There's no realtime DTS/DD encoding in 99% of on-board solutions available (hey, it's the same with PCI/PCIe cards unless stated otherwise in the card's specifications — see ASUS Xonar family for example). It's impossible to get 5.1 over HDMI or S/PDIF if it was not pre-encoded (it's called “DTS pass-through”). So you will get 5.1 over HDMI for bluray and such, but for regular audio, no dice.
No.. this DZ77RE-75K specifically stated that it supported Dolby Digital Live, and this is one of the many reasons why I chose it, although now I can't find where I read it at (maybe they took it down? lol) Either way, it doesn't matter as I confirmed it does indeed support Dolby Digital Live, but only through the S/PDIF outputs (both rear and on-board), not HDMI and probably not Thunderbolt either (although I've not tested that). It appears to be a Realtek thing and associated with their drivers. It's activated in the driver configuration and I'm using it now actually. 5.1 audio, one cable! Only thing I don't like about DDL is that my receiver is pretty old and there's usually a millisecond delay before it can sense and start decoding encoded DDL sound. This presents no problem in games, or listening to music. But it is kind of any annoyance with Windows sounds, which are actually just short audio clips, not constant sound, so they come through abbreviated. So I normally don't activate DDL on my receiver unless I really need it, ie for a game, which makes it respond much faster for some reason.
One other thing I noticed with HDMI audio as compared with DDL audio through S/PDIF. Unlike DDL, HDMI audio doesn't get abbreviated. After some investigation, I realized this was because HDMI audio is really a constant stream of sound even during silence. I could even hear a faint constant static in my speakers when it was suppose to be quiet, i.e. I wasn't doing anything in Windows and nothing was running. The Realtek audio doesn't do this, it stops the audio stream whenever there is silence, hence making my speakers totally quiet. So there are pluses and negatives doing it either way.. kind of makes me want to just use the old fashioned 3 stereo cable approach again, and I'd do that, but my OLD receiver only supports up to 4 speakers using that method (no center, subwoofer input, except through DD/DTS fiber or coax)