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I am interested in extending my multi-monitor setup, but I was wondering if anyone could shed a light on how external USB graphics cards work?

I have extensively searched but have not found an answer, any help would be greatly appreciated.

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Do you have USB 3.0? – cybernard Nov 9 '13 at 14:32
Any 3d graphics are out of the question. The USB 2.0 link it way to slow for any decent performance. USB 3.0 has a lot of bandwidth, but still the graphics chip is very weak and not suitable for any more than desktop work. – cybernard Nov 9 '13 at 14:36
I have USB 3.0 @cybernard – chris.fy Nov 9 '13 at 18:50
up vote 4 down vote accepted

Most of these are made by one company as far as I can tell - displaylink and its quite simply a chip that connects to the system over USB and appears to be a video card. Since the bandwidth available to USB isn't very much (though its better with USB 3.0), and any onboard processing is relatively anemic, rendering is done on the main system, then sent over to the chip in compressed form, and it de-compresses it and displays.

Most folk report that its acceptable for 'office' style word with very little full motion video, but you can forget about using it, for say watching videos on youtube, at least up to usb 2.0. Caveat Emptor - its a reasonably priced option, but forget about any degree of real performance out of it. In most cases a real graphics adaptor, even a cheap one, will run circles around it in every respect

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Thank you very much for your detailed answer. Do you or anyone else know whether using an USB graphics card is very taxing on the CPU? – chris.fy Nov 9 '13 at 17:34
Modern CPUs should handle it fine, and are unlikely to be the bottleneck. – Journeyman Geek Nov 9 '13 at 23:00

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