The man who invented the movie ratings system nearly 30 years ago thinks it works so well it deserves its own rating of JF, for "just fine," flaws and all.

"Until the parents of America say, `Do something else,' this is what we're going to do," Jack Valenti, president of the Motion Picture Association of America, said in a speech to the Los Angeles World Affairs Council on Thursday.Speaking to about 300 business, political and entertainment figures, Valenti acknowledged that the ratings system, which will mark its 30th anniversary Nov. 1, is not above criticism.

Filmmakers have long complained the 12-member ratings board acts arbitrarily and secretly and has more tolerance for violence than sexuality.

"Is this a flawless system? Of course not," Valenti said. "Does make errors? Of course it does. Why? Because it is all subjective. The lines between R and NC-17, and between PG-13 and R are smudged and ill-lit and not very finely drawn. I know that."

Still, he said, the industry hasn't found anything that works better. And he said polling has shown that 70 percent of parents with children under age 13 find the movie rating system very useful or fairly useful in choosing films for their children.

"Nothing lasts that long in this volatile and brutish marketplace unless you're doing something to help the folks that you devised the system to guide - and that's the parents of America," he said.

He added: "I think this rating system is doing just fine.

"I think you might say I'm in favor of it because I invented it," Valenti said.