No doubt the main reason WETA chose recently to tape an interview with the 79-year-old Todd Duncan is that the 50th anniversary of his debut in the title role of George Gershwin's opera, "Porgy and Bess," is approaching. But many of the high points in this fine 30-minute program (to be shown at 9:30 on Channel 26) are the memories of fighting segregation here in Washington, where generations of Duncans have long been pillars of the community.

The producers at WETA wisely limit the narration and even the music to a minimum, so rich are the recollections of the noted baritone and his wife, Gladys, who are interviewed at their home.

Duncan, who is retired but continues to teach young singers, recalls when he found out in 1935 that "Porgy," which is about life in the ghetto of Charleston's Catfish Row, would be coming to the segregated National Theatre and the stir that came when he refused to perform under those circumstances:

"They finally called my hotel and said they would allow Negroes to come in on Wednesday afternoon and Saturday afternoon, the two matinees. And I refused. Then after about five or six days, they claimed that Negroes could sit in every performance, in the balconies.

"And I refused, and that's when my union came into the matter, and my bosses in New York. And they said that I was not meeting the situation halfway. Well, I didn't understand any compromise on human beings. And yet I know what compromise means. To me it did not mean to sell your soul."

Duncan prevailed, despite threats from the union to fine him and revoke his membership, and blacks sat in every part of the theater.

He also mentions other artistic restraints on black residents of the city:

"I greatly missed concerts and the theater and opera. Negroes could go to Constitution Hall. But Section O was the only place they could sit. You go there right now and you can still see Section O."

Gladys Duncan adds her memories of the demonstrations that went on here for years before segregation was toppled. She was the president of the Americans for Democratic Action and vice chairman of the Democratic Central Committee. "We sat in restaurants and drugstores all over Washington." And she recalls one favorite trick: "If you could speak another language, they were afraid not to serve you, because they thought you might be diplomats."

Apparently Duncan was a hard bargainer not just with segregationists. He recalls the events that led to his being chosen as Porgy. When called to audition, he put Gershwin off for a week because of a commitment to sing at the Plymouth Congregational Church.

But he went to New York the next Sunday. "I sang 12 bars." And then Gershwin broke in and asked him "to start again and move around and look him in the face. 'Look straight at me as you sing.' I got to six bars. And he stopped me and asked, 'Will you be my Porgy?'

"And guess what I said? I had the gall, the nerve, to say, 'I don't know until I hear your music first.' He just adored it. To be that honest."

The next Sunday both Duncans returned to New York. They met with Gershwin, along with Ira Gershwin and representatives of the Theater Guild. "We all sat around on the floor . . ." and George and Ira sang through the whole opera, Duncan remembers, "and that was a day that I was in heaven."