In a gravelly voice, Homer Williams testified today that the green wall-to-wall carpet that makes up the centerpiece in the prosecution's case against his son, Wayne, was purchased in 1968, two years before prosecution experts say the carpet was manufactured.

Fiber experts from the FBI and Georgia Crime Laboratory contend that fibers from the Williams carpet match fibers found on the bodies of Nathaniel Cater, 27, and Jimmy Raye Payne, 21, the two young black Atlantans Wayne B. Williams is accused of murdering.

The experts also have testified that the carpet fibers match those found on the bodies of eight other victims in a string of 28 murders investigated by the FBI and a special task force. The prosecution has been permitted to introduce evidence from the 10 murders to show a "pattern or bent of mind."

Prosecution experts tracked the tiny fibers to carpeting manufactured by a Dalton, Ga., textile mill in 1970 and 1971, and sold to a limited number of homes in the state.

But Homer Williams, 68, dealt a blow to their case against his son when he waved a Dec. 7, 1968, newspaper ad touting three rooms of carpeting for $149 that he said led to the purchase of his wall-to-wall carpeting. "Attention, homeowners," the ad said. "Why pay more?"

"That really struck my chord," said Homer Williams.

The elder Williams also said his son was home in bed the night a key prosecution witness said he saw Wayne Williams holding hands with Cater outside a downtown movie theater the night of May 21.

Police contend that Williams dumped Cater's body in the Chattachoochee River early May 22, six hours later.

But Homer Williams, a retired high school science teacher and free-lance photographer, on the stand for the second day, testified that he drove the family car to a photo assignment at the Kiwanis Club on May 21, went on to snap pictures at a garden club meeting for The Atlanta Daily World, then returned home about 11:30 p.m.

He said he remembered that his son was in bed and had a stomach ache. Then, he said, Wayne got up and left the house, saying he wanted to use the car to retrieve a tape recorder he had loaned to the manager of the San Souci nightclub.

Later, about 4:30 a.m., his son returned, he testified, woke him up and said he had been grilled by police on stakeout at the river after he drove across the Jackson Parkway Bridge.

Police say they heard a splash in the water, then pulled Williams over for questioning. Cater's nude body was pulled from the river two days later, near the spot where Payne's body was retrieved a month earlier.

Williams' father visited the Atlanta Public Library to make a copy of the carpet ad that appeared in a local newspaper. He said he recalled the date because his wife, who spotted the ad, insisted the carpet be installed in time for Christmas.

The installer came out and left the carpet unrolled in the garage, he said. On Williams' lunch break, the family's new German shepherd puppy, Sheba, chewed up a large chunk of the carpet. Williams produced registration papers showing the dog's birth date as Sept. 13, 1968.

He said the dog had been docile for seven or eight years, contradicting a prosecution witness who said she saw Williams and Cater in a park with a "frisky" German shepherd.

Earlier, prosecution experts tracked fibers from the Williams' green carpet to fibers manufactured by Wellman Inc., a textile firm, between 1967 and 1974.

In 1970 and 1971, Wellman sold 850,000 pounds of the carpet to Westpoint Point Pepperell of Dalton, Ga., which dyed the fibers into 16 styles of carpeting, including the "English olive" line experts contend matches the Williams' carpet.

An FBI marketing analysis found that so little was sold in the southeast United States that the odds were remote that it would be found on more than a few floors in Atlanta. One fiber expert described the fibers as "microscopically and optically" similar to the Williams' carpet.

But prosecutors are expected to attempt to refute Williams' testimony that he bought his carpet before the English olive brand appeared for sale in stores by suggesting that another manufacturer using the Wellman fibers could have put its product up for sale about the time the Williams family start their shopping.

The elder Williams exhibited the same kind of fierce loyalty to his only child on the stand as he apparently has always shown his boy. The family's financial support of Wayne's basement radio station led into bankruptcy, yet failed to diminish the father's faith in his son.

But assistant prosecutor Jack Mallard tried to suggest that Williams had done so much for his son that he would lie to protect him. "Can you think of any area you haven't covered for Wayne?" he asked.

"I can't recall any," he replied.

He also explained that a bloodstain on his son's bedroom carpet came from a rat he killed with a mop handle. "I caught him pretty good," he said. "I hit him once and Wayne hit him twice."

He disputed police, who said a slapjack was found in the ceiling of Wayne's bedroom. He took it from a high school student in 1962, Homer Williams testified, and he kept it hidden in a closet. Wayne didn't know where it was kept, he said.