Home Box Office, the nation's biggest and most profitable pay TV operation, interrupts its flow of movies now and then for some new programming of its own. Most of it is godawful fluff -- the kind you flee to escape on free TV -- and scarcely worth paying for. Incredibly enough, the attitude at HBO is that viewers crave the sort of virus spread by regular TV; thus HBO squanders its satellite on mediocrities like a David Brenner comedy special, a concert by the dreary Melissa Manchester or another gratuitous addition to cable TV's enormous slang heap of unimportant sports.

Now and then, though, there's something exceptional, like last winter's documentary "The DeBolts Grow Up" (a sequel to a film first shown on commercial TV, as it happens) or, currently, the "Remember When" series of documentaries produced by Bruce Cohn, who did HBO's excellent nostalgia-thon, "Time Was," two years ago.

This month, the second edition of "Remember When" is one of television's rare looks at the tadpole from which grew a ravenous frog. "On the Air" is a remembrance of radio, from Marconi's first dots and dashes to the deejays who soothe the savage freeways of radio-hot L.A. Producer Cohn has ravaged the archives for footage to illustrate radio's history to bring its heyday back; the happy result premieres tonight at 8 on HBO, with repeats scheduled throughout the month.

Fans of old-time radio insist on the dewy memories of a medium they recall as more innocent and less monstrous than all-consuming TV. Actually, radio could be just as vulgar and cheap as television at its most Chuck Barrish. Film footage of antics from an old radio game show finds a husband peeking out from inside a prop doghouse and quacking like a duck while his wife, stationed on the roof, beans him with a newspaper, and all for a buck or a toaster or something.

But the memories evoked are usually on the cherishable side, from Fiorello reading the funnies when New York newspapers went on strike ("Now here's Dick Tracy . . . ") to the famous eyewitness account of the crash of the Nazi blimp Hindenburg ("Oh, the humanity!") to the pioneering, epochal broadcast journalism of Edward R. Murrow ("Hello, America, this is Edward Murrow speaking from London."). These images may be familiar from previous exposure in TV documentaries, but it doesn't seem injudicious to bring them up periodically, just for the sake of not forgetting. iAmerican broadcasting, what broadcast historian Erik Barnouw has dubbed "The Image Empire," might have been born the night a young telegraph operator in New York heard the distress calls of the Titanic from far away at sea. The operator was David Sarnoff, who later founded America's first network, the National Broadcasting Company. It chimed itself into American living rooms on Nov. 15, 1926, with a $50,000 party thrown at the Waldorf.

Making radio a visual subject isn't easy. Cohn had access to the old March of Time newsreels, which "covered" radio in the same fakey-fake style with which they covered most things (and a disclaimer at the end of the program admits to some "dramatic recreations"). But adding to the visual interest is host Dick Cavett wandering in and out of old newsreels and photos thanks to the UltaMat superimposition system Cohn used in "Time Was."

This gizmo can teleport people into other times and locations with almost creepy realism. Cavett really does look like he is wondering among the deck chairs on the Titanic, or asking a 7-year-old on "Quiz Kids" to imitate four kinds of finches, or tinkering around in a very early radio control room. Just before FDR launches into a fireside chat, Cavett tosses another log across the hearth.

Names and faces from the past come wafting back into the American living room where once they reigned supreme: Bing Crosby, Kate Smith, Freeman Gosden and Charles Correll as Amos 'n' Andy, Abbott and Costello ("Who's on First" is still funny, like "A Midsummer Night's Dream" is still funny), Maj. Edward Bowes and his Original Amateur Hour, and Fred Allen and Jack Benny carring on their famous phony feud.

But the essence of radio and mass communication, the real revolution of the 20th century, is captured in footage of a mine disaster to which, thanks to radio, the entire nation became a witness. That millions were on the edges of their couches over the fate of one previously unheard-of man suggests the stunningly unifying and galvanizing potential at the heart of this fantastic invention.

"On the Air" takes the rosy view of radio's past, not really noting how, like most man-made wonders, radio was abused and misused. As we stand on yet another threshhold (ugh, there've been so many), it's worth taking a more serious look at radio than "On the Air" takes, because all signs indicate we are going to become more dependent on electronic communication than ever. Some people think we are even going to thrive or perish on the basis of what we do with it. Oh . . . the humanity!