The images of the Watergate scandal have become part of our nation's collective memory: Richard Nixon flashing his trademark victory sign before boarding a helicopter to leave the White House after resigning in disgrace is just part of the scrapbook.
Nixon announced his resignation on Aug. 8, 1974, more than two years after the break-in at the Watergate complex offices of the Democratic National Committee during the 1972 campaign. By then, questions about Nixon's involvement in the Watergate break-in were coming from all directions.
"What did the president know, and when did he know it?" Republican Sen. Howard Baker demanded at the height of the investigation.
Today, on the 30th anniversary of the infamous break-in, the mysteries surrounding the event that led to the resignation of an American president are every bit as tantalizing.
After all the hearings, all the headlines and all the cover-up, we still don't know whether Nixon actually knew about the break-in in advance.
But another mystery might be cracked much sooner: that 18½-minute gap in one of Nixon's infamous tape recordings at the White House
"We have decided that the time is right and appropriate to determine whether that conversation can be retrieved or recovered," said Karl Weissenbach, a Nixon tape archivist at the National Archives. The tapes were last examined in 1974. But since then, the technology used to decipher recordings has improved dramatically.
The 18½ minutes in question is part of Nixon tape 342, recorded on June 20, 1972, three days after the Watergate break-in. On the tape, Nixon discussed the incident for the first time with his chief of staff, H.R. Haldeman. It is unknown whether the tape was deliberately or inadvertently erased for those 18½ minutes.
Amid the Watergate scandal, Nixon's loyal secretary, Rose Mary Woods, said she erased the tape while reaching for a phone call. Reporters were calling it the "Rose Mary Stretch." Back then, it was assumed that the 18½ minutes was gone forever, but technology may provide a surprise footnote to history.
"You never completely erase a tape," said Stephen St. Croix, a forensic audio expert with Intelligent Devices, near Washington, D.C. "You think you do, but you really don't."
St. Croix, a former musician, does most of his work for law enforcement, and like other forensic audio experts, he uses computers to help decipher old tapes, old recordings, or intercepted communications in which it seems impossible to understand what is being said.
The technology also allows for the retrieval of sound that has been erased, where the speech cannot be heard at all.
The key to such work is to find the scraps of sound left on the tape that can be made into audible voices, and removing everything else.
"It would take a human hours to find them," St. Croix said. "But a computer can find them in seconds."
The information comes in as analog data, from tapes, telephone or wire intercepts, and is converted to digital data in the computer. To decipher that data, forensic experts would use "bandpass filters" and other high-tech devices that look for frequencies that they do not need within a sound. It then throws them away, leaving just the frequencies of the human voices. Computers can also remove background noise.
St. Croix demonstrated how computers might be used to intercept the cell phone calls of suspected criminals.