Television has an open invitation to diddle with reality and rewrite history. The possibilities are simply inherent in the medium, and the profit motive encourages selective distortion in the interest of pleasing an audience.

The most we can hope for is that a truth will evolve from this manipulation of the truth -- an illumination, a revelation, or at least a peep of insight. ABC's sequel to the television landmark "Roots" promises, despite its merger of fact and fiction into what author Alex Haley calls "faction," to be the kind of bracing communal experience that the first one was.

"Roots: The Next Generations," to be televised Feb. 18-25, will bring an entire nation to attention all over again.

ABC publicity concedes that this "novel for television" is guilty of "mingling literal fact, probability and possibility," but to judge from the first twohour episode -- which starts 12 years from the point at which the first "Roots" ended -- and excerpts from the other six, the legerdemain justifies itself.

"It is a true story," said executive producer David L. Wolper at a press conference here, "but we have created certain characters who did not exist in order to tell it." Hmmm. The logic may sound wobbly, but then, we do not come to television for facts, figures and concrete statements. We come for impressions. Usually the impressions dispensed aren't worth the air they ride in, and most prove pitiful and frivolous beside the point. Any point.

Naturally, Wolper was asked if author Haley's legal troubles would affect the credibility of "Roots" on TV. Haley was sued for plagiarism over two paragraphs in the original book, and settled out of court. Wolper said the material involved has nothing to do with "Roots 2"; the sequel expands upon history that only took up 30 pages in the first book but was later amplified by Haley for the new series. Wolper also said Haley was so depressed about the lawsuit that he couldn't write. Wurely, he will soon recover.

People responded to "Roots" and they will open their hearts to "Roots 2" partly because there is so little else on television that makes substantial emotional demands on them and partly because there is no narrative form so satisfying as the saga of a family told against a background of historical events and social change. The two "Rootses" together are the "war and Peace" of television, and they may also comprise one last hurrah in the '70s.

It is a pity that NBCs effort at historical presto-changeo, "Backstairs at the White House," which begins its mini-series run tomorrow night, looms far less lofty on the living-room horizon. Though a cast of admirable actors brings a great deal of dignity to the project, the story, at least in the first three-hour chapter, tippy-toes along as if afraid of waking a sick uncle in the next room.

"Backstairs" went through considerable transformation in its progress from truth to television. Lillian Rogers Parks, the former White House maid who wrote the original book, has admitted she left out the nasty bits because she didn't want to tattle on the eight presidents for whom she worked from 1909 to 1961. When NBC bought the book it was perceived that the thing was too so-so to make riveting drama, so the original was amended after conversations with Parks; among other embellishments, a new character, played by Louis Gossett Jr., was invented.

Then the script was "novelized" and published as a paperback book. This may make it less a saga of the generations than an experiment in how many generations you can get from reality and still pass something off as real. Strangely, American television can't seem to get the rousing and skeptical grip on the leaders in its past that British television does with such elegant wingdings as "Edward the King." now running on local stations throughout the country.

An early title for "Backstairs" was "Backstairs at the Big White Jail." That was jettisoned not a moment too soon. A more appropriate one might be 'Blackstairs, Whitestairs," wince the production mixes soap opera and gossipy if basically reverential trivia and because even though the black servants are brought into the foreground, they are still portrayed mainly as awe-struck subservients to the whites in power. The presidents tend to remain stick or stock figures, so that when Robert Vaughn's Woodrow Wilson suffers a stroke, he looks a bit like an automated mannequin at Disneyland undergoing a stripped gear.

Characters were invented for "Roots 2" as well, but there is a core of authenticity and vitality that compensates for whatever compromises were made in the name of dramatic license. For instance, the fourth episode opens with a shattering incident that is not directly linked to the Haley family tree -- 13 black veterans of World War I are hanged by the Army at a camp outside San Antonio in 1917. Producer Stan Margulies says the inclusion of this scene is completely justified.

"It really happened," Margulies says. "It was the culmination of a series of incidents between black troops in Houston and the Houston police. As a result, the Army took action against the black troops. They were hanged in secret, and there was no chance for a review of the case by the War Department or the president.

"Now. Alex's father was in the service during World War I and he had told his son of the problems black soldiers had, and in the original script, the hangings in Houston were repeatedly referred to -- they were an offstage event. And finally Ernest Kinoy, who wrote the first three scripts and supervised all of them, had the notion, 'Why should this be an offstage event? If we put that onstage, it says more about black soldiers than we could say in 20 pages of dialogue.' So that's how we did it."

