Great debate: When is a commercial not a commercial?

Commercials on the sacrosanct channels of public broadcasting?

It's a possibility that has fascinated and concerned many viewers, because word has leaked out that ads are on the way in.

Somewhat surprisingly, reaction to this ''threat'' - from public broadcasting and even from viewers - has not been all negative, but generally mixed.

When Congress set up the Temporary Commission on Alternative Financing for Public Telecommunications a year ago to look for new ways to support public broadcasting, advertising was one of the options considered. And so an 18-month experiment was launched to see how noncommercial commercials would go down with public TV viewers.

Ten stations across the country are taking part. Seven of them are selling time between programs, accepting institutional, service, facility, and product advertisements. Two others are accepting ''enhanced underwriting credits.'' This arcane euphemism means that national or local underwriters show brief institutional or service messages about their companies.

The 10th station is broadcasting promotional and institutional spots about not-for-profit organizations in its market. It is also promoting discount services at these organizations to station members.

The question of advertising on public TV may generate more controversy among broadcasting officials than among viewers. Some recent surveys have shown that PBS audiences would not object strongly to commercials between programs. But some top public broadcasting officials make it clearly understood that they are against advertising in any form.

Lawrence Grossman, president of the Public Broadcasting Service, told the Monitor: ''As vigorously as I can say it, I am against advertising on public television. If the programming were supportable by advertising it would be on commercial television.

''Public television was set up with a different mission. I have nothing ideological against advertising. I spent most of my working life in it. But it doesn't belong on PBS.''

Edward Pfister, president of the Corporation for Public Broadcasting (CPB), sounded another warning. He told the Monitor: ''Advertising could be the death knell of PBS. It would certainly be a step backward. Once the camel gets its nose under the tent, the nature of what we do will necessarily change.''

But FCC commissioner James H. Quello, chairman of the temporary panel on alternative financing, says, ''I am very much for the experiment. In considering the opinions of PBS and CPB executives, you should keep in mind that there is an inherent amount of rivalry between PBS and CPB on the one side and the licensees on the other. The stations have payrolls to meet; PBS and CPB are dependent upon government financing for their very existence.''

John Jay Iselin, president of WNET in New York, adds that the ''experiment is needed. Our enhanced-underwriter identification is simply an extension of existing methods. Good taste and accuracy will control the messages.

''When you hear objections, they come mostly from men who head up agencies which are very much affected by a continuation of the current method of funding. They have a particular obligation to see that it is not jeopardized. Of course, I'm sure they voice their objections in good faith. . . .''

There may be lessons to be drawn from the experiences of some of the recently launched cable channels. CBS Cable was supposed to be an advertiser-supported cultural channel, but it has already announced plans to fold after losses estimated as high as $50 million in one year.

Bravo, a cultural pay-TV channel, survives - barely. The Entertainment Channel was intended to be a free, advertising-supported cultural channel. But it has premiered as a pay-TV channel with entertainment rather than cultural programming. PBS president Grossman says that the demise of CBS Cable only goes to prove that cultural programming needs underwriting, government or corporate, to survive.

Will the commission recommend some form of advertising? Will corporate underwriters accept PBS as a commercial station? Will viewers accept this?

Preliminary information suggests that PBS audiences are showing little resistance to ''enhanced-underwriter credits,'' but are showing major resistance to full advertising like that on commercial TV.

Many people are under the misconception that commercials on PBS would eliminate the constant on-air fund raising. But membership drives will continue, because stations will probably always have to rely to a great extent on membership contributions for day-to-day existence.

In a statement in the commission's first report July 1, commissioner Quello summarized the situation: ''. . . The ongoing television advertising demonstration . . . reveals that advertising holds significant promise as a means of generating revenue, at least in some communities.

''However, it is estimated that over 50 percent of the public broadcasting stations would be prohibited from commercial advertising, by either constitutional charter or opposition from individual boards. Also, advertising on publicly funded stations could change the character of the service and also generate business opposition, with possible negative effect on contributors and underwriters.

''The final report should thoroughly examine the feasibility of establishing a new category of public broadcasting stations - 'not-for-profit' as opposed to 'noncommercial.' ''

Whether the public will be able to tell the difference between a not-for-profit commercial and a commercial commercial remains to be seen.