On November 22 1963, a Dallas police officer’s motorcycle radio malfunctioned, and began to transmit more-or-less continuously. Several such transmissions occurred over a time period of about 11 minutes; the first encompassed the time period surrounding the assassination of the president. The sounds recorded during the transmissions consist chiefly of the motorcycle engine noise, various other ambient sounds picked up by the microphone, and radio communications originating from other sources. We use the term open microphone to refer to the transmissions made by the motorcycle radio.
If the microphone was in or near Dealey Plaza at the time of the assassination, then it may have detected the sounds of gunfire and these would presumably have been recorded.
II. TECHNICAL BACKGROUND
The Dallas Police Department operated two radio channels on November 22, 1963: channel 1 was used for routine radio traffic, while channel 2 was used by the motorcade. The open microphone was associated with channel 1.
Channel 1 was recorded on a machine made by the Dictaphone company onto a medium called a Dictabelt. The Dictabelt machine used a stylus to emboss a groove into flexible plastic belt, the groove being much like the groove in a phonograph record. Channel 2 was recorded using a machine called a Gray Audograph; this device recorded on a disk resembling a phonograph record, but unlike the phonograph it played from inside to outside and did not rotate at a constant speed.
Both machines were designed to record intermittently. In normal operation, they would begin recording when an audio signal was detected. They would stop recording upon the cessation of the audio signal and after a short time delay. In this manner, recording medium was conserved, and subsequent playback would be free of long periods of silence during times of sparse radio traffic. Recordings made by both machines were then copied to other media (primarily open-reel tape) by a variety of agencies...
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