I am working on an app that uses Airplay mirroring for a second display that has different content to the iPad itself. I'm using an iPad3 and ATV3 (i.e. released March 2012) attached to an LCD capable of 1920x1080 via HDMI.
With Airplay mirroring, the second screen's UIScreen is available through [UIScreen screens] array. I was able to get rid of the black borders by setting overscanCompensation to 3 but I can only display a 1280x720 image. The monitor is working at 1920x1080 (i.e. checked in its menu, set to "Just Scan", and the rest of Apple TV interface is at 1080p).
When I return the availableModes array from UIScreen it only has one item "1280x720"!
Does this mean that iOS 5.1 only supports 1280x720 on the new apple tv for airplay mirroring and not 1080p!? or am I doing something wrong.
Airplay itself only supports streaming in 720p. Here is what some clever guys did to get around it but you need to assemble some hardware.
http://appadvice.com/appnn/2011/03/airplay-1080p-format
EDIT Oops, apparently you CAN stream 1080p, but you can only MIRROR 720p (not enough memory?? I don't know)
来源：https://stackoverflow.com/questions/9952739/airplay-on-new-apple-tv-3rd-gen-for-second-screen-not-at-1080p