Video Asylum

Posted by michael w on March 14, 2000 at 13:54:50:

In Reply to: A naive question? What happens when a DD disc is processed by a pro logic decoder? posted by John C.-Aussie on March 14, 2000 at 00:06:29:

Hi John,

Depends upon which medium we are discussing.

A LD with DD will have Dolby Surround or similar on the digital tracks , so when fed into a Pro Logic decoder will playback as Dolby Surround.

The surround encoded stereo tracks, known as Lt and Rt are not the same as straight stereo as the surround info is mixed into them.

DVD is slightly trickier as DD can be anything up to 6.1 channels.
DVD has the option of downmixing the multi-channel track to 2 channel stereo or having a separate Dolby Surround track.

Both will have different results.
The downmixed track will combine down with a fixed algorithm, so that when played back the surround effects may not be as originally intended. Bit like those old Hafler style surround decoders which put all out of phase stuff to the surrounds.

Listening to the downmixed track should only used as a last resort !
You usually get all sorts of weirdness when you downmix, ranging fron frequency response irregularities to loss of bass.

A dedicated Dolby Surround track should give the more faithful experience.
Full Dolby Digital decoding should be even better :)

Hope this helps.

michael w