Season Two Research Files
Episode Sixteen
11:00pm-12:00am
AUDIO AUTHENTICATION
A forensic expert is trained to authenticate audio recordings in a number of ways. The technician can listen with proper headphones to the original tape using high quality analytical equipment. He first performs a preliminary overview of the original tape and notes recorded events including starts, stops, speed fluctuations, and other variations requiring further investigation. He then examines recorded events and categorizes them as environmental or non-environmental. After examining any anomalies, the expert analyzes background sounds. He listens for abnormal changes, absences, or differences in environmental sounds. The final phase of critical listening is an extensive audit of the foreground information. He concentrates on voices, conversation and other audible sounds. Anomalies here include sudden changes in a person's voice, abrupt unexplained topic change, or strong foreground interruptions that suggest ambiguity.
The next step is a physical inspection for tampering, followed by signs of magnetic development. This includes the search for track widths, type of recorder used, and the presence or absence of residual speech signals. Specialized computer equipment and programs can produce a spectrum analysis to characterize the acoustic quality of anomalies and identify their source. Another analysis is done on waveforms, which measures the signal return time and reveals how long a recorder had been turned off. He can then identify record-mode, including the measurement of record-to-erase-head distances, determination of the spacing between gaps in multiple-gap erase heads, and inspection of the signature shape and spacing of various record event signals.
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