(PHILADELPHIA, PA) -- Filtering software intended to protect children from exposure to pornography on library computers is doomed to fail despite its congressional endorsement as a viable safeguard, computer experts testified in federal court on Tuesday.

Two witnesses, called by plaintiffs at a constitutional trial that could overturn the latest bid by Congress to control online smut, said filters such as Cyber Patrol , SmartFilter and N2H2 cannot block sexually-explicit material on the World Wide Web without also denying access to innocuous sites.

That could prove crucial in the deliberations of a special three-judge panel that must decide whether to impose a permanent injunction against the Children's Internet Protection Act, or CIPA, on grounds that it violates free-speech rights protected by the First Amendment to the U.S. Constitution.

Worse still for parents who rely on filtering software for protection on home computers, witnesses said the products also consistently allow some Web sites containing objectionable material to get through to the viewing screen.

"My conclusion after reviewing 40 studies is that filters are systemically flawed," said Christopher Hunter, a communications doctoral candidate at the University of Pennsylvania who did research for libraries, library patrons and Web site operators that have sued to block CIPA.

He estimated software filters blocked benign Web sites 21 percent of the time while successfully stopping objectionable material 69 percent of the time.

Stanford University linguist Geoffrey Nunberg testified that the crude mathematical methods used to operate filtering software were no match for human judgment.

"These are problems that ... can't be overcome," said Nunberg, an expert in automated classification systems.

He said that even if filtering software were more than 90 percent accurate at stopping material deemed harmful to minors, the number of innocuous Web sites blocked would still be enormous given the breadth of the Internet.

CIPA, signed into law in 2000 by President Bill Clinton, was the third attempt by Congress to restrict access to the Internet and its estimated 11 million World Wide Web sites, about 1 percent of which experts say contain some sort of sexually-explicit material.

It would require libraries to install filtering software on computers or risk loosing millions of dollars in subsidies that provide library patrons with Internet access. The Justice Department which is arguing the case for the government, has postponed compliance until July 31 because of the litigation.

The first law to control online smut was thrown out by the Supreme Court as an infringement of free speech. A second remains sidelined by an injunction with the U.S. high court due to issue a final opinion by mid-year. Both would impose criminal penalties on violators.

Any appeal of the final ruling by the CIPA judicial panel, headed by 3rd U.S. Circuit Court of Appeals Chief Judge Edward Becker, goes to the U.S. Supreme Court.

Supporters say CIPA is the government's best shot at controlling online smut because it deals only with funding and not with direct restrictions on Internet access.

Hunter said many of the Web sites he found blocked by filters were operated by gay and lesbian groups.

Witnesses testified to the inconsistency of filtering software said a program might block the word, vibrator, but not, vibrators. Or they may block an online magazine article that asks whether President Bush's speaking style is sexier than former Vice President AL Gore's, but not full-color photographs of sex acts unless they are accompanied by suggestive or objectionable text.