GLORIOUS NEW AGE of television, like prosperity, is supposed to be just around the corner. Comes the cable revolution, our screens will overflow with culture, information and entertainment. So say many people, including the government's communications advisers at the National Telecommunications and Information Administration.

Well, don't hold your breath. Television is going to get worse, not better. As scores, even hundreds of channels open up through cable, satellite and other technologies, audiences will become fragmented and TV will become radio.

Back when commercial FM radio stations were first being planned, hopes were high that the proliferation of new channels would spawn new opportunities for classical music, opera, quality children's programs, public affairs documentaries, drama and other selective programs of interest to special audiences. Today, despite the fact that more radio stations crowd the airwaves than ever before, there are virtually no such programs on commercial radio.

Commercial radio offers scarcely any drama, comedy, children's programming, live performance or major documentary series. (These are available, but only on National Public Radio.) Instead the medium is dominated by low-cost entertainment and talk. The national commercial networks are all but gone. And most radio air time is filled with replays of pop tunes supplied to stations on discs and cartridges to promote record sales.

Commercial television is heading in the same bleak direction. As new cable and satellite distribution technologies multiply, the ABC, CBS and NBC combined audience share is being diluted, mimicking the experience of radio 25 years ago. In the past five years the TV networks' share of audience has declined 15%. And advertising agencies are projecting continuing network share losses as competitive TV channels proliferate.

New competition from local UHF independent stations, as well as from distant channels brought in by cable, has intensified the battle for survival. While the networks continue to offer some outstanding fare, in the stiffening contest for commercial success, prestige programs are fast disappearing. "Sunrise Semester," CBS-News' children service, NBC's "Studio 8-H," original drama, major performance programming -- the commercial network loss-leaders are becoming extinct species.

As for the cable revolution's new and wonderful pay program services, their entire financial base rests so far on recently released movies, a few championship boxing matches and soft-core pornography. Of the 10 or so pay cable program channels now in business, six are movie channels (HBO, Cinemax, Showtime, Home Theater Network, Spotlight and The Movie Channel). Two feature sex (Escapade/Playboy and Private Screening).

Bravo, the only pay cultural channel, has already cut back to the point where it has stopped producing original programs. And the newest entry, The Entertainment Channel, with a high percentage of BBC pop entertainment, indignantly refuses to be labelled cultural.

One interesting straw in the wind, according to a recent report in Forbes magazine: "Some executives at Home Box Office, the country's most successful pay-cable movie program service, have privately acknowledged . . . that, in each of the last three months, the disconnect rate has been running alarmingly above normal." For other pay program services the connect rate has been alarmingly slow.

Of the score or more basic cable program channels that have cropped up so far, every one continues to lose money, and none shows signs of turning a profit in the foreseeable future. That is especially true of the cable news headline services (Ted Turner's CNN 1 and CNN 2 and the Satellite News Channel), as well as the two basic cultural channels, ARTS and CBS Cable (reputed to have lost some $50 million so far). The rest of the basic cable program services are a grab bag of independent commercial stations featuring off-network reruns of local sports events, as well as religious, health, weather, pop music, women's, ethnic, sports, children's entertainment and game show offerings. In short, like the Holy Roman Empire that was neither holy nor Roman nor much of an empire, the wonderful new television revolution is neither wonderful nor new nor very revolutionary.

Why, despite all the promises, is the picture turning so depressing? The reality is that the average cable household is spending less than an hour a day watching all those basic cable services put together. When audiences are carved up into so many small segments, advertisers won't pay enough to provide for the production of quality programs. The result, as happened with radio, is the inevitable dilution, fragmentation and cheapening of television.

All this follows the pattern TV established more than 20 years ago when a two-network system first turned into an increasingly competitive three-network environment. Sitcoms and action-adventure series overran Playhouse 90, GE Theater, U.S. Steel Hour, See It Now, Studio One, Kraft Theater, Omnibus et al. Greshman's Laws worked on TV's "Golden Age" as effectively as it works in high finance: Cheap TV series tend to drive out quality television. And the cable revolution tends to accelerate that whole process. The greater success cable has in the future, the greater the commercial pressures will be to produce lowest- common-denominator programming to attract bigger, more sellable audiences.

To be sure, cable will play an important role. But we should have no illusions. So far there are no really new programming media at all, just new technological means of adding channels to TV sets. All this may very well do great things for distributing data, providing electronic bank deposits, introducing remote control shopping, playing video games and taking instant opinion polls. But it has nothing to do with good programming.

The irony, in fact, is that the audience for public television, traditionally a small fish in a big pond, has begun to thrive in the new cable environment. Public television's predominantly UHF station reception benefits from the spread of cable transmission, and its alternative national programs gain more visibility as commercial programming deteriorates. The latest figures show that public TV programs are watched 30 percent more in cable homes than in non-cable homes.

While commrcial network audience shares are declining, public TV's shares, though small by commercial standards, have been growing dramatically. In the past five years public television's audience has doubled. And indications are that it will expand further as commercial television programming continues to decline under the impact of the new technologies

Focusing on the continuing need for public television, FCC Chairman Mark Fowler recently compared it to the need for "public parks, museums and libraries, . . . religious and educational institutions, and the whole system of free public education."

As someone with an unqualified bias in this regard, I would like to thank the people who are bringing us the telecommunications revolution for making support of public television more and more critical to our nation. Larry Grossman, former vice president of NBC, is president of the Public Broadcasting Service.