The BDP-83 also doesn't properly "recognise" Dolby TrueHD 6.1, it calls it 5.1. What is the official name for this audio format, something like TrueHD 5.1 EX? I don't know, even the BD package calls it 5.1, but it's not (The Dead Pool, Clint Eastwood Dirty Harry Collection). This may in fact be a rather unusual audio format, I never noticed it with TrueHD before, even though the 6th channel really sticks out visually from a distance on my AVR's front display, compared to how 7.1 or 5.1 displays.

So, we've established to my satisfaction there are some "limitations" of the Oppo's chipset to recognising certain formats, specifcally those with a single (matrixed) rear channel. This is not a problem if you're sending the raw bitstream to your AVR, if it can decode it. I didn't check if the Oppo decodes this properly, but I'll guess not (based on the previous tests) until somebody tests it...oh, what the heck, the BD is still in the player and I'll try to remember to do it tonight...

FWIW, those with a mid-level and up Denon AVR will find it locks out all the back channel options you usually have in Standard audio mode with a 5.1 track, when receiving this 6.1 track via HDMI bitstream. On mine it applies PLII to that single back channel to drive the two back speakers/outputs. I have no way to tell if this would sound much different than applying the track's back channel to both back speakers directly (without PLII), which is kind of what I'd prefer (what I used to do with DVD EX tracks with a different THX-certified AVR). The Denon does correctly identify the raw 6th/back channel input as "Matrixed". So I will guess that the Denon does handle it correctly, as normally PLII derives the back channels, but in this case it just uses what it's given rather than re-derive the back from an already-existing back (and surround channels). I hope...

Edit: the BDP-83 does NOT correctly decode the back channel(s) for Dolby TrueHD 6.1 (or TrueHD 5.1 EX, whatever it's called).