can someone help me with what this setting does? what has it done to the track exactly? is it a stereo track? is it still a surround sound track? i don't really know. when i first started my collection, i must admit, i never wanted to get too deep into the "perfect settings" thing. i just wanted to get going and my priority was picture quality. since then i have kind of regretted my neglect for the audio part.. and i always set HD 5.1 sources into Dolby Pro Logic II "Core Audio AAC" setting. regretfully i only did the SD audio in "5.1" mix down AC3. i really have no idea what these differences are though, i'd like some one who is a bit of a guru in this area to help me understand which settings are ideal for films. both the Codec and the Mix down? i understand passthru keeps the original track but what are recommended settings for surround sound HD sources? AAC vs AC3? CoreAudio vs ffmpeg? and again, i only have a normal 2.0 tv set up, but I'm about to get my first 5.1 surround sound set.. what will my 5.1 speakers do with all of my Pro Logic II hd conversions? is it just going to be stereo played through all 5 speakers? have i basically ruined the surround sound affect of 5.1? thanks for anyone who can help.