More by concidence than design, public television has back-to-back essays on sublime and ridiculous human endeavor tonight, and both programs are as pleasureable as they are admirable: "Alexander's Bachtime Band," at 9, and "La, La, Making It in L.A.," at 10, on Channel 26 and other public TV stations.

It's hard to imagine two more divergent ways of skinning cats. "Alexander's Bachtime Band" is an hour of performance and rehearsal by a group of 66 gifted young musicians who earned the right to travel to New York for the December holidays and play together as a concert orchestra at Carnegie Hall under the direction of Alexander Schneider.

"La La," another in the thoroughly priaseworthy though erratically scheduled "Nonfiction Television" programs from New York's Channel 13, is a hip, flip, manic and depressive look at 55 very outgoing introspectors who came to the pop culture capital of the United States hoping to make it big with a capital L and a capital A.

Filmmakers Caroline Mouris and Frank Mouris simply sat each of these hopefuls against a white background and let them talk to the camera about themselves, which in most cases couldn't have requiried even a moment's coaxing. The interviews are intercut so that someone is always interrupting someone else, and yet the film never loses coherence or direction. Occasionally the whole thing is interrupted for montages of still photos accompanied by Clyde Lieberman and Gail Lopata songs about the city of the padded shoulders.

A montgage of L.A. license plates, for instnace, dissolves from "MR WARM" to "MRS ILL" to the appropriate and succinct, "MADNESS."

Most of the hopefuls are young and appear to have modeled themselves after models; they are a this type or a that type. But gray-haired Lenore Woodward says she started her acting career at the age of 57 because "there aren't that many old dolls working." When a producer tried to get even her on a casting couch, she recalls, she laughed.

The others talk about New York, relocation, the climate, joy breathing, rejection, cocaine, cauliflower, Rolls Royces, Mercedes Benzes and Jaguars. "Counting Jaguars gets to be dull," one of them notes. They use words like "laid-back," "mellow," "vibrations," 'relationship," "tactile," "incredible" and "total commitment."

It doesn't matter that some of them are doing what amounts to nightclub comic material, because that is revealing in the way it attempts to be protective. All of these people believe excessively in themselves-they have to if they're going to be waiters at Hamburger Hamlets all day while hoping there'll be good news from their answering service at night-but some are more assertive about this than others.

"I put out nothing but positive vibrations," one young woman announces. "I love any place I've ever been, because I'm there." Another ends her plans for the future with, "I'd like to retire with my own talk show."

Few of them appear genuinely pathetic-not even the fat girl fired from the doughnut shop for consuming profits - because they are all so slicked up and scrubbed; they radiate a cheap faith that makes your heart go out to them even though their values range from vaguely to outrageously deplorable.

Frank Mouris won the 1973 Oscar for best animated short subject with "Frank Film," his antic autobiography on celluloid. This picture has the same kind of inventive energy, and although its structure is actually rigid, it still floats and flows effortlessly. "La La" updates the American dream in ways not exactly uplifting, but its subjects never seem helpless dupes being clobbered by manipulative filmmakers. They wanted to be stars; so, they're stars, and "La La" is wonderful, hilarious and depressing.

"Alexander's Bachtime Band," a Ruth Leon production, is not as inventive in execution but is considerably more inspiring in content. At times it captures the eerie radiance of young musicians (aged 15-22) the way "The Turning Point" communicated the righteous fervor of young dancers.

We see the students in backstage bull sessions-during one of which a girl accidentally bops a boy on the nose with her violin-and in performance, as conducted and inspired by Schneider, who urges them to play the Second Brandenburg the way they would play the William Tell Overture, sort of. Other guest artists participating in the seminar's 11 days of joy and torture include Isaac Stern and Pinchas Zuckerman.

One might wish director Allan Miller had concentrated more on the young people than the old, and not relied quite so heavily on the stock balcony shot during the performance sequences in Carnegie Hall, but when style occasionally falters, substance stands up for itself.

Whether they are immersed in the haunting Air from Bach's Orchestral Suite No. 3 in D-Major, or cheering the prospect of a free meal at the Sun Lok Kee restaurant, the young musicians really are an inspiration; they have not chosen the quick and the easy, and they have not hopped on a bus to Hollywood in the hope of turning their faces into fortunes. "God bless you all," Stern says to them; it's an apt benediction for a glorious undertaking and a rousingly fine hour of television.

'Anatomy of a Seduction'

From somewhere in the vicinity of the mentalities that gave us movies like "Tea and Sympathy" and "All That Heaven Allows" comes "Anatomy of a Seduction," which is not a how-to-do-it piece but a tow-hour CBS time-killer at 9 tonight on Channel 9.

Costars Susan Flannery and Jameson Parker both have roots in daytime soap opera and the roots show in their portrayals of a 40-year-old woman who falls for the 20-year-old son of her best friend. You can imagine the geese to be cooked and horses lured back into barns before this affair is over.

Flannery's chilly demeanor suggests a cross between a plastic centerfold and an L.A. anchorwoman, and Parker's tooth looks awfully long for 20. But the whole mopey opus is given intravenous adrenalin by Rita Moreno as the kid's mother-a divorcee like her friend and fighting with every one of her unclogged pores the steady tick-tocking of the years.

"It's hard to be 43," she complains early in the film. Later she sweats through an exercise class and gives herself recorded French lessons at home. When she apologizes, late in the film, for blowing up at the thought of her best friend and son shacking up, she sobs to him, "Oh Eddie, I'm sorry I wasn't more progressive!"

Moreno steals all the scenes and, of course, scenes there are-big title matches when word of the affair filters down to interested parties. These bruisers are not quite long or embarrassing enough, but the film has its little payoffs, and one of them is a last-minute battle of the cliches as the woman and the boy decide to call it quits.

It's just like it used to be in the movies, except that now it's the man who cries.

At least Alison Cross didn't put a lot of cant about "my needs" and "your needs" into her screenplay, and director Steven Hillard Stern lavished no more attention than necessary, which turns out to have been somewhere between a smidge and an iota. CAPTION: Pictures 1, 2, and 3, In "La, La," from left, Lenore Woodward, Randy Bishop and Holly Johnston