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 approximately 100–700 Hz, Dolby Digital rear channels are specified to contain the full 20 Hz–20 kHz frequency content. The AC3 standard also has a separate subwoofer channel for the lowest frequencies.
Dolby Prologic is a four-channel system consisting of left, center, right and rear channel, (the single rear channel is usually played through two speakers).