i'm not sure what happened but something that i thought once worked, no longer works after resetting my Apple TV to solve a bunch of buggy behavior (per Apple Support's suggestion). my situation: ATV4k -> HDMI -> Sony 900e TV -> optical digital to Emotiva Processor. what should i be setting the Audio Surround Output to? - Best Available: if i do use this, the TV end up only feeding 2 channel 48k PCM data to my Processor - Dolby Digital 5.1: if i do this, for TV shows, i get 5.1 compressed data showing up on my Processor of course, the second choice "works" but it's not ideal because if i understand thing correctly, the ATV4k is compressing data into 5.1 format regardless of the source, and then sending it to my TV encoded as DD5.1 that is less than ideal since i like to use my ATV to stream lossless music from my Mac. and as is always preferable, i wish i could get the best possible sound output from my high end 7.1 setup (which is old enough that it doesn't support 4k or 4k pass-through) i could have sworn "Best Available" worked fine for me previously, but now it does not produce 5.1 output in my system.