People are laughing. Shucks, they're hysterical! They are whistling stomping, cheering; they're parting company with their marbles. And all for nothing, Either through manipulative tape editing or because they are all shot full of laughing gas, the audience at "The Great American Laugh-Off" is enjoying itself beyond reason.

Every act gets an ovation and every joke splits sides on this 90-minute comedy pilot, taped in San Francisco and airing tonight on NBC (Channel 4) in the "Saturday Night" timeslot of 11:30.

"Laugh-off" has nothing whatever to do with "Saturday Night," it's part of a deal producer George Schlatter made with the network when he agreed to do those lowbrow but highrated "Laugh-In" specials. "Off" features some of those zany "In" regulars - so called comedians with tediously mobile limbs and faces who endeavor to coerce and bamboozle one into being entertained.

And boy, do they have help - the most eager, willing, unctuous, risible, gullible, artificial audience since the synthesized crowd that used to double over for Dean Martin "Roasts." The program is a virtual texbook on how post production tinkering can turn a mediocre series of performances into a howling, shrieking smasheroo.

The noise level is relentless, most of it either manufactured or amplified; you find yourself yearning for one of the bright young things to be greeted with dumbstruck silence, just to give the show a little variety and oneself a moment's peace.

The producers of "Laugh-Off" don't leave anything to chance, nor trust us to decide what's funny and what isn't. The idea is to create the illusion of unmitigated triumph. It is time to turn down the laugh machines.

Some of the comics on the show have bright or promising material, though their styles are generally too intense for TV. A slighty innovational group called Duck's Breath has a high time with a simple box, but another of their sketches, a whimsical approach to sex education, has been censored from the program. Better by far they had dropped "Toad The Mime." Better still they had forgotten the whole thing. "Baby, I'm Back"

CBS tonight plays shuffleboard with its Saturday night schedule - which hasn't been successful this year anyway - to air the pilot for a possible pinch-hitter, "Baby I'm Back," at 9:30 on Channel 9.

The situation comedy, set in Washington - though obviously taped about 3,000 miles to the West - starts with a premise that may strike many viewers as unfit for comedic treatment. A black husband and father of two children returns to rescue his wife from remarriage after deserting the family seven years before.

Though this is hardly the stuff of laugh riots, and thought the comic tone of much of the dialogue is woefully coarse, "Baby, I'm Back" succeeds with the help of its leading players. Desmond Wilson, chubbier and hairier than he was for five years on "Sanford and Son," proves an actor capable of far more than being a straight man to a junk man.

And Denise Nicholas, who starred on the old ABC series "Room 222," is welcome and refreshing as the abandoned wife, though the character has been written along largely stereotypical lines. The most obvious cliche on the premises, however, is Helen Martin as Wilson's fire-breathing mother-in-law, Luzelle. Writers Lila Garrett and Mort Lachman shamelessly overuse her as a quick-laugh device.

When it comes to crowd-pleasers, nothing on the program can compete with Kim Fields and Tony Holmes as the two kids. They're not exactly actors, but kids on TV don't on TV don't have to be; they're just terribly cheering, credible little presences. The best scene on the show is the one in which the kids are alone together, when the other characters and their battles, and the show's itchy gas reflex, have been stilled.

May television be graced with many more such stillnesses; they could help keep us from going thoroughly out of our minds.