For 20 years, Hollywood did its best to ignore pop music, relegating it to movies for and about teenagers. In the past two years, however, Hollywood has found itself in the embarrassing position of finding such films as "Saturday Night Fever", "Grease", "FM" and "Sergeant Pepper" as by products of the records. The "Wiz" proves that Broadway and Hollywood still don't understand "rock 'n' soul".

A funky electric bass line promising good rhythm 'n'blues dance music opens the "Emerald City Sequence" on the film soundtrack "The Wiz" (MCA2-14000). But like so much else on the soundtrack, this promise is soon smothered beneath glaciers of strings and horns. Dozens of chorus voices swell up with all the personality of a sitcom laugh track.

A few songs later, Diana Ross eases into "Is This What Feeling Gets?" with a whisper that suggests an intense blues ballad. But the focus is soon destroyed by a mosquito swarm of violins and by Ross' inability to find the vocal power for a blues clincher.

Again and again, the jazz background of music supervisor Quincy Jones and his all-star musicians and the soul background of stars Diana Ross and Michael Jackson are betrayed by show music clinches. For all its jazz and soul flavorings, "The Wiz" is essentially not very good show music.

Broadway and Hollywood shows songs almost always have been designed to contribute only one clear emotion to the story flow. This formula rules out the irony and dramatic conflict within a song that have enriched the folk-blues tradition and the jazz, rock and soul that came out of it.

What show music can achieve is clever word play and enchanting melody, as Harold Arlen and E.Y. Harburg proved in their score for the original "The Wizard of Oz". Charles Smalls' Broadway score kept the lyrics straightforward and the closest he came to a fresh melody was the chorus to "Ease On Down the Road". That song stands out so much that it's used three different times with only minimal changes on the soundtrack.

To expand the single album stage soundtrack to a double film soundtrack, Quincy Jones dropped three numbers (including one of the best, "I Was Born on the Day Before Yesterday") and added six new songs, seven instrumental segments, several carbon copy reprises and a long overture.

In the souvenir lyric program that comes with a poster in the album package, Jones boasts that he worked with nine principal singers, 105 backing singers, 300 musicians, three conductors and nine orchestrators. Heshould apologize. Those numbers insure that the personnel have to be directed like a battalion and the music sounds like it.

Jones never misses a chance for an overkill. Whenever a talented performer like harmonica-player Toots Thielemans or singer Theresa Merritt establish some intimacy, Jones comes in with a symphonic tidal wave and wahses it away.

Jones has Steve Gadd, one of the best trap drummers in the world, and Ron Carter, one of the best acoustic bassists. He also has solid jazz musicians like Jon Faddis, Steve Khan, Howard Johnson, the Brecker Brothers and members of Stuff. But he wastes them. He could just as easily have used a good union musician.

A welcome contract is the 1975 Broadway soundtrack, "The Wiz" (Atlantic SD 18137), produced by Jerry Wexler, who produced Ray Charles' and Aretha Franklin's greatest records. The songs aren't any better, but at least Wexler encourages the ryhthm 'n' blues side of the music with gospel-arranged harmonies and spares accompaniment.

The film soundtrack also reveals how overrated Diana Ross is a singer. In retrospect, her records as leader of the Supremes have held up less well than those of her Motown contemporaries, Martha Reeves, Marvin Gaye and David Ruffin. Her voice had a certain childish sexiness, but no real power or character. Her voice was simply an easily controlled instrument for the classic songs of Eddie Holland, Lamont Dozier and Brian Holland.

She is unable to transcend the less worthy material by Smalls, Jones, Nickolas Ashford, Valerie Simpson and Luther Vandross in the film. On "Be a Lion," she trembles certain notes and overstates others. She has neither the restraint to set up big moments nor the timing and power to pull them off.

One example of her weaknesses comes on "Believe in Yourself." She whispers and fluffs her way through it. Two tracks later on the record, Lena Horne gives the same song a performance that is exhilarating by contrast. Horne's voice rumbles with dramatic restraint in the beginning and then bursts with trembling release into an ecstatic climax.

Other enjoyable moments on the record: Michael Jackson - of Motown's second generation - does a creditable job of leading the disco swagger of "Ease on Down the Road" and the gospel celebration of "A Brand New Day."

Mabel King from the Broadway cast gives a roaring stride blues vocal on "Don't Nobody Bring Me No Bad News." Nigthclub veteran Nipsey Rusell doesn't so much sing as jive his way like a street corner Maurice Chevalier through the dixieland "Slide Some Oil to Me."

Until Hollywood realizes that it requires more than just adding some new motifs to the same old formulas, they won't contribute any important music to the genre.