When iOS 4.2 debuted, Apple changed the name of AirTunesthe feature that let you stream music from iTunes to an AirPort Expressto AirPlay, and in the process upgraded it considerably. In addition to streaming audio from iTunes on your computer, you could now stream from any AirPlay-enabled iOS appyou could even stream video to Apple TV. In fact, as of iOS 5, you could actually mirror the screen of an iPhone 4S or later, or an iPad 2 or laterwhatever that screen displayed, you could view on your TV through your Apple TV.

AirPlay mirroring was such a great feature that people wanted it for their Macs. And in Mountain Lion Apple has delivered: You can now send your Macs screen to any second- or third-generation Apple TV on the same local network and mirror it on any connected TV. The only catch: Your hardware must be relatively new. Specifically, it requires a mid-2011 or newer iMac, Mac mini, or MacBook Air, or an early 2011 or newer MacBook Pro.

If you have the requisite Mac, using AirPlay to mirror its display is simple: Mountain Lion automatically detects if a compatible Apple TV is on your local network. If so, an AirPlay Mirroring pop-up menu appears in the Displays pane of System Preferences. (You can opt to have an AirPlay Mirroring menu appear in the menu bar whenever an Apple TV is available.) Choose your Apple TV, and in a few seconds your Macs screen appears on your TV, with the Macs audio playing through your TV or home-theater system. Apple says the video stream is encrypted for security and optimized to give you the best image quality without stalling or experiencing glitches.

High-res mirror

Mountain Lion gives you a few options for choosing the best screen resolution, though your options depend on where you set them. From the system-wide menu, you can choose whether your Macs screen appears at its standard resolution on your TV or changes to match the TVs native resolutionthe latter option offers the sharpest image on your TV. In the Displays pane of System Preferences, you can choose your screens native resolution (Best For Display), the best resolution for streaming to your TV (Best For AirPlay), or any of the other resolutions supported by your Mac (Scaled). The Displays pane also lets you enable overscan correction.

(Note: Apples Mountain Lion features page says that AirPlay Mirroring supports up to 1080p HD, but Mountain Lions own Help system says that mirroring sends a 720p video stream with stereo audio.)

There are a few nice touches here. For example, if you play a video in iTunes while mirroring your Macs screen, the AirPlay Mirroring feature automatically switches the video to full-screen mode on your TV. And if you just want to listen to your Macs audio, you can use the Sound pane of System Preferences (or Option-click the system-wide volume menu) to choose any Apple TV or AirPlay-enabled audio device; youre no longer limited to iTunes audio for audio streaming.

In addition to being useful at home for watching movies, playing games, and surfing the Web on your big-screen TV, the new AirPlay mirroring feature is a welcome addition for presenters and teachers. With an Apple TV and either an HDMI-compatible projector or an HDMI-to-VGA adapter such as the Kanex ATV Pro ($60), you can project your presentations and live demos from anywhere in the roomwithout wires.