Chromecast's ultrasonic device pairing is much simpler than it sounds
It turns out that there's more than one way for guests to join in on your Chromecast party. In addition to accessing Google's streaming stick via an onscreen PIN code, friends can pair their devices to your living room entertainment setup via inaudible ultrasonic frequencies. Apparently, all one needs to do to enable this is allow the Chromecast to support nearby devices, and it'll push the necessary tones through your flat-screen's speakers, which said gizmos will receive and sync with. If it sounds simple, that's the whole point. As Gigaom tells it, Chromecast engineering manager John Affaki says that this is an effort to make using the HDMI dongle in a social setting much easier. Whether you can trust friends enough for them to not stealth-add Slayer's "Angel of Death" to the next party playlist is up to you, though.
