached.  Some more initialization, such as firmware loading,
must take place first.  If any of these devices are former LG-H06xF
users, something must be done to disable this probe or enable I2C before
attaching the tuner.



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