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Topic: AC3 meta and encoding phase shift options (Read 4088 times) previous topic - next topic
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AC3 meta and encoding phase shift options

I am using the Dolby AC3 Pro encoder with Vegas 8.0a to encode a mix of a music concert for a DVD.

There are several options designed to provide some backwards compatibility for Dolby Pro Logic surround players when down mixing to two channels.

After reading the Dolby AC3 documentation, the "90-degree phase shift" option appears to actually shift each of the rear channels 90 degree. As this occurs in the actual encode, it alters the actual 5.1 mix even when played on a full 5.1 dolby digital system. This makes it unappealing to me.

There is also another a optional meta-tag for "Lt/Rt Downmix preferred". As I understand it, this performs a 90 degree phase shift and then mixes the resulting signal in a Pro Logic friendly way into the left and right channel.

My audio contains a lot of natural reverb in all of the surround mics. Mixing the left and right surround speakers into the front centre out of phase and in the center could therefore cause some problems in the stereo mix. However, I am going to force a choice in the DVD menus so the user has to decide whether to go for a dedicated stereo or the 5.1 mix. Therefore it may be worth me retaining the Prologic compatibilty through unobtrusive meta tags in the ac3 5.1 mix.

My question is, if I switch off the "90 degrees phase" option and switch on the "Lt/Rt Downmix preferred" option, will I be producing mixes that retain Pro Logic compatibility without altering the original signal?