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When I connect an external monitor to my new MacBook Air via Mini DisplayPort to HDMI, I see the extended desktop but there are snow-like artifacts. When I change the resolution to 720p, the screen looks normal.

I have tried other standard resolutions (1024x768 and higher) with the same effect. I have also tried toggling between 50hz and 60hz, 1080i and 1080p, each of which causes the same issue.

I get the feeling it has something to do with my relatively cheap adapter cable, but am unsure. Thoughts?

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The adapter might be defective. I have an Apple adapter and a similar monitor, and I had a problem similar to this one: ofter I got snow as soon as it was switched on, and after a random amount of time it started working (and never stopped working until it was shut down again). I didn't have this problem at first, and it worsened with time, plus touching/tweaking the adapter sometimes fixed it, so in my case it most definitely was the adapter itself. –  Lohoris yesterday
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2 Answers 2

up vote 3 down vote accepted

Likely to be the adaptor cable if it's cheap - might not be able to handle the HD throughput (although outputting to HDMI, you'd think it'd be able to).

While this sounds counter-intuitive, try moving around the alignment of the second screen as this might ditch the 'snow'.

Also, have you tried the screen with any other HD inputs?

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I have two identical displays and both exhibit the same problem. I am unable to manually adjust the picture alignment (with the exception of an overscan setting which has no effect). I've also used my Apple TV (2) on both screens and have successfully played HD video, so the screens are 1080-capable. It looks like it must be the cable. –  Paperjam Jan 30 '12 at 22:09
    
Definitely sounds like crosstalk or capacitive interference in the cable. Since increasing the resolution alters the frequency at which the digital signal is transmitted, this in poorly shielded cables will induce a feedback signal causing strangely interpreted data on the screen –  Stu Wilson Jan 31 '12 at 16:43
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You may have mirroring on.

With mirroring it seems to only match the resolution of your internal display (1440 x 900, native).

This doesn't always scale up the best to a 1920 x 1080 monitor and at 23" you might start to see some pixelation when you zoom in.

I noticed this on my 22" Acer with my 2012 13" MacBook Air.

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