Soundcard interference but only over KVM?
July 10, 2007 6:12 PM   Subscribe

I need help figuring out a strange audio interference issue with my computer setup, involving headphones, a KVM, and an apparent contradiction.

The setup: iMac, custom-built gaming PC, nice pair of headphones, plenty of cables. Until today, I had speakers hooked up to the iMac and the headphone jacked into the PC, and everything was fine. Today, wanting to use the headphones on both machines, I bought an ioGear KVM which includes audio switching, hooked it up, and now the PC - and only the PC - is generating irritating background noise.

So, with headphones on, I have the KVM set to the iMac. The connection goes headphones -> KVM -> KVM cable -> iMac. The sound is perfect. I then switch the KVM to the PC's port, and get a decently loud background hum/buzz noise, including various additional sounds when the mouse moves, hard disk spins up, network activity, etc.

Plugging the headphones straight into the PC results in perfect quality, as before. So if it's not the sound card (or the problem would exist with the direct connection) or the KVM cabling (or the problem would exist with both machines) then what is it?

Additional caveats: I think I can rule out nearby cable interference and the sound card, as both KVM connections have other cables nearby, and the issue happens with both my onboard and expansion sound cards (but to a lesser extent with the onboard). I also recall a similar issue with other KVMs in the past.
posted by cyrusdogstar to Computers & Internet (7 answers total)
First thing I would try would be switching the Mac and PC on the KVM, if you haven't tried this.
posted by 6550 at 7:17 PM on July 10, 2007

I think you probably have a cold solder joint or a bad crimp joint on one of the connectors in the KVM, probably on a grounding pin, that is allowing the KVM to float above ground. But you may have grounding problems in the PC itself.

To check this out, you could look for ground problems between the PC case and your house ground with a DMM (digital multimeter), or you could connect external ground wires between the KVM and the computer chassis and the house ground. You might want to open up the KVM and check its internal wiring, too.
posted by paulsc at 7:23 PM on July 10, 2007

Are the KVM and iMac plugged into the same electrical outlet, and the PC on another? You may have a ground loop condition. Try putting them all on the same power strip.
posted by contraption at 7:30 PM on July 10, 2007

Response by poster: Sorry 6550, that must've been discarded during edits - I've tried two different KVM cables between KVM and PC (one expansion cable and one built-in cable - the KVM has 2 of each) and it changed nothing.

paulsc, how would a grounding issue in the KVM cause problems with just one of the connections?

contraption, the KVM isn't a powered variety, but everything on my desk that is powered is plugged into a single power strip.
posted by cyrusdogstar at 8:47 PM on July 10, 2007

Just to doublecheck you've also tried switching the KVM connections as well, as in if the Mac is hooked up to "A" put the PC on that one. Sorry, but it wasn't quite clear from your description.

Assuming you've done that what happens if you take the KVM out entirely except for the audio connections? If you don't get noise then it's likely interference with another signal.

You do say it's not a powered variety but does it get power from USB? If it has any electronic switching it has to get power from somewhere. Can you post a link to a pic or info on the model?

Regarding Paul's reference to grounds, grounding problems/loops/etc can cause all sorts of weird and hard to troubleshoot behavior.

If both computers are plugged into the same strip along with the monitor try unplugging every other device to see if that changes anything.

Good luck! I know these types of problems can be extremely frustrating.
posted by 6550 at 10:23 PM on July 10, 2007

"paulsc, how would a grounding issue in the KVM cause problems with just one of the connections?"

As 6550 points out, if the KVM is using electronic switching, it is using power from something even if just USB from the PC or Mac. The "right" way to avoid grounding issues is to A) make every element in a system reference the same electrical ground, and B) to make sure that every component is properly grounded to a physically common point. Up to the limit of a branch circuit's capacity, you can make sure of B by plugging every thing into grounded power strips from the same outlet, if that outlet is itself properly grounded, and all the devices use grounded plugs. But some computer devices don't require grounding to achieve safety protection, because they are double insulated or isolated from AC voltages by outboard AC-DC converters, such as those that ship only with "wall wart" or "brick" type power supplies, which have only 2 prong connections. In these devices, to take care of A, you may have to interconnect them for chassis ground, or optically isolate them. If they are connected to devices which have small leakage currents that normally drain to ground, but which for some reason aren't being grounded properly, they can quickly elevate to 10s of volts above ground on their chassis, which is enough to swamp signal ground. Your PC for example, might have a 3 prong power cord, and require a grounded outlet for safety and noise protection, whereas your iMac and your KVM may be 2 prong devices.

To make all devices in such a setup play nice, you may need to ensure that chassis ground is, in fact, common electrical ground, for all devices.
posted by paulsc at 10:41 PM on July 10, 2007

really does sound like a grounding issue, headphones (and speakers) can sometimes connect directly to the casing instead of the audio jack's ground. there's also various wiring errata when you add devices like a KVM, each socket, cable and connection can be a problem. with "background" noise from the PC,

a quick fix, if you have a newer PC, with front port audio connectors, use that connector instead, its likely to be better insulated.

it could be that the KVM's connectors aren't insulated on the PC side, or vice versa, the imac side, and the electrical noise from the PC is being sent along the audio ground in the KVM's setup. if you've OK'd the cable being used, and the connectors are all secured, check the internals next.

have you checked the audio port of the KVM ? i.e. plugging in an ipod/casette/whatever for the pc input side, see if the "noise" is still there through the headphones.
posted by toliman at 11:06 PM on July 10, 2007

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