USB Analog & Digital Audio Adapter 

The Micro II USB sound card adds a stereo analog output and an optical digital output to a Mac® or PC. The stereo output drives powered speakers, headphones or an external recording device. Because it uses a USB digital connection, the Micro II isolates the audio signal from the noisy electronics inside a PC or laptop to provide higher-quality sound. A built-in amplifier delivers crisp, clear sound on headphones or ear buds.

Inserting the included TOSLINK adapter into the stereo jack converts it into a S/PDIF digital optical output, allowing you to connect  your Mac or PC to the optical digital input on home theater systems, digital speakers and other digital audio devices. This digital connection lets you enjoy DVD or streaming digital movies on your Mac or PC with 5.1 channel Dolby® Digital or DTS™ surround sound on your home theater system.