Is Dolby TrueHD compatible with regular Dolby 5.1?

[BIslander]

Originally Posted by odin24

Regarding hidden DD 5.1 tracks... absolutely not true, if it exists on the disc there will be an option to select it.

I think this is a semantics issue. Unlike DTS, where the lossless track is constructed using the lossy core, Dolby uses two separate tracks. As you noted, the DD 5.1 track is embedded in the TrueHD package. That's why it is often called a hidden track. It's there, but can't be selected from the audio menu. Some studios such as Warners include a selectable DD 5.1 track on their TrueHD discs, although I don't know whether that's simply a visible link to the embedded track or a separate version.

HDDVD TrueHD tracks on the other hand are slightly different, they do not have an AC3 track embedded, so there will be a separate AC3 trakc available... most likely a DD+ (E-AC3) track.

HD DVD players also transcode TrueHD or DD+ to DD 5.1 or DTS on the fly. As best I recall, the HD DVD spec has a 448 kbps bitrate limit for DD 5.1, the same as DVD. That's what you get when you play the DD 5.1 track. But, transcoding allows for a 640kbps DD 5.1 output.