DVI Audio Noise – High Pitched Whine

So, while working on Return to Northmoor (northmoor.spookyouthouse.com) this week, I’ve been troubleshooting a very annoying high-pitched whine that has found it’s way into my audio stream.

It sounds a lot like high-speed Morse code.
I was able to determine that it was only present in the output from my computer and not while recording, which was good, but it was still driving me mad.
Eventually, by plugging and unplugging every component of my PC, I was able to determine that the noise was coming from my video card.  When I unplugged the cable to my LCD monitor, the noise about doubled.  I swapped out my high-end eVGA video card for a cheap one, and voila, no noise.  Still, I didn’t want to give up my good card, so I did some more research.  I’m not sure if what was going on was a ground loop, or DVI noise being directly conducted into my USB/FireWire stream by the video card (which sits right above one of the USB sockets on the motherboard), but switching to balanced audio cables killed off the noise.
The noise was getting in between the audio interface (an M-Audio Profire 610), and my powered monitor mixer.  I had been using nice (Monster Cable) unbalanced cables to connect them, but for whatever reason (conduction or ground loop), they were susceptible to the noise from the video card.
Even though the ProFire only has 1/4″ outputs, they can accept a balanced Tip-Ring-Sleeve (TRS) plug.
A balanced cable uses two wires + ground to carry the audio signal instead of one + ground.  Equipment that can work with balanced cables can detect if there is identical eletromagnetic interference on the two wires, and reject it.
Here’s a picture of one of the cables I am using now:
You can tell that it’s a balanced cable because the 1/4″ plug has two bands on it, hence tip, ring (between the white bands) and sleeve (the rest of the plug and the connector housing).
You may have seen connectors like this for stereo audio cables.  If you use them for two channels of audio (stereo), then it’s two unbalanced channels in one cable.  Whereas I’m using two cables like this, one for the Left channel, and one for the Right.  So I have to use two cables, but each one is balanced, and this less-susceptible to interference and noise.
The other end is an XLR connector that plugs into my mixer.  XLR connectors are a hallmark of balanced cables, they are almost always present in a situation where you are going from a mic to a mixer or pre-amp, because mics have very low signal levels, and thus noise and interference can entirely swamp a signal.  Not to mention that by the time you amplify the signal a great deal, the noise gets amplified as well.
So lesson learned, used balanced cables where ever you can around computer equipment!
Tim

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