AirPlay Mirroring

One of the most useful features of Mountain Lion is AirPlay Mirroring. For those unfamiliar with AirPlay Mirroring (previously only available on certain iPhones and iPads), this feature allows you to wirelessly stream both your computer's screen and audio to any television that has a second-generation or newer AppleTV connected to it. This is a game-changer for conference and meeting rooms.
Assuming you have the requisite hardware (configured properly), using AirPlay Mirroring is a breeze. Simply click the AirPlay mirroring icon in your menu bar and select your AppleTV. Video and audio will then be mirrored to the television. In our testing and use of this feature, there is only a small 'lag' in the streaming of video and audio; it's great for presentations, but not ideal for watching movies (although other AppleTV features allow for movie-watching already.)
AirPlay Mirroring is only available on certain Macs running Mountain Lion. Older Macs do not support this feature, even if they have Mountain Lion installed. The list of supported Macs is:
”• iMac (Mid-2011 or newer)
”• Mac mini (Mid-2011 or newer)
”• MacBook Air (Mid-2011 or newer)
”• MacBook Pro (Early 2011 or newer)

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OS X Mountain Lion - Apple