Dolby Digital 
Dolby Digital is a proprietary technology used for creating and reproducing digital surround sound (also called AC-3, or 5:1). The 5:1 format refers to the way digital sound is recorded on six separate tracks: front left, front right, front center, rear left, and rear right, with an extra track reserved for very low bass. This method mimics the three-dimensional quality of sound in real life. Although analog surround sound (called Dolby Pro Logic) also often uses six speakers, the analog format simply splits a traditional stereo signal into front and rear components.