I think the "official" CD/DVD recordings were done in 1980 except Götterdämmerung which was in 1979. As far as I know the DVDs and the CDs are the same performances, which were done an act at a time in an empty theatre. So the '76 and '77 recordings would be the same production, with a similar but not identical cast, and "actually" live in front of an audience. I've only heard the Philips CDs. I wouldn't want it as my only Ring but I'm glad to have seen YouTube videos of the whole thing and heard the CDs.