AirPlay is Apple's technology for streaming media over a local network. Specifically, it lets you stream audio from any Mac or iOS device to any AirPlay-enabled audio system, or video from an iOS device or recent Mac to an Apple TV. AirPlay works over any modern ethernet or Wi-Fi network—The sending and receiving devices just need to be compatible with AirPlay.
Here's a quick look at the most common AirPlay setups and how to configure them. I'm assuming here that your devices are already connected to a local network.
Setting up your AirPlay devices
Before you can use AirPlay, you need to configure your AirPlay receiver: a speaker or audio receiver with AirPlay built-in, an AirPort Express base station, or an Apple TV.
Many recent AirPlay-enabled speakers are simple to set up: For Wi-Fi, just use your iOS device's USB-sync cable to connect the speaker to your iPad, iPhone, or iPod touch. A dialog on the device's screen asks if you want to share your network settings with the speaker; tap Allow, and the speaker is automatically configured to join your network. Ethernet is even easier: You just connect an ethernet cable to the speaker.
For an AirPort Express that's already on your network, you launch AirPort Utility, select the Express, and then click Edit. Next, click the AirPlay tab, check the Enable AirPlay box and give the Express a descriptive name; you can also enable an access password. Click Update to save your changes.
To enable AirPlay on your Apple TV, navigate to the Settings screen, select AirPlay, and then make sure that the AirPlay is set to On. For security, you can optionally configure either an onscreen code or a traditional password.
(You'll of course need to connect speakers or an audio system to the AirPort Express or Apple TV for audio, or a TV or home-theater receiver to the Apple TV for video.)
Using AirPlay for audio
Streaming audio over AirPlay is easy. On your Mac, you can stream iTunes audio by clicking the AirPlay button next to the volume slider in iTunes, and then choosing the desired AirPort destination. To send audio to multiple AirPlay-equipped devices simultaneously, click Multiple and then select the speakers you want to stream to; you can control the volume level of each speaker independently in this menu.
You'll need third-party software to stream audio from a specific Mac app other than iTunes. Rogue Amoeba's Airfoil for Mac lets you stream audio from any currently running app to the AirPlay destination(s) of your choosing.
If you're running OS X 10.8 Mountain Lion, you can stream all of your Mac's audio by opening the Sound pane of System Preferences, switching to the Output screen, and then selecting your AirPlay destination in the list. Alternatively, you can hold down the Option key and click the systemwide volume icon in the menu bar, then choose your AirPlay destination under Output Device.
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