''Nine'' brilliantly applies the techniques of a Broadway musical to Federico Fellini's ''81/2,'' its original source and inspiration. Beginning with Mario Fratti's adaptation of an Italian script, librettist Arthur Kopit and composer-lyricist Maury Yeston have written the words and music for a satirical tale about ''a man who wanted everything,'' a noted Italian film director, and the women in his life. Raul Julia is giving the kind of performance that makes legends and matinee idols. Tommy Tune's spectacular staging abounds in sumptuous pizazz.
The new show at the 46th Street Theater is the most original Broadway musical since ''A Chorus Line'' - and this is not to overlook the moral malaise that feeds the story or some gratuitous offensiveness.
''Nine'' hits its stride and makes its first bow to Fellinism when the black-clad figure of Guido Contini (Mr. Julia) rises from his seated position at the center of the stage. Baton in hand, this conductor of countless affairs instantly quells the bedlam of belles (and not so belles) dames before him. In ''Not Since Chaplin'' and ''Guido's Song,'' the first of Mr. Yeston's richly atmospheric musical passages, ''Nine'' begins displaying its melodic and harmonic prowess. The range of musical invention and reference ranges from romantic ballad to jazz, opera, liturgy, and Neapolitan folklore. The performance conducted by Wally Harper (with Jonathan Tunick orchestrations and Mr. Yeston's choral composition and continuity) is a matter for constant delight and wonder.
In his first song, Guido, the incorrigible egocentric, carols: ''I would like to sing a duet with myself.'' For her part, the long-suffering but rueful Luisa Contini (Karen Akers) rationalizes the behavior of her philandering spouse with, ''My husband makes movies - to make them he lives a kind of dream.'' The image invests the filmlike fantasy of the tale. The hollow-eyed, world-weary Guido is haunted and sometimes shadowed by the small figure of his nine-year-old self (Cameron Johann), whose parochial schooling was accompanied by some seaside sex education of which the teaching nuns and his mother were belatedly aware.
Notwithstanding all of the show's populous movement and vivid stage imagery, Mr. Julia and Miss Akers - with their director's intuitive assistance - manage to present a relationship that can somehow survive. It is this tender feeling that provides the core element of the dizzying fantasy. ''Nine'' reaches a kind of opera buffa climax in Act II as Guido, his personal and creative life simultaneously collapsing, decides on a burlesque version of the adventures of Casanova. The farcical shambles lead to a denouement as poignant as it is sudden.
The Yeston score rewards all of the show's principals with solo flights. In addition to the extraordinarily effective Miss Akers, there are dazzling performances by Anita Morris and Shelly Burch (as two of Guido's mistresses), Liliane Montevecchi (his impatient producer), Taina Elg (his mother), and the aforementioned Master Johann. The ensemble vocal work is outstanding.
To accommodate the constant flow of action, designer Lawrence Miller has created an enormous tiled spa overlooking a backdrop of Venice. The spa's tiered benches provide the only furniture. In Mr. Tune's imaginative uses, and under Marcia Madeira's play of shifting lights, they serve any number of incidental locales. ''Nine'' has been dressed to the nines - and beyond - by William Ivey Long, whose Broadway fashion show includes a stagewide black feather boa for the irresistible Miss Montevecchi.