A fine performance of "Tannha user," Richard Wagner's eloquent testament to the irreconcilable demands of profane and sacred love, will be broadcast at 8 tonight on "Live From the Met" (Channel 26, with simulcasts on WETA-FM as well as WGMS-AM and FM). It was taped Dec. 20 and is sung in German, with English subtitles.

This production, one of the prides of the Metropolitan Opera repertory, is directed for television by Brian Large, the man also in charge of the terrific television "Ring" from Bayreuth. But there any resemblance between the two ends.

The television "Ring" is a starkly staged allegory of the Industrial Revolution. By contrast, in "Tannha user," the Met goes all out to re-create the medieval world of the 13th century in loving detail. The "Ring," after all, is just a legend; but the director of "Tannha user" must bear in mind that the plot's pivotal moment, the second act's song contest, was an actual historical event, one that occurred at the Wartburg, near Eisenach, in 1208.

The contest is set in the lofty Minstrels' Hall, a crowd scene that is the production's high point. The costumes alone must have cost a fortune. And Large uses his cameras with great imagination in displaying this feast of opulence. Musically, the scene is just as memorable, with the on-stage brass and the large chorus superbly coordinated by conductor James Levine.

Once again, Levine proves himself to be a remarkable Wagnerite. The Met orchestra is in splendid form.

Vocally, the two principal women, representing the opposing sides of love that torment the minstrel Tannha user, dominate this performance.

Virginal, devout Elisabeth would seem to be a shrinking violet--but she must have a whale of a voice; in her first moments on stage, at the opening of the second act, she must sing "Dich, teure Halle," one of Wagner's more taxing soprano arias. Eva Marton tosses it off with thrilling assurance. Long a promising artist, Marton shows an amplitude here that is new.

Tatiana Troyanos is a suitably celebrated Venus. She sings well, and dramatically, she is the most believable of the main characters.

Tannha user's rival minstrel knight Wolfram is done splendidly by baritone Bernd Weikl, including a wonderfully lyric "O du mein holder Abendstern," more popularly known as the song to the evening star. John Macurdy is a solid Landgrave.

Alas, there is one weak piece of casting, and unfortunately it is Richard Cassilly in the title role. Tannha user is a part that comes easily to nobody--spanning as it does sweet romantic lyricism and rigorous Wagnerian rhetoric. At this point in his career, at least, Cassilly simply does not have the voice. The sound is pushed, wobbly and sometimes downright gritty. His notion of acting seems limited to an occasional arm gesture and a randomly raised eyebrow. Also, he is physically ungainly, and suffers from the frequent closeups.

Do not expect the degree of dramatic believability that director Large got from his singers in "The Ring." Yet, for about three-quarters of the time, this is quite a memorable "Tannha user."