Last year Nickolas Ashford and Valerie Simpson got a phone call from their old friend, Quincy Jones, who was supervising the music for the film version of "The Wiz." Jones needed some new songs to expand the stage score to movie size. Would they write a couple of songs with him, including one for Diana Ross' grand entrance?

"We thought the idea was great," recalled Simpson backstage after their Constitution Hall concert Saturday. Ashford and Simpson had written several big hits to help establish Ross' solo career right after she left the Supremes. And Simpson had sung background vocals for Jones before she and Ashford became singing partners and later marriage partners.

"But we were kind of scared," Simpson said, "because we had never written with anyone before: we had always written with each other. Also, writing a song for a movie is different from just writing a song. But Quincy fit like an extra arm."

"All I remember," added Ashford, "was being locked in that room to write the songs. Sidney Lumet [the film's director] wanted something special for Diana. He went to great pains to tell us what it should be and what it shouldn't be. We kept thinking we could make a Top 10 song out of it. He said that's nice, but stick to the movie."

The resulting song, "Can I Go On?" is a centerpiece of "The Wiz" movie version. A second song, "Is This What Feeling Is?" is played as an instrumental in the film but appears as a vocal on the soundtrack.

While working on "The Wiz," Ashford and Simpson also put together for Warner Brothers, their sixth album, "Is It Still Good to Ya?" Both the album and the single, "It Seems to Hang On," have all the appearances of a breakthrough. While last fall's album, "Send It," took seven months to go gold, their new one has already sold the necessary 500,000 units in five weeks.

"This one is putting us more in a crossover market," Ashford claims. He denies, however, that they changed anything to go after rock and pop audiences. "We want people to like us for what we are," he said. "It would have been very easy for us to write crossover songs. But we write from a very special place inside and if people don't like what we're doing, we don't want them."

The 6-foot Ashford, 35, towers a full 12 inches over his wife, 31, whose hair is braided into dozens of thin strands. Backstage, the couple had changed from their shiny black and glittery silver stage outfits into subdued gray and tan suits. In contrast to their athletic choreography and evangelist singing on stage, their demeanor is quiet and subdued. They refresh themselves with tea and fresh fruit.

Ashford and Simpson met in a Harlem church in 1964 when he was 21 and she was 17. Simpson was a pianist and melodist, and Ashford had a way with words. They said "Let's Go Get Stoned" to Ray Charles in 1966 and were signed by Motown records as house writers.

They wrote songs in New York and traveled to Detroit to sell them. "We were always in competition with somebody else," said Simpson. The competition was stiff - the writing teams of Holland-Dorier-Holland and Whitfield-Strong and even better known names like Stevie Wonder and Smokey Robinson. But eventually Ashford and Simpson scored hits with "Ain't Nothing Like the Real Thing" for Marvin Gaye and Tammie Terrell and "Ain't No Mountain High Enough" for Diana Ross.

"We worked out of these little houses," Ashford said, "and the studio was in a basement. You couldn't believe this million-dollar operation was coming out of there. But it was some of our best training. We learned how to write fast and still get quality. It's paid off for us."

After seven years at Motown and two solo albums for Simpson, the duo recorded their first album, "Gimme Something Real" in 1973 for Warner Brothers. They've also continued writing for other performers; Denice Williams and Johnny Mathis, Chaka Khan, Quincy Jones and the Brothers Johnson have recorded their songs.

Their writing style has shifted from the snappy Motown sound to the currently fashionable silky disco. But their concern with inspirational man-woman relationships has been constant. "There are so many different kinds of music," noted Ashford," and we've tried to cover one area very well - the area of love and the spiritual uplift that comes from a relationship."

They married in 1974 after a couple of years of living together. "As you grow, you change," said Ashford, "and it's reflected in your music. We work very spontaneously now, where we used to labor over things. Val will play something on the piano and a vibe will hit me and the song will come. We never worry where the next song will come from anymore. We've done it so long, it just comes to us."

When Nickolas Ashford and Valerie Simpson were house writers for Motown's singers, they were capable craftsmen of the style, but hardly exceptional. Now that they're writing disco love songs for their own duo act, they're still lovable, but less than overwhelming.

Though they're better known as songwriters, Ashford and Simpson's singing at Constitution Hall Saturday night easily outshone their compositions. The songs they've written for themselves, such as "By Way of Love's Express" and "Don't Cost You Nothing," are interchangeable with dozens of other glossy soul love songs. But on stage their voices chase each other up the scales into distinctive, trilling falsettos.

When they reached the top on "It Seems to Hang On," Ashford held a note in a shimmering vibrato Simpson's wispy voice jumped around. They sounded like two Eddie Kendrickses trying to outdo each other.

Their latest project has been some additional songs for the film of "The Wiz," whose choreographer, George Faison, has worked with the couple on its stage act. Ashford and Simpson pranced and slithered erotically in shiny black outfits glittering with rhinestones and silver linings.

One genuine moment cut through the evening's glitter. The two sat down on the piano bench for a different arrangement of their hit, "Send It." Simpson played the block chord and the two sang in unadorned gospel voices that filled the hall.