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one other than presidential biographers and historians af- ter he left office. However, it quickly turned into a political and legal bombshell of historic proportions in 1973 when White House aide Alexander Butterfield revealed during his congressional testimony that there were secret audiotape recordings of conversations between the President and his advisors! After all, this was in the midst of the Watergate investigation and the widespread public concern about the veracity of various White House officials who had testified before Congress and in federal court. President Nixon even- tually agreed to release edited transcripts of the various con- versations recorded by the secret taping system and later the tapes themselves.
In 1974, interest turned to a particular recording of a con- versation between President Nixon and his Chief of Staff H. R. Haldeman recorded in 1972 in the EOB. The investigators were suspicious that the recorded conversation included re- marks about the Watergate cover-up, but when the record- ing was examined, the investigators discovered that 181⁄2 minutes of the recording were obliterated by an unexplained gap consisting of audible buzz sounds but no discernable speech. Investigators suspected that someone had deliber- ately erased or recorded over that section of the tape to de- stroy the originally recorded conversation, perhaps with the intention of eliminating incriminating remarks.
John J. Sirica, Chief Judge of the US District Court for the District of Columbia, determined that the potentially altered tape required expert analysis beyond the routine capability of the court (McKnight and Weiss, 1976). He requested that the Watergate Special Prosecutor and the counsel for the president jointly nominate a group of six outside technical experts (including several ASA members) to form a special Advisory Panel on White House Tapes “...to study relevant aspects of the tape and the sounds recorded on it” (Advisory Panel on White House Tapes, 1974).
The Advisory Panel analyzed the physical tape itself and the electrical signals observed on playback and, ultimately of greatest importance, performed magnetic development us- ing ferrofluid to reveal latent magnetic domain patterns on the tape and the magnetization signatures of the recording and erase heads installed in the tape recorders known to be present in the White House. The magnetic development of the tape led the Advisory Panel to the conclusion that the 181⁄2-minute gap consisted of several overlapping start-stop erasures performed with a specific tape recorder available in the White House but not the same device that was used
to make the original recording (Advisory Panel on White House Tapes, 1974).
The procedures employed by the Advisory Panel became the standard for audio forensic investigators: (1) examine the physical tape, reels, and structural housing, documenting their characteristics, total length, and mechanical integrity; (2) verify that the recording is complete and continuous and does not exhibit any erasures, splices, or stop/start sequenc- es; (3) listen carefully and critically to the entire tape; and (4) use nondestructive signal processing as needed for intel- ligibility enhancement.
Modern Audio Forensics: Digital
“Good” and Digital “Bad”
For a few decades after Watergate, the discipline of acousti- cal forensics revolved around two common requests: estab- lish the authenticity of a tape recording and identify the talk- ers whose utterances are audible in the recording. The work generally focused on analog audio because up until recently, the most frequently encountered recording medium in fo- rensic cases was analog tape.
Analog magnetic tape had a variety of drawbacks including comparatively poor signal quality, stability, and storage ca- pacity. However, as found with the 181⁄2-minute gap in the Watergate tape, the physical medium itself could provide useful forensic information about edits, splices, stop/start/ erase sequences, and other alterations affecting authentic- ity. Our contemporary digital recorders can provide much greater quality, stability, and storage capacity, but the ability to read, alter, and resave the contents of a digital bitstream and even to manipulate the file’s date and other file system data without leaving a physical trace in the flash memory or computer disk file has caused serious concern about our ability to authenticate digital audio forensic recordings. If a forensic examiner cannot detect any evidence of tampering, it is still conceivable that the digital data could have been manipulated in some undetectable manner.
One proposed approach to address digital audio authenticity is based on detection of a tell-tale hum in the recording due to interference from alternating current (AC) power leak- ing into the audio recording (Grigoras, 2005; Cooper, 2008). The electrical network frequency (ENF) of the AC power grid actually varies slightly from its nominal 60-Hz (US) or 50-Hz (Europe) frequency by a time-varying deviation that depends on the instantaneous balance between power gen- eration and consumption on the grid, which fluctuates from
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