Keep America Singing (PBS, 8-9:10 p.m.; please check local listings, since schedules vary widely during pledge week): If the virtuoso harmonizing seems a far cry from renditions heard at the corner barbershop, it's because the groups performing are international quartet champions.
Acoustix from Dallas, for instance, recently made its Carnegie Hall debut and is the 1990 winner of the International Gold Medal of Barbershop Quartet. The 1991 winner is the Ritz of Toledo, Ohio, which sings on three continents. Rural Route 4 of Kansas City, Mo. - the 1986 international champions - includes two generations of singers and offers a touch of rustic humor.
Presiding over the homespun event - which takes place in a Victorian parlor setting - is the knowledgeable host, Mitch Miller, whose relentless ``sing-along'' records and TV shows became big in the 1950s and influenced at least one generation of pop listeners. He began as an oboist with George Gershwin in the original ``Porgy and Bess'' stage musical, and here he is today - same guy, same beard, same faintly Mephistophelean grin.
Nightline (ABC, 11:35 p.m.): For two consecutive nights, the program is devoted to ``Castaways,'' a revealing and in many ways inspiring glimpse at life on Penikese Island off Cape Cod, Mass., where a staff of four try to rehabilitate eight young offenders. It is a hard, remote place with no electricity, indoor plumbing, or TVs. We see kids hooded up against the cold, on boats, cutting wood, and generally learning - through hard work and cooperation - that actions have consequences and that they are capable of constructive achievement.
The ex-marine running the place has no illusions about ``turning them into choirboys,'' as he says on the show. He merely hopes that if, for instance, a kid commits robbery again, he might be content with taking the money and not also decide to beat the victim. But the show does manage to convey what has been called, with some realism, the rehabilitative ``magic of the island.'' (Part 2 airs Thursday at 11:35 p.m.)
Please check local listings for these programs.