I'm confused about the way the AppleTV3 handles Surround sound (and I do understand a lot about this stuff) so would appreciate some clarification - if anyone can help.... AC3 - I'm fine with this no problems. Its passed through on the Optical output so my Yamaha sound-bar can decode the discrete channels. I'm also guessing its on the HMDI cable as well (?) Great. AAC 5.1 - This can be contained in MP4/M4V files and is a discrete format with better compression than AC3. How does the AppleTV3 deal with this ? I've read in various places that its down mixed to a stereo 2.0 PCM signal and that this 2.0 contains the matrixed DPLII data that my yamaha sound bar can still decode for a surround effect. I've also read that it down mixes the LF & RF discrete signals ONLY into a 2.0 PCM signal that would (probably) not reveal any surround info when passed through a DPLII decoder I'm guessing this signal is contained on the HMDI cable, so what signal is on the Optical cable ? I"ve googled this many times, and its quite conflicting and I'm still none the wise so any help and practical advice would be great. The reason I'm asking is I have a lot of AAC5.1 movies that I'd be happy to watch if its down mixed to a DPLII signal - just try to figure out how to set it all up.