Nicolaus Harnoncourt, who can sometimes be the enfant terrible of Baroque music, is on his best behavior tonight for a reverent, stylistically authentic and generally well-performed interpretation of Part I of Bach's "Christmas Oratorio." The first three of the oratorio's six cantatas, telling of the birth of Christ, the angels' appearance to the shepherds and the shepherds' visit to the stable, will be shown on WETA (Channel 26) and Maryland Public Television stations from 9 to 10:30, with stereo simulcasts on WETA-FM (91) and WBJC-FM (91.5).

Part II, which tells of the circumcision and the visit of the Magi, will be seen next Friday. Ordinarily, it would be unthinkable to split up a work of art in this way, but this treatment is legitimate for the "Christmas Oratorio," whose six cantatas were actually intended to be sung on six different days of the 12-day season between Christmas and Epiphany.

The main theme of Part I is a common Christmas motif: the contrast between the grandeur of Christ's origin and mission and the humble circumstances of his birth, in a stable surrounded by animals and visited by shepherds. The music begins and ends in a burst of pure glory, with trumpets and drums. This sounds like music for a royal occasion, and in fact much of it is. Bach, who was both overworked and artistically thrifty, recycled a lot of his musical pomp and circumstance from two secular cantatas (Nos. 213 and 214) originally composed to celebrate birthdays in the royal house of Saxony.

At the other extreme from this music (which essentially celebrates power) are a lullaby for the new baby, a sweet and simple pastoral symphony that serves as the overture to the second cantata, and several of the plain, strong Lutheran chorales that represent the voice of the people -- the congregation -- in Bach's music. One of these may come as a slight shock to people familiar with the St. Matthew Passion; it is "O Haupt voll Blut und Wunden" ("O Bloody, Wounded Head"), which repeatedly punctuates and comments on that somber narrative of suffering and death.

Even with different words, appropriate for Christmas, it may seem out of place in the celebration of a humble but joyful birth. But it also gives depth and perspective to the Christmas story, subliminally looking ahead to the end even in the beginning. Another great chorale, which Bach uses several times in different forms for this work, is the Christmas carol "Vom Himmel hoch" ("From High Heaven I Come"). Musically, this is one of the world's finest Christmas carols, but it is relatively unfamiliar to Americans because (unlike "Silent Night" or even "In dulci jubilo") it has never acquired a set of English words generally and spontaneously adopted by the public. The three exquisite performances it receives tonight are, almost by themselves, worth the time invested in watching this performance -- as is the Vienna Choir Boys' performance of "Silent Night" (in German) inserted at the end to fill out the time.

As was the rule in Bach's time, no women sing in this performance. For choral parts, this works well, with the Tolzer Boys Choir handling the soprano and alto parts effectively. However, the solo arias by the boy soprano and alto are less effective than those by the superb tenor Peter Schreier and bass Robert Holl. The expressive limitations of the boy alto are particularly notable in the aria "Schliesse, mein Herze" ("Lock In, My Heart") in the third cantata because of the beautifully played violin obbligato that accompanies the voice. But this is the way Bach expected the music to be performed.

In many telecasts of nonoperatic music, the FM simulcast all by itself seems quite satisfactory without pictures. In this case, however, the video dimension does add something, with the camera lingering over details of the Christmas scene in Baroque sculpture at appropriate points in the music.