ABC TV president Frederick S. Pierce has prophesied that the second "Roots" will have more impact on American awareness than the first "Roots" did. This could be dismissed as hubris aforethought, but Margulies, who produced both, thinks there may be some truth to it.

"In the second one, we deal with things that are much closer to the audience," he says. "The audience cannot say, 'Oh, that happened 150 years ago.' You could divorce yourself from the subject of slavery, but now we are dealing with things quite relevant today."

Could this mean that, recent memories being more painful than distant or vicarious ones, "Roots 2" will scare off large sections of the audience? "If we set out to say 'we are now going to give you 14 hours of preaching and praying about what a tough life blacks have had,' we'd scare everybody off," says Margulies. "But the focus of this story is still on the family and on the very human moments within the life of that family, and that's what counts."

From 1882, the story proceeds through 1960, when Alex Haley, played in the last chapter by James Earl Jones, goes to Africa, learns of his heritage and makes the comic leap back to Kunta Kinte, the child kidnaped by slavers in Africa generations earlier. In the last episode Marlon Brando makes his TV acting debut as American Nazi party leader George Lincoln Rockwell, whom Haley had interviewed on assignment from Playboy magazine.

Brando's sequence lasts 8 1/2 minutes and ends with his rendition of the racist ditty, "The Jews are Through in '72, Parlez-Vous." Exactly how "Roots 2" will end isn't definite yet since Margulies says the film is still in the lab being processed, but he would like to conclude with a montage of faces in Haley's past, starting with LeVar Burton, who played Kunta Kinte in the original but does not otherwise appear in the continuation.

Obviously, at this point, an audience that has made it through all 26-hours of "Roots" and "Roots 2" will want a chance for a huge emotional release. The failure to provide one at the close of "Holocaust" was one of the oddest errors of that production on NBC last season.

In certain respects, the second "Roots" is a substantial improvement over the first. According to Wolper, the first cost $6 million to produce and the second $16 million. The additional money -- only possible because of the astronomical ratings earned by the original -- shows in every physical aspect of the production. It is inordinately handsome and lavishly cast.

More importantly, the second "Roots" involved many more black actors, craftsmen and technicians than the first. There were two black writers, two black directors, and for some episodes a camera crew composed entirely of blacks -- the first time ever for a TV film production, according to actor George Stanford Brown, who appears as Tom Harvey in the first two episodes and himself directed the fifth.

Unfortunately, the first "Roots" did not change history in one of the most positive possible ways. Though it exposed or introduced a host of impressive black performers, there was no revolution in Hollywood about giving more parts to blacks or more sensibly depicting blacks on the screen. With most of the cop shows shot down by anti-violence crusades, there were even fewer of the demeaning criminal and street-wise parts to play. The best integrated series of the fall season, ironically enough, was "Battlestar Galactica," which is set in another galaxy altogether.

"Usually, the kind of success that 'Roots' had is exploited," said Brown at the press conference here, "but this time, mysteriously, it wasn't." Brown, strikingly authoritative in the first chapter of the new "Roots," said most blacks on TV continue to come off as "buffoons."

"I hope we don't have to wait for another 'Roots,'" Brown said, "because I think this is the last one."

"Roots" has changed television, however, if not in all the ways one might have hoped. Network executives now foresee that most Tv/ drama is going to be socially conscious or what we might call reality-oriented stuff. The idea of television as a theater in the home, as a medium of art for art's sake, is dead among the commercial networks and must now remain the province of public TV. We are in an era of functional fiction that inches toward documentary and of escapist news that Leans toward jazz and jollies.

Much is amiss, all right, but it's now being commonly predicted that eventually network television will abandon its national jester roll -- commit zanicide on "Laverne and Shirley" and send "The Waltons" off to a funny farm -- and become principally a news and information service. At yet another press powwow here, ABC News vice president Richard Wald, formerly a president of NBC News, said he thinks that in the '80s, because new technologies like videodiscs and tapes will siphone off the entertainment audience, "network television will turn more and more to live, simultaneous, nationwide programming," leading to much more news and sports.

And ABC News and Sports president Roone Arledge chimed in, "I think that will become the main reason for networks."

In the meantime, if the truth be told -- well it won't be. But in February, a persuasive facsimile, "Roots: The Next Generations," will show how powerful skill-fully souped-up pseudo-history can be, and it is very likely that millions upon millions will again respond. Lightning is about to strike a second time.