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I am looking for a way to have a third monitor on my 2008 iMac (iMac 7,2).

I have found some USB solutions that extend one output, lets say, the iMac miniDVI port, so the image will fit two monitors. I don't like this, for several reasons:

1) this solution requires both monitors to have the same resolution and the two monitors I want to use, have different resolutions and I don't want to reduce one to be like the other.

2) this solution will split one image to fit two monitors, so OSX will think it is one monitor and start doing annoying stuff as showing alert boxes on the middle of screen (half of the window on one monitor and half on the other) and stuff like that.

Said that, I ask you nice guys: there's some solution that can create a new "virtual" output that is seen by the OS as a third output? As this monitor I want to use is VGA, the solution have to work with this output up to 1280x1024 pixels.

thanks

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Is an external USB graphics card not what you want? e.g. http://www.gefen.com/kvm/dproduct.jsp?prod_id=8863 Although I've never used one, they ought to behave just like internal graphics cards - I see no reason why they'd need both monitors to have the same resolution or split one image. Are you sure you're not thinking of something like a Matrox GXM? That does have those limitations, but it's connected with DisplayPort - essentially it presents itself to your iMac as a monitor twice the size of one of the individual monitors you're connecting to it.

In short, though, you will need an additional graphics card output. Since changing the internal graphics card in an iMac would require (at the very least) cutting extra holes in the case, it's not really a practical proposition. Your only option would be a USB graphics card. I doubt all of them have the limitations you describe, even if you've found one that does. Shop around, look at different models, and, if you can, go to a physical shop and get it demonstrated to you.

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this is the specification of the gefen model you mentioned: "The extra displays can be configured either to mirror the primary display or to show the extended desktop, allowing additional visibility of more applications that can all be seen at the same time"... here we go. When they call "extended desktop" means, X monitors that are seeing by the OS as one and X monitors that have to be configured with the same resolution. You cannot extend a window to several monitors and have different resolutions. It is like having a 5000 x 1024 display that is cut in X monitors. Will not work for me. –  SpaceDog Jan 18 '11 at 14:59
    
... continuing... this is why I have posted it here, because I have shopped around in zillions of sites and found nothing. –  SpaceDog Jan 18 '11 at 14:59
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@Mike - That's not what extended desktop means, at least in Windows...I have no experience with this stuff on Macs so drivers may vary, but every USB->DVI connector I've used in Windows has behaved EXACTLY like an internal graphics card would (albeit one with lousy hardware acceleration), which is to say you can have multiple displays at multiple resolutions, all totally independent. –  Shinrai Jan 18 '11 at 15:28
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Are you sure that's what they mean by "extended desktop"? I've seen that term used - mainly with regards to Windows - to simply mean non-mirrored displays. I believe the Windows preference is labeled "extend my desktop onto this monitor" or something similar. –  Scott Jan 18 '11 at 15:30

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