[mythtv-users] new video card causing interference with cable signal

Greg Zornetzer gzornetzer.lists at gmail.com
Sun Dec 6 22:23:43 UTC 2009

On Sun, Dec 6, 2009 at 7:50 AM, Brian Wood <beww at beww.org> wrote:

> On Sunday 06 December 2009 08:34:17 am Rod Smith wrote:
> > On Sunday 06 December 2009 02:12:59 am Greg Zornetzer wrote:
> > > Hi all,
> > > I recently got an MSI GT 220 512mb card to upgrade my myth box in
> > > perparation for 0.22.  While the new card does very well with decoding
> > > VDPAU, I've noticed a disturbing problem with several of my QAM HDTV
> > > signal quality - I see a lot of blockiness (as if the tuner is getting
> > > some kind of interference.  Removing the GT220 and going back to the
> > > motherboard's onboard graphics seems to cure the problem.  Does
> anyone
> > > have advice for some kind of makeshift shield to block interference
> from
> > > the graphics card? Or other ideas?
> > >
> > > My tuner card is a Pinnacle 800i (using the latest xc5000 driver) and
> I'm
> > > using 0.21-fixes for right now.
> >
> > I've run into this sort of thing before, but even worse: On my main
> > backend/frontend system, a pcHDTV 3000 is next to useless because of
> > interference. I tried at least three different video cards in an effort
> to
> > fix the problem. I did find significant differences, but the problem
> still
> > existed with all the video cards I tried. I've never tried a physical
> >  shield, so I don't know if that would work. I did try moving the tuner
> to
> >  different PCI slots, to no avail. It's possible that swapping out the
> >  video card for another one would improve matters for you, so if nothing
> >  else you could try removing the video card in favor of another model.
> Even
> >  one from a different manufacturer but based on the same chipset might
> be
> >  an improvement. FWIW, I ended up replacing the pcHDTV 3000 with an
> AverTV
> >  A180. The A180 still has problems sometimes, but not nearly as bad as
> the
> >  pcHDTV. I eventually put the pcHDTV in another computer that functions
> as
> >  a slave backend, and it's flawless in there, so it's definitely not a
> >  defective card -- it's a defective interaction between the card and
> other
> >  components in the backend.
> >
> Not saying it will help, but make sure you have checked all the obvious
> things:
> Make sure you have used *all* of the motherboard mounting holes, and
> have proper length screws that don't "bottom out" before making a good
> contact with the mobo ground plane. I have seen systems where the builder
> used only 1 or 2 mounting screws, which can cause problems like you
> describe.
> Make sure you are using a good quality PSU, cheaper ones often leave out
> de-coupling capacitors and filter inductors, resulting in common mode
> interference problems.
> Don't use "modular" PSUs, the connectors can make less-then-perfect
> connections, which can cause rectification of high frequency crud if they
> behave in a non-linear fashion.
> Make sure all cards are seated properly.
> Dress cables as far away from cards as possible.
> Use a good quality cable from the graphics connector (on mobo or a discreet
> card), preferably one that has ferrites on both ends.
> Don't use anything in the case that's not necessary for system operation.
> Fancy lights and other "cosmetics" should be avoided.
> Don't use cases with "windows". If you can see the inside of the machine,
> stray signals can take the same path as your eyesight. Computing is not a
> spectator sport.
> I know these are all obvious things, but they all have caused troubles for
> people ignoring them.
> Hi Rod and Brian,
Rod - I'm at least glad to hear that I'm not the only one.  I guess this
could be rendered moot by Comcast's impending scrambling of the QAM
signals.  Either that or I may just have to make a separate myth backend...

Brian, those are some really useful points.  Luckily, the computer is a
home-build, so I think the motherboard mounting is pretty solid.  I don't
currently have ferrite cores on my HDMI cables.  I'll give that a shot.  The
case is an Antec HTPC, so it does have some connectors on the front as well
as an LCD, but I cannot pull those.

FYI, I found a post on a NZ HTPC board about homebrewing a shield out of
plastic-wrapped foil.  I tried it with aluminum foil in a plastic sandwich
bag - no improvement.

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