The Supersonic Bluetooth Wireless FM Transmitter lets your stream music and calls directly from your Bluetooth device to your car's FM stereo system. The built-in microphone provides you with an amazing hands-free function. It also features a USB and SD Card reader that lets you play your favorite MP3 and WMA files on your car's stereo system. Play music files from an external device by connecting to the auxiliary port. You can also charge your smartphone using the dual USB charging ports.