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Old 2007-05-30, 19:23   Link #1
evangelic04
Junior Member
 
Join Date: Nov 2006
Question ProLogic / Surround Sound Q please?

Excuse the bonehead question, but might anyone be kind enough to clarify something about Dolby ProLogic for me? I understand that it is used to splice and upmix stereo tracks into faux 5.1 surround, but if I use it to process something with legitimate 5.1 surround am I losing any quality?
My setup is a USB SPDIF out mixer card, connected to an external surround decoder. Use AC3Filter set to DolbySurround/ProLogic as output setting. Then use VLC to play the video.
I can play both stereo mix xvids and 5.1 mix MKV's, and both output sound through all 5 speakers, but should I be switching back to 3/2+SW 5.1 Channels as my output setting in AC3Filter before watching something I know has true surround sound?
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Old 2007-05-31, 15:47   Link #2
Jinto
Asuki-tan Kairin ↓
 
 
Join Date: Feb 2004
Location: Fürth (GER)
Age: 33
As you figured out yourself already Prologic does encode the surround sound into a stereo signal and not even digital at that (its analog encoded). That causes that you don't have full channels for surround (rear usually only a very short frequency band and center being the monomix of the stereo signal).

With SPDIF and AC3 you can forward 6 channels digitally in dolby surround (however, not every digital dolby signal is neccessarily AC3 encoded - thats only one (most common) of the many codecs used for dolby surround).

To answer your question now... dolby digital is backward compatible. By choosing to decode a dolby prologic stereo mix with dolby digital 5.1 channel you'll still get the stereo signal in dolby prologic (just that only 2 of the 6 channels are used for the signal). If your receiver/amplifier is properly equiped it can detect that a dolby prologic (stereo) signal is send (or maybe you need to switch the dolby prologic switch on manually).
When you play sources with dolby digital 5.1 the same decoder will forward 6 channel audio now, and your receiver/amplifier will automatically use the appropriate surround decoding (assumes you have a decent receiver/amplifier).

So all you really need is the 5.1 channel mode (which is backward compatible with prologic).
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