Dolby 4.0 vs 5.1

A recent DVD release that I'm after (Miller's Crossing) appears to have
"Dolby Digital (4.0) Surround" for region 2 and Dolby Digital 5.1 for Region
1. By the look of it, DD5.1 would be better...but presumably only if it's
mixed to use all 5.1 channels. As I've not come across DD4.0, does anyone
have any idea:

1. Why this difference should exist.
2. Whether there's a difference (or does "DD4.0" = DPL)?
3. Which I should get.

Ken.

"Ken Catchpole" <k.catchpole.TakeThisOut@ich.ucl.ac.uk> wrote in message
news:bnir6s$lt2$1@uns-a.ucl.ac.uk...
> A recent DVD release that I'm after (Miller's Crossing) appears to have
> "Dolby Digital (4.0) Surround" for region 2 and Dolby Digital 5.1 for
Region
> 1. By the look of it, DD5.1 would be better...but presumably only if it's
> mixed to use all 5.1 channels. As I've not come across DD4.0, does anyone
> have any idea:
>
> 1. Why this difference should exist.
> 2. Whether there's a difference (or does "DD4.0" = DPL)?
> 3. Which I should get.
>
> Ken.
>

4.0 is Left, Right, Center and dual-mono Surround, with no LFE mix. It
isn't exactly the same as DPL as it is "discrete monophonic" and there is
no 4:2:4 encoding on the rear channels. i.e. the surround channel is
independant of the front and not encoded within it.

The 5.1 mix will have the dedicated LFE channel (the .1) and stereo rear
channels.

Dan