Commercial broadcasters would like us to believe that cable TV's promises of greatly expanded diversity in programming are so much pie in the sky. But it just so happens that millions of Americans are already partaking of said pie in their own little living rooms.

In operation now, and beamed to hundreds of cable systems via satellite, are two full-time children's programming services, a 24-hour-a-day sports channel and a channel devoted to live coverage of the House of Representatives. Soon to come: Ted Turner's 24-hour all-news wingding (June 1, 1980), a Spanish-language channel and an all-day service for older viewers called "Channel 50 Plus."

Suddenly there are pioneers in mass ccommunication again, and one of the bravest is Robert L. Johnson, who with perspicacity, determination and a few hundred thousand dollars has launched Black Entertainment Television (BET), a cable network Johnson says will offer black viewers, and viewers of other colors, too, the kinds of programming commercial and public TV can't, don't, or won't.

Johnson is starting small -- two hours a week, on Friday nights, beginning in early January. But he thinks big. "I want to be a communications giant," he says. "This thing has to succeed, and I'm going to make it succeed."

That isn't only bravado; Johnson's plans are based on exhaustive analysis of the market and its potential. "It takes guts to do what he's doing," says one cable insider. "The secret of this industry is entrepreneurs like Bob Johnson, who are willing to take a risk. He could fall flat on his nose."

Flat on his nose is not where Johnson, a mere 33, expects to land. This week he announced in New York that Tele-Communications Inc. (TCI), the third-largest multiple systems cable operator in the country, has become a minority investor in BET. Johnson is putting up some of the rest of the "seed money" himself; he hopes BET will show a profit by its third year.

BET programs will be offered to all U.S. cable systems free of charge, and will be free to viewers as well; Johnson hopes to support himself with advertising. So far only one major sponsor, a beer company, has signed up, but Johnson thinks others will come aboard.

"The black consumer market is a $75-billion consumer market," he says. "It generates more revenues than 114 nations in the world, and television sells products to that black audience -- and yet blacks still have trouble getting jobs in television."

What do black audiences want from television that they don't get now?

"What black audiences want is, I think, pretty much the same thing as white audiences: more choice. They want to see J.J. of 'Good Times,' but they want a sensitive drama about blacks, too. You take a show like 'Family.' Why couldn't that family have been black?"

Johnson is not exactly a crusader against the networks and the way TV shows depict blacks, but he notes of ABC's "Benson" that if Benson is that bright, why is he still a butler? Why isn't he an executive" The primary purpose of BET is not to provide idealized images of blacks, however.

"It's not that there's too little black programming on TV, it's that commercial television is limited to mass-appeal programming, which means it has to be bland and it can't be specialized," Johnson says. "We'll have a special channel that will showcase the full creative range of black entertainment, whether it's soap operas, game shows, sitcoms, dramas or Grambling College football games. We'll provide an option that's not there."

In cable TV, going up on the satellite, or bird, is called uplinking, and the extent to which homes are wired up is called penetration. It is commonly assumed that cable penetration of black households is not yet very substantial and won't be much more so in January when Johnson uplinks to the bird.

"No one's really done a nose count of how many black cable households there are," he says. "But we do know blacks watch more television on the average than whites. And we know that blacks like to watch programs with black actors and actresses in them.

"With that knowledge, and knowing that cable has only one place to grow and that's in urban America, where you have a large number of black households, I think cable with a black channel is going to very attractive to people. When the cable industry in 1983 or '84 reaches 20 million homes, 15 percent of those will be black homes."

Besides, he expects a high white-viewer tune-in as well; "black entertainment from the beginning has always been attractive to white audiences, and it will continue to be so."

Johnson's initial programming does not exactly constitute a Harlem Renaissance. At this point he cannot afford to produce his own material, so he has to acquire films and TV shows elsewhere. And so viewers will be less likely to see the prestigious Negro Ensemble Company than they will a kiss-and-kill bloodspiller like "Willie Dynamite," one of a number of films Johnson has bought.

"Some of the product I have happens to be so-called 'blaxploitation,' 'Johnson concedes. "But this is a one-step-backward, two-steps-forward kind of a thing. I'll be able to use that stuff to build an audience and, three, four, five years down the road, we'll be showing our own, original, made-for-Black Entertainment Television programs. I think that will completely overshadow what we're starting with."

And it won't be all Superfly or Fist-face at the start, either. For years a black motion picture industry existed parallel to Hollywood's, turning out films mainly for segregated theaters. The films were not lavish extravaganzas, but some preserved on celluloid key performances by legendary black entertainers. Johnson has acquired some of these for BET, though he says the American Film Institute, which owns a bunch of them, has refused to cooperate with him on getting more.

A former vice president of the National Cable Television Association here, Johnson became a cable lobbyist without knowing "anything" about cable. But who did? Now, unmistakably a go-getter, he has an office in Georgetown, an emphatic corporate logo, and the energy of the truly zealous.

He made a pilgrimage to Hollywood to drum up support among black professionals in show biz and "the reaction was overwhelmingly positive," he says. So far, some of the biggest cable operators in the country -- Warner Cable, United Cable TV Corp., American TV and Communications Corp., TCI and others -- have signed up for BET.

"It's a good, American apple-pie kind of service for a cable operator to carry," Johnson says. Lining up advertisers has been harder. Johnson isn't just after Afro-Sheen and all the other products in the Soul-Train medicine cabinet. He wants major advertisers to see his service as essential to their campaigns.

"Network prices are going sky-high, and advertisers are looking for alternatives," he says. "They're after the biggest bang for the buck; if it can be returned by advertising in Ebony, then they buy Ebony. I think the idea of a sort of 10 percent set aside for 'my special minority buy' is somewhat dead."

Some of Johnson's futurology may sound lofty to the point of naivete, but when he gets revved up, there's no doubt that you're in the presence of a dedicated daredevil. It was trail-blazer types like him who pioneered American broadcasting in the first place, risking the loss of countless shirts in the hopes of striking it rich.

One can only hope that this time -- as TV is reinvented for the '80s -- it will turn out better.