IF you hadn't seen public TV for a long time and tuned in today, you might think something was wrong with your set. The dial could be turned to a public station, but you'd be seeing what looked an awful lot like commercials.Not just "This program is made possible by followed by the company title, but wordy descriptions that generically sound something like this: a dedicated group of companies where people work together to bring safe products and good living to people of all ages through ... blah, blah, blah." The organizations named would be mainly companies - often multinationals with vague catch-all titles. But there'd also be groups like the Corporation for Public Broadcasting, local public stations, the National Endowme nt for the Humanities, and on and on. If mystified, you should ask yourself a question: When is an ad not an ad? Answer: when it's on public TV. Years ago, of course, that medium stopped merely naming funders of programs and began adding phrases. But please don't call the result commercials. They're "exhanced credits." It's just one part of a promotional bent that appears out of control on many public stations. You get the feeling you're constantly being pitched to, even though no one's making much money on those stations and public TV has p erennial budget problems threatening one of the last, best hopes of broadcasting. Yet if the medium is the message, on public TV that message should be more than a menu of shows unavailable elsewhere. It should also create a different context where people can have an implicit trust that some limit will be placed on opportunistic attention-grabbing of the kind seen on commercial stations. Where is public TV going to place those limits? Right now station breaks are a thicket of promos - for upcoming shows, for the station itself, for fund drives. There's so much of it, I don't even want to hear anymore of those credits for myself that are often thrown into the mix: thanks to the generosity of our viewers." Pledge weeks, held three times a year, are also getting to be a viewing menace. They're not even really "weeks" in all cases. In March they last about three weeks. Then there's auctions on local stations - and please don't remind me how important those are. I've been a volunteer auctioneer on my local public station - and an enthusiastic one - for so long I'm thinking of putting in for retirement benefits. I do it because public TV deserves all the attention it can get without betraying its own nature. Its shows, after all, are facing a struggle for survival in a media world full of too many unworthy things trying to catch your eye. Starting next Monday PBS has its "Showcase Week," offering an impressive array of specials and premieres that easily justifies the plugs that go with it. These are comments by celebrities like Kevin Kline and Kareem Abdul-Jabbar about public TV. They last only five to ten seconds and are designed, in PBS's own language, "to inject a mixture of vitality, humor, and surprise into station break messages." That translates into things like Ed Asner saying "I like Bill Moyers, Alistair Cooke, and Oscar the Grouch. But not necessarily in that order." The celebrity spots have been packaged by an ad agency and there's talk of doing more of these on PBS. They're probably fulfilling a modified Parkinson's law: Left to their own devices, promos grow in length or number to fill the available space, like nature abhorring a vacuum. By themselves plugs like the ones on Showcase Week are innocuous enough. But add all the other public TV pitches, and the psychological impact becomes indistinguishable from commercial TV. The difference is conceptual rather than perceptual. You have to remind yourself that while, yes, they sound like commercials and seem to come as thick and fast, it's nonprofit, so they're really not. But the dynamics are the same: vested interests on the airwaves using language and images designed to convince you of something. Maybe that something is good - I think it usually is - but let's not pretend the mental means used to persuade you of this is not at heart the same as in a Toyota ad.