The problem with the Kanex XD is that we're annoyed it exists. This isn't Kanex's fault, and in fact its device works well. As advertised, the Kanex XD bridges the gap between HDMI-based video devices and the Mini DisplayPort input on Apple's 27-inch iMacs. That means you can now use the 27-inch iMac as a home entertainment hub, but in a more convoluted manner than large Windows all-in-ones with HDMI ports built-in. If you have a need for such a device, we can recommend the Kanex XD, even if its $150 price tag feels a touch high. We just wish Apple hadn't made the device a necessity if you want to take full advantage of its already expensive iMacs.
The Kanex XD hardware consists of a small (1.2 inches high, 2.2 inches wide, 3.25 inches deep) silver-colored metal box. Kanex is also kind enough to provide power, HDMI, and Mini DisplayPort cables. There's no software to install, but setup still requires a few steps.
First, you need to ensure that the device you want to connect to your iMac is set to output at 720p, as well as 60Hz, if that setting is exposed. The 720p requirement comes because the 27-inch iMac will only support two input resolutions, its native 2,560x1,440 pixels, and 1,280x720 pixels, aka 720p. Apple hasn't responded to questions as to why this limitation is present, but after trading a few e-mails with Raymond Soneira, president of display testing outfit DisplayMate Technologies, we have a pretty good guess.
According to Soneira, "This is common with very high-resolution digital displays. The reason is that the display's internal processor would have to digitally rescale the image to the native resolution of the display and that requires a lot of processing power. The higher the input resolution and pixel clock, the harder it is to do. Now 1,280x720 is a special trivial case because it is exactly half of the 2,560x1,440 resolution, so the processor just needs to do pixel replication."
Kanex says the Kanex XD can support 1080p should Apple ever issue an update. For now, depending on your device, setting it to output 720p might involve connecting it to another display first. We set a PlayStation 3 to 1080p and found that it was able to detect the iMac as an unsupported screen, and then automatically switched to 720p to allow us to lock-in the settings. We also had luck with the "Auto" resolution setting on a Sony HD camcorder.
Once you establish the proper resolution output, you need to connect the cables in order: plug the HDMI device into the Kanex XD, connect the Kanex XD's power cable, and then finally run the Mini DisplayPort cable from the Kanex XD to the iMac. The iMac should be turned on when you make the connection, and the signal will switch over automatically. For the most part the external signal remains separate from OS X and the hardware. You can still adjust the volume and screen brightness with the iMac's Bluetooth keyboard. You can also toggle between the external signal and OS X by typing Command-F2.
We tried a number of different devices with the Kanex XD and had almost universal success. We connected a PlayStation 3, an Xbox 360, a standalone Blu-ray player, a satellite cable box, and the Sony HD camcorder. Connected to the iMac individually via the Kanex XD, all worked as you would expect. The only quality issue came via the Blu-ray playback, both via the PS3 and the standalone player, where the picture quality drop-down to 720p was noticeable. Movies were still watchable, but the image was less detailed than we expected. In all cases, the audio passed through without trouble, although you only get stereo output from the iMac.
The only other problem came when we put an HDMI switch in between the Kanex XD and source devices. We had luck swapping between an Xbox 360 and a Blu-ray player. Our issue came with the satellite cable box, which couldn't establish a stable signal. We suspect the copy protection signal check didn't like making so many jumps between the source and final output. Switching manually isn't that much of a hassle, but it does add yet another damper to the convenience factor of using the iMac as a home entertainment hub.
The Kanex XD isn't the only Mini DisplayPort-to-HDMI adapter on the market. Altona Technologies sells one for $150, and we expect a promised device from Belkin will materialize before too long. We wouldn't be surprised to find others on the market as well, which will hopefully push prices lower. We expect that a determined enthusiast won't find the $150 too large a hurdle, but given that even $700 Windows all-in-ones offer built-in HDMI input, adding $150 to the already elevated iMac price tag stings a little.