[mythtv-users] Interference lines in all recordings

Tim Gray tim at p-a-a-i.com
Tue Feb 13 18:03:39 UTC 2007

I found a solution to my problem. 

After lots of tests, adding in aluminum shields, soldering the shield 
cans, and not getting rid of the interference I discovered the problem 
that may help others.  Regular TV's and the ReplayTV have no sensitivity 
to this so it has to be a PVR-350 specific problem...

My signal coming in the house is too high. Specifically on the lower 
channel numbers.  I had a 18+DB difference between channel 2 and channel 
96  adding in a tilt compensator where the cable comes in the house 
eliminated the problem by reducing the signal of the lower channel 
numbers and not affecting the higher channel numbers signal strength.  
Granted I have tools at my disposal that most of you do not such as a 
NTSC/QAM signal strength meter/ cable plant analyzer but adding in a 
$8.95 tilt compensator to your cable may solve some interference 
issues.  It certianly did for me.  I am actually planning on adding a 
overall attenuator to my cable line as well to reduce the signal another 
3 DB across all channels as it is still higher than reccomended.

Tim Gray wrote:
> I was silly and built a nice tiny setup. 2 pci slots only and it's a 
> micro itx with everything built on board as it's a backend recorder 
> only. I have not tried the move the wores around although at the start I 
> tye wraped all wires away from the cards as far as I could go assuming 
> that anything inside the case will mess with the cards to begin with.  I 
> will try soldering down the can caps, I am sure it will help if it does 
> not fix the problem.  I also never thought of the over/underclocking 
> thing.  I'll give that a try and see what happens with the recordings.  
> I was also thinking of cutting a piece of aluminum and grounding it, and 
> wrap it in electrical tape and slide it in-between the two cards just in 
> case they are interfering with each other.
> the 3 motherboard screws are tight, the rest are those teflon standoffs.
> Thanks a ton for your help and ideas, I'll see what happens with all 
> these different suggestions and report back.
> Brian Wood wrote:
>> On Feb 9, 2007, at 1:14 PM, Tim Gray wrote:
>>> Thanks a bunch, I already ran that route...
>>> Cables are hand made RG6quad with firmly crimped connectors, they are
>>> good.  they go into a single splitter to take the signal into both  
>>> cards
>>> whuch works good when you view from a TV on the same.  I also have  
>>> +10DB
>>> CATV signal coming in as far as my NTSC/QUAM cable analyzer is telling
>>> me, although channel 96 is only +3DB so there is a little bit of
>>> difference from one end to the other.
>>> signal coming into the tuners is verified to be 100% interference free
>>> by hooking up a replayTV and viewing recordings that are 100% clear.
>>> which leads me to be sure the interference is coming from inside  
>>> the PC.
>>> Has anyone found a way to shield the cards better or ground them  
>>> better
>>> to get rid of PC generated interference?
>> Great, it sounds like you know what you're doing, but I had no way to  
>> know that at first :-)
>> I assume you've tried moving your PVRs to different slots, if you  
>> have them. I'm guessing you're getting interference with the baseband  
>> video on the PVRs and not RF into the tuners, but it couldn't hurt to  
>> make sure the can tops are tight, or even solder them.
>> What sort of PC is this? Could you re-route internal cables to try  
>> and get them away from the PVRs?
>> Otherwise you are talking about trying to put shielding material  
>> around the cards, not any fun but it might be your only solution.
>> Also make sure all the motherboard mounting screws are tight, loose  
>> connections can act as detectors and cause trouble.
>> If there are any cards in the PC that you can get rid of do that as  
>> well, but I suspect you've tried that if it's even posible.
>> If there's any way you can narrow things down to what portion of the  
>> PC is generating the interference it would help. For example if you  
>> could boot without a graphics card and SSH into the machine to make a  
>> manual recording you could prove or eliminate that card as the source  
>> of the problem.
>> This one's a little off the wall: If you can change the frequency of  
>> the interference you might cause the beats to be moving fast enough  
>> to at least be less disturbing. Under/Over clocking the machine by  
>> 5%-10% might cause a significant change in the perceived if not the  
>> measured interference.
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