YESTERDAY'S godsend has a way of becoming today's disappointment. And tomorrow's blight. Cable TV, not so long ago, was going to be the miracle that rescued us from television. Eventually, we may all be looking for something to deliver us from cable TV.

Although cable has hardly reached its official day of reckoning, it has definitely passed out of the zone of amazement. Cable is now a highly potent economic force in mass communications, with yearly revenues fast approaching $1 billion; it has helped to reduce sharply the networks' share of the total viewing audience; and it is settling in to the national consciousness as plain old television did long ago.

But the shimmer is dimming in two ways: For entrepreneurs, the gold rush days are over, because the cable market is becoming glutted with more available program sources than existing cable systems can handle, and some, perhaps many, are sure to fail. In addition, the thrill must be fading for millions of cable subscribers who thought a vast and rewarding new world would open up to them when the almighty wire reached their homes, only to find that world far more vast than rewarding.

The great promise of cable was that it would eschew commercial TV's slavish courtship of the fat mass audience and that "narrowcasting" would rescue people disenchanted with, if not disenfranchised by, broadcasting. Now it appears cable is a boon mainly for those whose idea of nirvana is the ability to watch a soccer game at 4 o'clock in the morning or see a motion picture clinker whenever their hearts desire. Home Box Office, the money-green giant of pay cable networks, grovels to the mass audience just as devotedly as the networks do. It provides viewers with hours and hours of generally terrible movies instead of hours and hours of generally terrible TV programs.

What cable needs is a gale of fresh air, more bold innovators, and some class. It will get at least the last of these--and maybe some of the first two as well--from Arthur R. Taylor, the former CBS Inc. president who is now chairman of the Entertainment Channel, an ambitious and costly new cable network that beamed up to its satellite transponders on Friday and from there to a small number of cable subscribers Taylor feels certain will quickly grow.

The Entertainment Channel differs from other "premium" channels (those that cost the cable subscriber something extra each month in addition to the basic cable fee) in that its foundation is not movies. Its foundation is the prestigious and productive British Broadcasting Corporation (BBC), most of whose programs previously entered the U.S.A. via the Public Broadcasting Service. Under a new 10-year agreement, the Entertainment Channel gets first dibs, and two years of exclusivity, on all imported BBC shows.

"We call ourselves a premium television network for those people who have outgrown ordinary television," says Taylor, who is even more consummately natty than one would expect a former CBS president to be. "No one's ever tried before--and it's an interesting try--a premium service that's a network in orientation. It has lots of different things, series and movies and mini-series and specials and so on, all beamed at a part of the commercial network audience for a very simple reason: Our research, reinforced by the Gallup's, indicates that about a half of the American television audience is totally unhappy with what they're seeing. Do they stop looking? No. Are they dissatisifed? Yes. We have a real potential within that group."

The BBC material is only part of what the 24-hour channel will offer. Among its opening-month attractions: Ben Vereen in a taped performance of the Broadway musical "Pippin"; a six-part BBC serialization of "Great Expectations" by Charles Dickens; the premiere of "The Animal Express," a daily visit to the San Diego Zoo; and "Two's Company," the original British model for a later CBS sitcom, "The Two of Us," about the conflicts between a brassy American journalist (Elaine Stritch in the British version) and the proper English butler she hires.

If it sounds better than the usual commercial network fare, some of the programs previewed look better, too (although "Animal Express" is excessively talky and trots in incongruous guest stars like Brooke Shields). But ABC, CBS and NBC are not Taylor's competition. His competition includes all the other new cable services vying for the limited amount of channel space available on systems throughout the country. A Hollywood TV producer who attended a recent cable convention counted 19 premium channels being hawked--despite the fact, he says, that "most existing cable systems don't even have 19 channels," much less 19 to apportion to premium services.

Taylor says the competition among cable networks, especially the movie services, is ferocious, that it's "dirty, dirty pool all the time," but the Entertainment Channel won't be trying to knock movie services out of systems; instead, it will be pitched as a supplemental channel, one for cable viewers--especially those who get more than one movie service from their local systems--tiring of such treats as "The Hand" or "Loving Couples" or "Dirty Tricks" or innumerable other available lemons offered umpty-two times a month.

"One of those movie services," Taylor says, "has a 70 percent churn rate a year, which means they replace their subscribers 70 percent annually. All that is is subscribers saying, 'I'm getting the same stuff from more than one service , and I'm paying $20.' In February, you could have watched 'Kramer vs. Kramer' 200 or 300 times if you'd taken HBO, Showtime and The Movie Channel."

Another thing The Entertainment Channel has going for it is money, $100 million of it, according to Taylor, put up by RCA and Rockefeller Center (the service was originally to be called RCTV). "People are finding out cable is not a $15 million investment," says Taylor. "Unless you're prepared to put in $100 million, you really can't even play the game."

Taylor also predicts that soon some of the premium channels that sell themselves as "commercial-free" will have to start accepting commercials in order to stay afloat. "I think advertising will be on pay cable." In five years? "It will be less than that. HBO is making money, a lot of money, but the other two movie services are either not at break-even or just recently at break-even, and the cost of motion picture programming is rising so tremendously . . . that if one is to hold the monthly charges within an even band, it could be a lot less than five years." Cable subscribers start to balk when their cable bills get beyond $30 a month, Taylor says.

