Dolby Digital

Is a five-channel system consisting of left, center, right and left rear, right rear channels. All processing is done in the digital domain . Unlike Dolby Prologic in which the rear effects channels are frequency limited to approx. 100-700Hz, Dolby Digital rear channels are specified to contain the full 20-20Khz frequency content. The AC3 standard also has a separate subwoofer channel for the lowest frequencies. Source: http://www.owenscorning.com/around/sound/glossary.asp