The ZP90 is a digital wireless transceiver that is capable of bringing all your music sources into any existing music system with an auxiliary input. These sources will usually be your hard drive music library such as iTunes, but can also be (i) Internet Radio, (ii) online libraries such as Rhapsody, and (iii) a source directly attached to any other zone, which might be a CD player or a DAB+ or AM/FM radio, for example. You could say that the ZP90 turns any existing system into an independant Zone controllable by the Sonos controller. Note: You'll probably also need a Zonebridge to connect to your router.
Print