Why Are All DVD Disks Labeled "Dolby Digital" But Some Do Not Play in 5.1 Channel Surround?

Some DVDs play back in 5.1 channel surround, some in Dolby Surround, and some in mono or stereo. All DVD discs, however, are labeled Dolby Digital because Dolby Digital was selected as the primary encoding system for the DVD standard. But Dolby Digital and Dolby Digital 5.1 do not mean the same thing. In this case, Dolby Digital is the "carrier" or vehicle that contains the sound track. The sound track can be anything from mono to 5.1 channel sound, but it is always encoded in Dolby Digital on a DVD disc. Some disc packages identify the sound track contained on the disc, some do not. Dolby Digital is capable of downmixing a sound track. For example, if the DVD has a 5.1 channel sound track, it will also be able to play in Dolby Surround, stereo or mono. But if the disc only has Dolby Surround, it cannot "upmix" to 5.1 channels.  more

Some DVDs play back in 5.1 channel surround, some in Dolby Surround, and some in mono or stereo. All DVD discs, however, are labeled DD because DD was selected as the primary encoding system for the DVD standard. But DD and 5.1 do not mean the same thing. In this case, DD is the "carrier" or vehicle that contains the sound track. The sound track can be anything from mono to 5.1 channel sound, but it is always encoded in DD on a DVD disc. Some disc packages identify the sound track contained on the disc, some do not. DD is capable of downmixing a sound track. For example, if the DVD has a 5.1 channel sound track, it will also be able to play in Dolby Surround, stereo or mono. But if the disc only has Dolby Surround, it cannot "upmix" to 5.1 channels.  more