RECENT STUDIES show the nostalgia gap is narrowing. What recent studies? Why, recent studies by the Armchair Observers of Troublesome Societal Phenomena, a group that could very well exist but does not.

In the mid-'60s people were nostalgic for the '30s - a three-decade gap. In the late '60s people were already nostalgic for the '50s, we are experiencing a surely premature fascination with the late '60s, that tiresomely turbulent time now understandably considered exciting by those living in its apathetic aftermath.

If this escalation continues, we may come full circle and by 1984 actually be nostalgic for 1984.

The lag between vantage point and object of yearning has been narrowed by television, perpetual paean to the short attention span and dogged recycler of anything that might help fill empty acres of temporal space. "Now is not so very far from then," sings the JImmy Thudpucker character on tonight's "Doonesbury Special" on NBC, and television is the medium that has accustomed us to being bounced from now to them in the twinkling of its eye.

Tonight, NBC is bringing back the '60s, and in more ways than one: two. "A Doonesbury Special," at 9:30 on Channel 4, follows "The Hobbit," at 8 o'clock on Channel 4. The realistic cartoon characters in "Doonesbury," from the most successful comic-strip since "Peanuts," are '60s expatriates trying to mobilize what remains of their committed little souls. The fanciful grotesques of "Hobbit" are the goofy, mythic figures who entranced hordes of psychedelirious '60s escapists in the "Lord of the Rings" trilogy by J.R.R. Tolkien.

One sings; the other doesn't. Turning refugees from the '60s into animated cartoon characters proves an ideal distancing device - the '60s were a cartoon, in a way - and "Doonesbury" also manages to maintain the singular soft-acerbic flavor of the printed strip it is based on. But the hobbits, goblins, monsters and sages dreamed up by Tolkien have already been turned into animated characters in the minds of those who read and loved the books, and these people will have little choice but to be disappointed by the essentially mundane materializations on the NBC special.

"Doonesbury" extends and enhances our own conceptions of the Garry Trudeau characters and their world, but "Hobbit" scales down the Tolkien creatures to something less than liter-size. Worse, the fantasy lacks depth and mythic resonance. It doesn't help, either, that Bilbo Baggins, the reluctant Hobbit hero, looks a tad too much like Buddy Hackett.

The two programs also demonstrate two approaches to animation for television: right to wrong. "Doonesbury" is true to Trudeau's drawing style, but with backgrounds slightly more detailed and of course with color and a soundtrack added. The color doesn't intrude; in fact, it adds a compensating warmth the strip often lacks. The voices, including those of actress Barbara ("Nashville") Harris as Joanie Caucus and Rev. William Sloane Coffin (himself) as the radical Rev. Sloane, are perfect.

In addition, "Doonesbury" is the work of real animators and "Hobbit" the product of program packagers. John and Faith Hubley, the Oscar-winning team whose short films have not been widely seen on TV (they are now popping up on public TV now and then), began work on their half-hour special a year ago. John Hubley died in February and the program was completed by his wife, in collaboration with Trudeau. It reflects the concern of craftsmen whose goals included high fidelity in adaptation.

But "Hobbit" comes from the cartoon factory of Arthur Rankin Jr. and Jules Bass, who have produced innumerable lackluster cartoon specials, often seem on local stations prior to holidays, and some of those Saturday morning kiddie bashes. Obviously more time and effort have been expended on this program than on routine fare, but the animation shows some of their usual weaknesses - inexpressive characters, monotonous backgrounds, occasional monochromatic stretches, general lack of ingenuity.

Neither show is really "full" animation in the grand old Disney sense. It has been said that the beauties of such elaborate (and expensive) animation are lost on TV audiences anyway, since television transmits minimal visual information and images of limited detail. The fact is, however, that the old Disney stuff looks better than the new short-cut stuff even on TV; surely even children notice the difference between characters that move like living things and characters that move like shooting gallery ducks on a treadmill.

With "Hobbit," we are all too aware that we are looking at a speeded-up series of sequential drawings, partly because the characters rarely stand out from the backgrounds; everything blurs together in one wan mass. The Hubleys were able to stay in a stylized milieu and yet make the "Doonesbury" characters distinct. They also played entrancing games with perspective and illusion, as when the camera takes a fly's-eye tour of a rock band, zooming up from among the instruments and out among the audience.

"Hobbit" looks especially barren when compared not only to the Hubley show but to the eye-tickling hyper-kinetics in the new breed of animated commercials, especially those for Levi's and for 7-Up. It's obvious that the people who made "Hobbit" had a consistent, watercolorry,faithful-in-spirit look in mind, but it's not a look one feels comfortable with; there's something musty and fussy about it that is quickly wearying. A few highlights like the burst of fire from an irked dragon's mouth help, but an unfortunate passing resemblance between the little 14-man army of hobbits and elves and the seven dwarfs in Walt Disney's "Snow White" works only to the program's disadvantage.

Tolkien's stories of life in his imaginary Middle Earth were a campus rage in the late '60s, about a decade after their first publication. Some have tried to attribute allegorical powers to the novels, but their appeal was more likely escapist - escapism with honor, as it were, since the acquiring of a taste for them and the joining of the Tolkien cult were considered marks of sophistication. As fairy tales, they were basically unsentimental; naturally for TV, they've been sentimentalized, and the program is regularly interrupted for a ballad about "The Greatest Adventure of All."

As it happens, the TV "Hobbit" may not be definitive even by default, because director Ralph Bakshi ("Fritz the Cat") is now preparing a feature-length version of "The Lord of the Rings" for release by United Artists in June of 1978.

"Doonesbury" deals with the '60s more directly. Its characters are the compatibly misfit residents of a trendy commune. They now live in the '70s, where remnants of '60s rebelliousness and value-rejection have been formalized and institutionalized by a culture that swallowed the counter-culture whole. The air is mosquito-pocked with phrases like "affirmative action program" and "role model" and the pregnant perorations of ostensibly raised consciousnesses. People are heady with the guilt that's inevitable when every remark is a potential policy statement.

Trudeau's script catches the cadences of this condition, this quirkily transposed embattled idealism, in a shrewdly skeptical way. "Doonesbury" depends heavily on spoken words, as animation theoretically should not, but the words aren't stumbling blocks; they are in harmony with the drawings in their bright, smart slant on our me-directed Now and our far more passionate Then.

So it is that Joanie Caucus, in her Javertian pursuit of The Right Thing, not only teaches at a day care center but tries to steer a little black girl there toward a career in the racially imbalanced construction industry. "But", the little girl protests, "I don't want to be a building contractor!"

Once these people fought the world with pious placards and blazing anthems. Now they are armed with - RECENT STUDIES.

NBC is currently preparing a TV series based on the book, "What Really Happened to the Class of '65?" It isn't altogether foregone that the real live people on that show will be able to seem more real and live than the Trudeau creations in the "Doonesbury" special, nor come up with more pungent insights into nows and thens. Eventually, of course, the era will be analyzed to death and metabolized from still-painful memories into just another collection of rock 'n' roll's greatest hits. At the moment, though, the period remains tantalizing - frighteningly near and frighteningly far away as well - and "Doonesbury" proves one of TV's best treatments of it.