Just yesterday Google announced that it would soon allow users to send video and other entertainment items to a nearby Chromecast even when they're not connected to the same WiFi network, with the backend relying on location data for verification. It looks like there's some even more interesting technology going on behind the scenes. GigaOm reports that the upcoming update will allow Chromecast and Android devices to authenticate each other using ultrasonic waves.
Here's how it works. You're at a friend's house, and you've just got to show him the latest Honest Trailers video. If his Chromecast has the upcoming "nearby devices" option enabled, you won't need to get on his WiFi. Just cast from the app as normal, and the Chromecast will emit a series of noises from the television speakers that are above the human audible range. The phone will "hear" these sounds and verify the Chromecast, make the connection over the mobile network, after which the device will stream directly from the home WiFi. The Android user should still be able to pause, advance, and what have you from the phone or tablet. There's no reason this functionality couldn't also be added to other Cast-enabled devices.
Keen observers will recall that Google acquired SlickLogin back in February. This startup used phones as authentication tokens for logging in to websites on a standard desktop web browser, with extensions that authenticated the user with the same ultrasonic sound waves verified by the phone. Google didn't mention this particular acquisition when talking about the upcoming Chromecast feature, but it's a safe bet that some of the talent and/or tech went into it.