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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?

I do not know what connector you currently have in your MacBook Pro (it will depend on the model generation).

However, I carry something similar (older version) to this on my business trips for my laptop presentations, since I can encounter many types of displays - it works perfectly with either a MacBook Pro and my Lenovo laptop with a Mini-Display Port connector: VicTsing Mini Displayport Thunderbolt to HDMI & VGA & DVI TV HDTV Video Cable Converter - White [DP V1.2 Version/4Kx2K] - 632423280846 … an inexpensive product!

Here is the image of the product:

It connects to the Mini-DisplayPort on my laptop on the cable end, and allows me to connect to just about anything I am encounter for a display (projector, large monitor, Smart TV, etc.).

It also works perfectly well with my LCD TV at home using at 1920 x 1080 resolution with a relatively long (10 feet), standard HDMI cable.

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it could be the video is loosing synchronization in that mode.

long shot but from the display of the 2nd monitor open

>System Preferences>Displays

Hold down OPTION and click Scaled and you should see additional video resolution options. try a different resolution on the 2nd monitor.

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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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