Actually, there are no true 7.1-channel discrete recordings at all, even on Blu-ray. This is a FACT.
All 7.1-channel encoded audio recordings, mixed by the studio sound engineers, are all derive from 5.1 audio stems, to create the two additional rear surround channels. Ask any professional recording studio engineer, and they will confirm this fact.
* Here's few names that you can contact: Roger Dressler, ex-audio engineer at Dolby Labs, Kal Rubinson, Stereophile writer, Marc Fishman, professional film mixer,
Chris, from Audyssey head department, and many more...