What type of picture interference am I seeing here?

[mvdude]

I have a Sapphire Theatrix 650 capture card installed in an Acer desktop with an AMD Athlon 64 X2 3800+ processor. I'm running WinXP SP2. I'm also using the SageTV software.

Picture on all channels is great except for one issue. I get the problem shown below on channel 24 (Comcast US HRC) and also on channels 17, 18, and 19. In addition to the white bars, I get occasional lines of horizontal distortion in the picture.



I know these channels are susceptible to CB / Ham radio interference, but I don't believe that's what this is. Here is my troubleshooting so far:

* High-pass cable filter: no improvement.
* Portable tv on same cable line: does not show the problem.
* Connect main cable antenna directly to capture card: no improvement.
* Different PCI slot: no improvement.
* Checked cable and connectors: all in good condition.
* Different versions of ATI's WDM driver: no improvement.

So far, this leads me to believe the problem is in the driver, or in the capture card, or being induced by my system hardware. I do not see examples of the problem shown on any of the picture quality guides on the internet (this one, for example.)

Does anyone recognize this type of interference?

[mvdude]

I'm going to answer my own question in case this helps somebody else. Those white bars are classic video hum bars caused by a ground loop. Lots of info here:

http://siber-sonic.com/electronics/GLoopwhatis.html

Thanks to the SageTV forum (great product, by the way) for helping me out on this. Now that I know what the problem is called, I can fix it.

[Alex_ander]

What you see comes from 60Hz sine mains voltage added in small proportion to analog video signal. The reason is that if a coax cable shield is grounded from both ends, AC interference current can flow via that shield between 2 differently grounded devices (e.g. grounded by protective earth wire in mains cable). This results in voltage difference between the 2 ends of the shield wire and that voltage is directly added to video signal divided in half between 2 termination resistors (75 Ohm). That's how ground loop appears (I found the explanation in the referenced page not as clear). To reduce the problem, isolation video transformers (an expensive solution), or differencial amplifiers can be used on reception end. This breaks the circuit for interference AC current.

[mvdude]

I'm planning on trying this cable isolator device. Supposedly it will break the ground. I'll report back with the results.

[mvdude]

To make a long story short, my particular problem is not a ground loop. It is probably some type of induced AC interference, but ground loop is not the source. I tried isolating the cable and also tried isolating the AC ground. Problem is still there.

About the cable isolator device:

Either the connectors or shielding on this device are poor and are allowing RF ingress. I was surprised that it is encased by plastic and not metal, but perhaps there is some shielding inside? At any rate, adding this device to my cable antenna makes my TV picture look exactly the same as when one of my coax connectors is loose: bad herringbone interference over multiple channels. Also, the threaded connector seems to be loose and it spins around in the casing. This makes it very difficult to properly tighten the F connector. It also makes me question the integrity of what's inside the black plastic case. Could something be broken if the connector is rotating freely?

I've contacted the company to get their thoughts, but I'll probably end up doing a return. Not because it didn't fix my hum bars (the device has other benefits) but because it seems to be poorly made. But maybe I just got a faulty unit.