[mythtv-users] Strong signal, ugly artifacts...

Andrea Giuliano sarkiaponius at alice.it
Tue Jun 23 00:45:34 UTC 2009

Dear Brian,

thanks for your kin answer, but I guess there is nothing wrong with the
amplifier: it has been working perfectly fine for many months, and (you
may or not believe me) the problem disappears completely if I run a
2.6.18 kernel!

I made some tests with other kernel, too: 2.6.25-2, 2.6.26-1 and
2.6.26-2 all prevent me from getting good recordings, while 2.6.18-6 let
me record exactly as I've always been doing for many months with the
same equipment.

The only thing I have changed is the graphics card: currently I have an
nVidia GeForce 4000 MX, with the legacy driver. The 2.6.18-6 kernel is
not yet configured for this card, so I could record only using mplayer
from the command line. But mplayer, just like mythtv and every other
player, records very bad movies when running the other kernel mentioned
above. Please note also that femon reports very bad values (and a
strange error message, too) with the "bad" kernels, and normal values
(and no errors) with the "good" kernel.

Since I've been using those kernels for months without problems, the
different behaviour cannot be just a matter of drivers: my DVB-S card,
or something else in my Linux box, may have some problems that more
recent kernels cannot handle (and maybe they're right doing so), while
the old one can.

It's all so strange!

Best regards.

On Sun, 2009-06-14 at 15:25 -0600, Brian Wood wrote:
> On Sunday 14 June 2009 14:33:16 Andrea Giuliano wrote:
> > Well, I checked cables and dish, nothing changed.
> >
> > One thing I should have mentioned before is that I have an in-line
> > amplifier that should give a gain of 20 dB, but MythTV always reports
> > 4.8 dB despite the amplifier is present or not. In the latter case,
> > MythTV reports a signal strength less than 10%.
> >
> > If MythTV is not fooling me, the amplifier is gone: 4.8 dB is less than
> > the gain it should give!
> >
> > On the other hand, the signal strength is dramatically affected by the
> > amplifier.
> You may be overloading the amplifier, either by too strong a signal input or 
> by having too many channels.
> Make sure the amp is designed for the channel loading you have, and that it's 
> push-pull if you are using the mid-band (a single-ended amplifier will throw 
> second harmonics from the low band into the mid-band, a PP amp will not do 
> this (as much)).
> Amplifiers are happiest with all the input channels close to each other in 
> level. If you have a strong local station (or even strong local FM stations) 
> you will overload the amp. You may need to attenuate the local station(s) to 
> prevent this.
> So double-check the amplifier specifications and make sure you are operating 
> within them.

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