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Google has officially started to roll out guest mode for Chromecast, making it possible to connect to a nearby Chromecast without being on the same Wifi network as long as the network owner permits it. Chromecast product manager Jagjit Chawla explained the feature this way in a blog post:
“You’re having friends over and before you know it, a battle has ensued in your living room over who can show the funniest YouTube video. Now, it’s even easier for your friends to cast to your TV without first having to connect to your WiFi. With the new guest mode feature, anyone with an Android device can cast to your TV as long as they’re in the same room. ”
The new feature is starting to roll out Thursday, and requires the most recent version of the Chromecast app for Android. Here’s how the feature is explained within that app:
[company]Google[/company] first announced guest mode for Chromecast at its Google I/O developer conference in June. Back then, Chromecast engineers revealed that the streaming stick is using a pretty cool hack to pair with nearby devices: Instead of relying on NFC or other technologies that would have severely limited the number of devices capable of using guest mode, Chromecast playing an ultrasonic sound through the TV’s loudspeaker.
These sounds are inaudible to the human ear, but can be used to transmit small bits of information to a nearby mobile device that will then be used to pair the two devices. This kind of ultrasonic networking was pioneered by Google engineer Bori Smus, who released an experimental web app capable of sending emoji via ultrasonic sounds from one device to another in 2013.