The problem with "Tannhauser," which will be telecast tonight on PBS (Channel 26 at 9), and simulcast on WETA-FM, is that it starts with an orgy-ballet before going on to more weighty philosophical, psychological and moral issues. After those close-up shots of athletic, writhing young bodies entwined in positions uncharted by the "Kama Sutra," it is hard to avoid a sense of anticlimax when you get down to the hard questions of sacred versus profane love, sin and salvation. But Wagner managed it in this opera, and the 1978 Bayreuth performance preserved in this telecast stages a strong third-act comeback after a spell of post-coital lethargy in Act II.

This is the first telecast here of a production from Wagner's own house, and despite a few problems, it whets the appetite for more. The PBS production sensibly aims it at the largest possible audience. Subtitles are provided -- not complete, but adequate and more readable than most printed librettos -- and Martin Bookspan supplies some local color and a lucid plot summary before each act. Hard-core Wagnerians may feel that all this is unnecessary. They are wrong.

Colin Davis conducts with his usual unassuming expertise; the orchestra and chorus are exemplary, and the cast is very strong, particularly Spas Wenkoff in the title role and Bernd Weikl as Wolfram. Gwyneth Jones sings both Venus and Elisabeth, embodying the two kinds of love that struggle for the control of Tannhauser's spirit. This double assignment was the sort of thing Nilsson could do in her prime, but it seems a bit demanding for mere mortals; Jones does show fleeting moments of vocal discomfort in the Venus role. But by the end -- and particularly in Elisabeth's magnificent final aria, "Allmacht'ge Jungfrau," she is convincing both musically and dramatically.

The photography represents an interesting compromise between an allout film production (such as Losey's "Don Giovanni") and the more static norm when a live performance is filmed, as this one was. The camerawork is highly skilled, with excellent close-ups, a remarkable variety of angles for a three-camera job and a fine sense of where to point the camera at a given moment -- frequently away from the singer, to catch another player's reaction to what is being sung. Tonight's "Tannhauser" should win a good many new enthusiasts, not only for Wagner but for opera itself.