What would cause ripple interference seen on on a display whereby the computer is attached to it via VGA?

By ripple interference I mean that the edges ripple and the display flickers. It is occasional and intermittent and it may occur for a few seconds to a little longer.

My set up - the problem is seen with a 2010 17" MacBookPro attached to a KVM switch via a DisplayPort to VGA adapter. 2 other machines are attached to the KVM switch.

The problem has only begun recently, up until then this arrangement worked very well. The other 2 machines (Windows 7 and XP-based) don't show this problem when they are using the display.

I don't want answers advising to move to a HDMI or DVI based KVM switch for 3 reasons: 1) because I am reluctant to make additional purchases and 2) this arrangement has worked well for at least 6 months now and 3) One of the machines only supports VGA.

So to recap, what would cause interference on a VGA signal? Could it be a hardware issue?


It could be poor grounding of the computer or monitor, cheap cables, cables not screwed on tightly, or the KVM beginning to go bad.

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    Did you make any changes to the way power is wired? Cables? Power strip? Different outlet? – David Schwartz Feb 8 '12 at 9:23
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    It may help to plug the KVM, monitors, and computers into the same power strip. It may also help to add a ground wire between power strips if you use more than one. Make sure no power plugs are loose. – David Schwartz Feb 8 '12 at 9:54
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    Maybe things that were plugged into the same strip/outlet before now arne't? Or maybe something isn't grounded. – David Schwartz Feb 8 '12 at 10:35
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    Check for any power cabled 'wrapped' around your KVM/VGA cables. – HaydnWVN Feb 8 '12 at 10:48
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    Good thinking! Even a long parallel run can do it if they're really close together. You don't want your power lines and your video lines running together. – David Schwartz Feb 8 '12 at 10:49

I was also having this problem but i wasn't taking it serious until both of my bluetooth trackpad and keyboard also malfunctioned. I found out it was a problem caused by the interference of the signal from my router, so i test out the best frequency that would make all of them work together by changing the channel on my router, and it worked.

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    rippling in the VGA is VERY unlikely to be caused by a router.. – Simon Sheehan Nov 8 '12 at 2:38

I haven't seen the problem since after 1) Getting my MacBookPro serviced (while I watched) at Genius Bar at my local Apple Store, Southampton, UK, the servicing being that they ran some diagnostics (didn't find any problems) but reset some low level stuff for me, though I still saw the problem after that. And 2) Issuing this command in the Terminal app on the Mac:

defaults write NSGlobalDomain AppleFontSmoothing -int 2

And rebooting my Mac.

This second piece of advice (i.e. that command above) is the answer to my same question on apple.stackexchange.com (a more specific forum for Macs that I had to resort to as I had tried everything else suggested (thanks!).

The question is here: https://apple.stackexchange.com/questions/45671/interference-on-external-vga-monitor-intermittent-ripple-only-when-booted-into

And the solution is here: https://apple.stackexchange.com/a/45672/4018

I still can't pinpoint the exact root cause but it has vanished. The best conclusion I can make is that the combination of the Genius Bar servicing and the command shown fix the problem. Now, that command doesn't seem to me much to do with display configuration however its setting may cause a different code path to be executed that does make the Mac behave in a favourable way (i.e. the ripple goes away).

I'm sorry @David Schwartz from moving the accept away from your answer. Your answer is definitely definitely on point and suggests things that one must do and to eliminate. However the problem prevailed after that and this answer seems to have fixed it and therefore is closer to the root cause. I would also want others with same problem to see this answer as the accepted. I have +1 upvoted your further comments where I can to compensate :)

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    Accuracy of information, complete answers along with a solution for the OP are the main aim's here... It's not really about points! Many thanks for updating your question, answer and adding the complete solution. Your information will greatly help others! :) – HaydnWVN May 14 '12 at 9:00
  • +1 @HaydnWVN totally agree. Thanks to you and everyone else for input. – therobyouknow May 18 '12 at 15:04

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