Question Weird a/v "noise" ?

klandingham

Distinguished
Jun 10, 2011
3
0
18,510
0
Greetings Hardware Gurus.

I hope you're out there...

This one's got me stumped:

I have several PCs, some of them a bit old, hooked up to a Startech 8-VGA-port KVM. This KVM uses special cables which carry the USB bus for the connected PCs on unused VGA pins. Each input on the KVM consists of a single (special) VGA connector and a standard 3.5mm stereo jack (and nothing else). On the PC end, the KVM cable has both a VGA connector and a USB connector. The audio connections are made using ordinary male-to-male 3.5 mm stereo cables.

So here is the mystery: When I switch to one particular machine, I hear a whine in the audio, and the screen shows what look like faint horizontal beat bars which float up and down erratically. The whine isn't very loud and the audio is still working, but with the whine constantly in the background it's hard to listen to anything for very long.

The beat bars have soft top and bottom edges, they're very faded, and the video is otherwise fine, but you can plainly see the bars in the background. The audio whine "warbles" a bit, about once a second. It sounds like a triangle wave of some sort, and it's around 600 Hz. Changes in the screen seem to modify its frequency, but not by much.

The machine in question is running a Gigabyte G31M-ES2l motherboard. If I hook it up normally (without the KVM) it is fine: the audio is quiet and clean and there are no discernable beat bars on the display. Testing with the KVM, with everything plugged in on the PC end, I found that the noise starts as soon as I connect the VGA cable to the KVM, and immediately stops if I disconnect that cable.

Things I've tried:

  • checked the grounding on the KVM and the machine...all fine
  • plug the problem machine into a different input port on the KVM - same behavior
  • swap the KVM cable with that of another machine - no difference (other machine fine, problem machine still whining)
  • connect a different machine to the "problem" KVM port using the "problem" cable - no issues
  • tried different audio cable - no difference
So it seems that something is happening because of some weird interaction happening between that particular PC (motherboard?) and the KVM switch, but what could it be?

Any ideas?
 

klandingham

Distinguished
Jun 10, 2011
3
0
18,510
0
emi interference
try ferrite beads
mm-hmm. But, not sure where I'd be placing ferrite beads. The special KVM VGA/USB cables are sealed.

As I continue to ponder this, I'm starting to suspect there may be some kind of high frequency oscillation happening in the audio output circuitry of the motherboard, and I'm hearing (and seeing) some low-order harmonic of it.

Thanks for the suggestion.
 

ASK THE COMMUNITY