mic in= microphone input: plugged into the microphone line in= line input: input MP3 and other audio to the sound card for synthesis and editing on the computer front out= front output: 2 channels of earphones or speakers with more than 4 channels of speakers inserted into the front speakers rear out= rear output: turn on more than 4 channels to produce sound / insert rear speakers cen/bass out= Center/bass output: 5.1 speaker dedicated interface SPDIF IN= optical input: optical audio input to the sound card, synthesizing and editing in the computer SPDIF OUT= optical output: generally output to optical power amplifier or decoder (digital signal)