AC-3
From MythTV Official Wiki
Dolby Digital is the trademarked marketing name for Dolby Laboratories' "lossy" AC-3 codec. The common version contains six total channels of sound, with 5 channels for normal-range speakers (Right front, Center, Left Front, Right Rear and Left Rear) and one channel for the LFE, or subwoofer. The Dolby Digital format supports Mono and Stereo usages as well. [1]