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Old 05-20-2008, 11:46 PM   #1
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Weird Noise in my Speakers..


Hey everyone, my first post here! I am really in a pickle!

I am running a recording studio and my monitors are exhibiting a windy/papery sounding noise whenever I plug in my external flatscreen monitor, or my digital piano via USB. It seems like the noise comes whenever I plug in anything with its own 3pin AC power cord.

So far ive swapped out the speakers, the flatscreen, and I put the whole system on a backup battery and shut the power off to the building to rule out EMI from lights or something. The noise persists!

The setup currently is stripped to the bare minimum for troubleshooting:

-MacBook Pro Laptop connected to power strip with 3-pin AC cord
-External Firewire BUS powered audio interface (this is plugged into the MacBook Pro via FW400 and it plugs into my speakers via one 1/4" instrument cable each). It is not grounded independently.
-Speakers are plugged in to the surge protector/power strip via 3-pin AC cord
-Flatscreen LCD Monitor which is plugged into the powerstrip via 3-pin AC cord as well as plugged into the laptop via DVI connector.

Here is a sample of the noise I am getting. It is not effected by the volume knob on my audio interface or the laptop.

http://www.megaupload.com/?d=7UJQ3LSE

Thanks everyone!!!! I have been fighting this problem for 2 weeks! I'd really like to get it fixed and I think you all are my best shot =)

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Old 05-21-2008, 08:16 AM   #2
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Weird Noise in my Speakers..


Well, when I follow your link to MegaUpload, I just get a home page. Lot's of nice looking girls though!

I drew a picture of what the equipment arrangement is based on your description and it looks like the culprit may be a ground loop. Does the sound go away if you disconnect the HDMI cable from the Mac? Also, is the monitor close to the speakers and/or 1/4" cable?

Let us know,
Jimmy

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Old 05-21-2008, 09:20 AM   #3
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Thanks Jimmy!

If you go back to that page, there is a spot in the upper right (to the right of the MEGAUPLOAD logo) that you have to put in a 3 character code, then it takes you to a page where you have to wait 45 seconds for the free download link to appear. Sorry, I wish I had webspace =)

Here are two images of my setup, it might be better than me explaining things.

The noise only happens when I plug in my flatscreen via DVI. Once unplugged, the noise disappears. The noise also happens when I plug in my digital music keyboard via its USB cable. Both the Screen and the Piano have their own 3pin AC cords.




Also, the noise only happens when the laptop is up and running while the above criteria are met. However, the screen can be turned off (as long as its still connected to the laptop and to power).
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Old 05-21-2008, 09:30 AM   #4
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Weird Noise in my Speakers..


What if you plugged in things to different circuits? It could be a ground loop. Also, does the mac have a microphone? Pointing a microphone at a speaker will get you feedback.
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Old 05-21-2008, 09:56 AM   #5
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Originally Posted by darkecho View Post
The noise only happens when I plug in my flatscreen via DVI. Once unplugged, the noise disappears. The noise also happens when I plug in my digital music keyboard via its USB cable. Both the Screen and the Piano have their own 3pin AC cords.

Also, the noise only happens when the laptop is up and running while the above criteria are met. However, the screen can be turned off (as long as its still connected to the laptop and to power).
Ground loop. I used to see this a lot when I did serial-based SCADA work. Sometimes we had to cut the signal ground to break a loop. Unfortunately, I don't know that much about the HDMI standards yet.

There appear to be FW isolation cables available. Evidently, they provide some form isolation to the extent that you cannot use bus-powered devices downstream. I don't know the details of these so I don't know if it would help. Does your monitor have component video inputs? If so, you may want to try using them although you'd need some sort of HDMI to component video converter is the Mac doesn't have comp. video outputs(as I wrote this I checked and found some converters although I also found a website that says that it cannot be done [digital to analog conversion] due to copyright reasons).
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Old 05-21-2008, 11:01 AM   #6
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I thought a ground loop happens when you have two devices on different circuits connected together? Doesnt a ground loop sound like a loud "MMMM" versus a statiky/paper/wind noise?

Were you ever able to listen to the sound clip?

How might I test to see if it is indeed a ground loop exclusively?
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Old 05-21-2008, 11:13 AM   #7
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I thought a ground loop happens when you have two devices on different circuits connected together? Doesnt a ground loop sound like a loud "MMMM" versus a statiky/paper/wind noise?

Were you ever able to listen to the sound clip?

How might I test to see if it is indeed a ground loop exclusively?
The MMMM is 60hz but remember that the HDMI is passing digital data and far more excessive rates. In short, we're probably dealing with a ground loop at the serial level (BTW, you'll find a lot of similar discussions if you google "HDMI isolator."). You could try looking up a pin out diagram on-line and removing the ground fingers (I think there are multiples and possibly shield/drains as well) from one end of the cable.