So--cable TV is being taken over by the same kind of gigantic corporate interests that have always controlled broadcasting in this country, and people will soon have to sit through commercials even on channels they pay to receive. Then what is the big difference going to be between cable TV and regular TV? The amount of sex and violence available, perchance? Taylor says the difference is that cable will cater to more specialized tastes than regular television.

Specialized, yes, but not as specialized as some hopeful souls may have thought. The cultural cable networks now in existence--ARTS, Bravo, CBS Cable--are all suffering chronic financial strain, and it's likely only one, if any of them, will survive. Anyone who dreamed of being able to watch on cable opera, ballet, great drama and the kind of high-toned fare they remember seeing on free TV in the '50s, may be pipe-dreaming. Taylor, for his part, shrinks from the "culture" label and says his competitors have tried to force it on him in an effort to discredit the new channel.

"What 'culture' does say is that you're relegated to a very small part of the audience. We did a lot of research on the performing arts, and they haven't jiggled up one-tenth of one percent, in terms of interest. Absolute numbers have increased, because the population has increased. But at the bottom of the scale are, essentially, musicales, then going to symphonies, and opera goes up a little bit, and with dance, ballet, pretty good. But still, you are only talking about anywhere from one percent to 4 1/2 percent of the audience."

A certain irony about Taylor's drum-beating for his new company (he is an investor as well as an officer) can't be overlooked; while at CBS, from 1972 through 1976, Taylor was one of the loudest industry spokesmen decrying that vile encroaching gorgon, Pay Television. Oh, what could be worse for this great nation of ours? Taylor has a standard answer for how he so radically changed his spots: "When Bat Masterson was an outlaw and the leader of his gang, they were very successful, but then he became marshal of Tombstone and cleaned up the old gang, and everybody understood that. I don't think it's so hard to understand."

It is suggested to Taylor after this little parable that he has gone the opposite route--from marshal to outlaw. Denigrated though the networks may be, broadcasting has a respectability cablecasting lacks. Cable operators are moneymakers with no FCC-enforced mandate of public service. In city after city, the competition among firms bidding for valuable local franchise rights has become so politically sordid that Newsweek pronounced it "a national scandal."

Taylor concedes he does not walk as tall as a cablecaster as he did as a braodcaster. "There's a difference. I have not personally seen any of the corruption, but there sure are an awful lot of stories around." Aren't cable system operators, as a group, a sleazy crowd? "I won't quibble with you on words. I think there are great variations. We're really in the first generation of cable operators; some of them owned television set stores, some of them were installers, and some of them just sort of blundered into cable at midlife because they didn't know what else to do.

"The ones that have been the best are as brilliant and shrewd as you can find. But virtually everybody has done well that's been in the cable business. And now we see a change. The system operators are gobbling up systems and there's a different breed of people coming along." Better or worse? "It's not clear. I have a feeling we may be replacing the mullets with the sharks, but I'm not sure." Webster's definition of a mullet: a fish having "a small mouth and feeble teeth." Small mouths and feeble teeth won't get you diddly in cable these days.

No one in the Washington area will get a gander at the Entertainment Channel because the few cable systems now in this vicinity haven't signed up for it. John Evans, executive vice president of Arlington's Metrocable system, says, "We are considering it. We're still doing an evaluation of it and balancing that off our market research. There are potential problems. It will rely very heavily on BBC programming, and whether that is sufficient to drive a pay TV service, I'm not sure."

Metrocable, which has a 35-channel capacity, is about to add John Coleman's Weather Channel (a 24-hour national weather service), a health channel and the Playboy Channel, which Evans says is "driven by sensuality" and thus is expected to succeed. The Playboy Channel offers not pornography, precisely, but R-rated movies, living centerfolds, and other sex-themed fare.

Some in the cable industry think R-rated material is the cable wave of the future. Taylor says, "I think that's ridiculous. The statistics that are available just won't support that. Escapade which will become the Playboy Channel tails off very sharply after the third month in terms of the number of homes that retain it. They're hard R movies, is what they are. They come into the home on Friday night, by Saturday night you get a little tired of them, and by Sunday night, how many people will look at that stuff? And yet everybody has decided that if you really want to be sure, you put on dirty movies."

Taylor says the National Cable Television Association would love to have the Entertainment Channel's presence here, where congressmen and others in the mover-shaker establishment can see it; it's considered good public relations for the industry.

While Evans says he is skeptical of the Entertainment Channel's potential for success, he also says, "I am very impressed with Arthur Taylor. If anybody can pull it off, he can pull it off." Taylor obviously thinks so, too, and what he feels he can bank on is not only the lustrous quality of his own programming, but the lacklustrousness of the old commercial network stuff that, figures indicate, Americans are wearying of in record numbers.

"Our best work indicates that the total prime-time share of the three networks, now about 75 percent--down from 90 percent 10 years ago--will fall to about 60 and stabilize there," says Taylor. "Now a 15 percent drop in market share will have a very very profound effect on return-on-investment of networking. The networks will either have to accept lower profitability or cheapen the product in order to keep the profitability up. One of the silly things is that if they had been willing to sacrifice profitablity prior to this, a lot of what you and I are sitting here talking about would never have happened. What made it possible was, there were important constituencies of audience out there who grew dissatisifed.

"What we are offering is a dependable consistent quality. It's like starting a restaurant for people who have certain tastes. We think that will command not a giant audience, but a large enough audience, particularly among those people who sort of turn off the set at night and say, 'Jeez, I really wasted the evening.' There are," says Arthur Taylor, "a lot of people like that."