[mythtv-users] Interference lines in all recordings

Brian Wood beww at beww.org
Fri Feb 9 19:59:30 UTC 2007

On Feb 9, 2007, at 12:35 PM, Tim Gray wrote:

> I know this is not mythtv specific.  But someone out there has had to
> deal with this and I cant find anything out there that talks about it.
> I have a mythtv box with 2 PVR-350 cards  It records great except all
> recordings have a diagonal set of interference lines recorded in them.
> each channel has the lines at different angles some have them very
> faint.  How can I get rid of these?  I am sure it has to be a problem
> with the PC generating it as nothing else in the house shows those  
> lines
> when watching the same channels live.
> copy the file to a laptop, see the lines, so they are getting  
> recorded.
> Is there a way to get rid of the interference from the PC getting into
> the tuners?

First do all the obvious stuff:

Make sure you are using good quality shielded cable with the correct  
connectors crimped properly (ie: not fishing line crimped with a pair  
of pliers).

Make sure you don't have any extensions  with 300-ohm twinlead  
running under a carpet or the like (don't laugh, I've seen it).

Make sure that all connectors are tight, either very firmly hand- 
tight or very gently wrench tight. The connectors on capture cards  
can be hard to reach and get properly tightened but you have to make  
sure they are.

Make sure any unused ports on splitters or couplers are properly  
terminated with a 75-ohm termination.

Make sure your computer case is properly closed up with the screws  
in, not running with the side off or the like.

In other words make sure all your shielding is tight, both on the  
cable system and the computer itself. It sounds like radiation from  
somewhere is getting into your PVR tuners, it may be from your  
computer or something else but your supposition is likely.

A tightly shielded system going into the PVR tuner, which is  
shielded, should not pick up outside sources of signal (much).

If all this is OK you might make sure the press-on top of the shield  
can on the PVRs is tight. I once had one where I had to put a dab of  
solder on each side of the can lid to clear up a similar problem to  
what you describe. This is probably not your problem since it's  
happening on 2 cards but I thought I'd mention it

A monitor cable can sometimes be a source of radiation, using a good  
quality cable helps, especially if you are using an extension. If you  
are feeding a TV set separate from your computer monitor try  
disconnecting the monitor cable from the computer and see if it helps.

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