BTW, when you wire up instrumentation in power plant or process control applications, you typically use a shielded cable but you never ground more than one. Double-ended connections of shields/drain wires can cause ground loops that corrupt/interfere with the low-level process signals (i.e. 4-20mA, 0-10vdc, etc.). Same principle.
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Old 05-21-2008, 11:24 AM   #8
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Using an extension cord, I plugged the flatscreen into a different outlet in the house and got a different noise, it was more like the humm at 60hz you mentioned. But when everything is plugged into my powerstrip and then into one wall outlet, the noise changes back to the highpitched windy sounding one again.

Is there any way to break the ground loop without a safety risk?
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Old 05-21-2008, 12:03 PM   #9
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Is there any way to break the ground loop without a safety risk?
Do NOT defeat any of the grounding on the AC plugs.

I'm guessing that the most straightforward approach would be to plug your flat screen into an isolation transformer (Tripp lite is just one manufacturer). Unfortunately, there can be an issue with capacitive coupling due to the chassis/shield and the coupling increases with higher frequencies. That being said (and again, I know very little about the characteristics of the HDMI signals), this may not work.

As I mentioned, you may want to check out some of the home theater forums as this appears to be a topic of discussion and someone therein may have a cleaner solution.
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Old 05-21-2008, 12:07 PM   #10
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Thanks Jimmy

I dont know if this helps or not, but the same noise happens when the I plug my digital piano into the system, (screen is disconnected completely from the system, so its not interacting with anything at this point) So maybe that takes out the variable of the HDMI/DVI stuff you were talking about. The digital piano is connected to the computer via USB and the wall via its AC cord.
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Old 05-21-2008, 12:45 PM   #11
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I brought my laptop and one of my speakers with me to work for a little test today.


I plug the speaker in via its 1/4" unbalanced instrument cable into a 1/8" adapter into my MacBook Pro's headphone jack.

I then connect an LCD Flatpanel to my MacBook Pro via a USB cable (allows me to use the USB hub built into the screen)

Then I plug the speaker and screen into the same power strip.

So I have two devices plugged into the same power strip, and then both of those devices connect to my computer through their respective cable.

I get the same noise I get at home (This rules out the possibility of the noise being interference by any equipment at my house). When I unplug the USB cable, the noise goes away. Silence.

I work in an IT room, so I decided to swap out the MacBook Pro for a Dell D600 sitting across the room.

There is some hiss when I plug the speaker into the Dell's headphone jack, but when I plug/unplug the USB cable into the Dell, the noise doesn't change. This leads me to believe that the Dell simply has a noisy headphone jack, and that the problem in my system is the MacBook Pro!

I did this test with several other PCs, interestingly, I found that a regular MacBook has the same issue. I wonder if there is something going on with their internal design. The MacBook and MacBook pro are similar ages and are from the same year (mid2007). I was thinking that the MacBook Pro's aluminum case might be an antenna for EMI or something, but the regular MacBook, which also exhibits the same noise, has a normal plastic body.

Very interesting.
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Old 05-21-2008, 02:13 PM   #12
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Thanks Jimmy

I dont know if this helps or not, but the same noise happens when the I plug my digital piano into the system, (screen is disconnected completely from the system, so its not interacting with anything at this point) So maybe that takes out the variable of the HDMI/DVI stuff you were talking about. The digital piano is connected to the computer via USB and the wall via its AC cord.
Whether you are plugging in the piano or screen, you are connecting back to the computer and therein creating the loop.
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Old 05-21-2008, 02:39 PM   #13
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hey Jimmy,

Ok I see now..

What do you think about my most recent findings? I think the evidence strongly points to something being amiss in the MacBook Pro specifically.
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Old 05-21-2008, 04:42 PM   #14
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What do you think about my most recent findings? I think the evidence strongly points to something being amiss in the MacBook Pro specifically.
I hate to beat a dead one, but I still strongly believe that it's a loop. The fact that disconnecting the data cable to the TV kills the noise seems to prove this theory as that would be opening one of the redundant paths. If it was a problem with the sound card in the Mac, wouldn't you think that the problem would persist regardless of the connection of the monitor?

Besides, my wife is a devout Mac user. I'd never suggest that her hardware was the cause of any problem she was having!

I did think of one thing that you could try. Get one of those "2 to 3 cheaters" and plug your monitor into it. Now plug that into your rubber-plugged extension cord and the cord into the strip (you don't necessarily need to do it this way. You're simply trying to break the ground path without removing the pin). See what happens. If the noise goes away, you will definitely know that it's a ground loop. Once the experiment is over, go back to the normal configuration.

Let me know what you find,
Jimmy
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Old 05-21-2008, 04:47 PM   #15
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I know what you mean about mac users and their hardware =)

I just find it strange that it wouldn't be laptop specific if the noise disappears when I use the 6 other laptops.

Just to clarify- I am connecting the laptop to a flatscreen computer monitor, not a TV. Everything is running off of the same powerstrip in the same room.

You still think its a ground loop even though the noise goes away when I swap the macbook out for one of 6 other laptops under the exact same circumstances?

Maybe its a ground loop, and the macbook just doesnt handle the noise as well as other brands of laptop?

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