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At one time, decades back, Chicago police were on FM at 800-something MHz, if I recall correctly. A transistor oscillator / diode mixer could move this down to the AM band, where you could listen via slope detection with an AM radio. A friend built such a device. First time I heard some of the transmissions I wondered what they were talking about; "I have a TV at 5th and Main" until I was informed "TV" meant traffic violation.
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Old TV literature, New York World's Fair, and other miscellany
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