Let me come right out and say that the latter is better. The USB2HDMI adaptor may require two USB ports, but it produces a much more responsive picture and goes up to 1920×1080 resolution providing your computer will support it. In fact, I found it almost good enough to play games via, but it was just little bit sluggish to best a good ol’ directly connected monitor. This is par for the course, naturally, as both devices squeeze a video signal through USB using compression, and both devices therefore have delay.
The latter, the USB2HDMI, is compatible with Mac OS X, Windows and Linux. Although your mileage may vary getting it up and running with Linux. It uses HDMI for the video connection, which is very easily adapted into DVI using an HDMI->DVI cable. It also has built-in sound so you can get both your sound and picture through the HDMI cable, an absolute must if you plan on using it for movie watching.
USB doesn’t natively have the bandwidth for full-screen displays, this is well known and documented by DisplayLink. Compression is used, and thus a higher CPU/GPU load is incurred when using a DisplayLink device. User-level CPU use (OSX) is dramatically higher when playing a youtube video on the DisplayLink display, versus a native display. This is not much of an issue, however, as playing video via the DisplayLink display isn’t exactly the best of experiences anyway. Video will always be a little choppy, and thus is best watched on a “native” display connected via VGA/DVI/HDMI/DisplayPort or otherwise.
There’s some respite for Windows users, however. On Windows, a WEI score of 4.5 or better is recommended, and the checking of the “Optimize for Video” setting in the GUI is required for full screen video playback. This “Optimize for Video” option isn’t available on the Mac at all, so choppy video is the norm. Ultimately you’re only going to need to use these settings if you plan on a bit of video watching on a TV using the HDMI adaptor. This is not a use I’d strongly recommend the USB2HDMI for, however, despite it being the most obvious. It’s simply not as good as a native HDMI connection.
Windowed video isn’t quite as bad as full-screen, however I noticed some tearing- a visual artifact that appears when the video refresh rate and the screen refresh rate are at odds with each other. Tearing was even more prominent in full screen video on OSX, and this wasn’t something I could find a fix for.
When using the USB2HDMI adapter the aforementioned necessary compression was noticeable and is not dissimilar to the sort you’d see in compressed JPEG images. The compression also caused a subtle but noticeable delay in the screen response, but it’s still absolutely usable for browsing, programming and other day-to-day productivity. As mentioned, whilst it’s possible to play a full-screen game this latency will make the experience less than stellar, so online, twitch-gaming FPS titles are probably a no-go.
To get the best out of a DisplayLink adaptor it’s usually necessary to ignore the out-of-date driver offerings of the manufacturer, and head straight to the DisplayLink website. I managed to pick up Lion compatible drivers for the USB2HDMI which weren’t included out of the box. Linux users can take advantage of a fully open-source (GPL) driver and a wealth of support and guidance available libdlo.freedesktop.org/wiki/.
Sound is the final thing in the USB2HDMI adaptors armada. No HDMI connection would be complete without the capability of sending audio to a connected television. The USB2HDMI can do just that, and provides a USB audio interface through which your system audio can play, over HDMI, to your TV. Because of the less than brilliant video performance, this feature is probably a little redundant, but if you want to hook your laptop up to a high-definition TV and have only a VGA port (such things exist!) then it’ll do the job fairly admirably considering the limitations of USB2.
Ultimately the USB2HDMI adaptor is best used in Windows, and best used for productivity. It can do video, and it can do gaming, but neither results in a particularly smooth experience. The DisplayLink drivers offer OSX and Linux compatibility, with limited features. This is good news for us alternative-OS users and does make the USB2HDMI a handy way to add an additional monitor to a MacBook Pro.
I’m currently running a second 24″ monitor from the USB2HDMI, although my MacBook Pro graphics card isn’t capable of supplying more than 1680*1050 to this tertiary display. I use an ageing Dell 2407WFP which has a built-in 4-port USB hub. This is handy, as two of those ports serve to provide power and data to the USB2HDMI which I can tuck away behind the monitor. A third USB cable running from the monitor to my laptop then handles the picture. The net result is that I now have a second 24″ monitor (albiet only running at 1680*1050) which works the instant I plug in a single USB cable to my laptop. This is handy, as I take my laptop home every day and have never had a problem simply plugging in the two monitors every morning. More high-end monitors would do well to have this DisplayLink technology simply built in. Sadly, linux users will most likely face trouble replicating such a plug-and-play experience.
Ultimately, it’s a product that serves a purpose well, but not really the one it was designed for. HDMI suggests full-screen, 1080p video and gaming but alas the bandwidth of USB2 is not great enough to deliver performance befitting your average gamer of video connoisseur. Buy it for the office, avoid it for gaming or movie watching.
For those of you with USB3 ports, you’ve got a much better chance of getting an acceptable full-screen video and gaming experience. A Startech USB 3 to HDMI adaptor is already available.