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The War Department's 1944 guide to FM radio is still a great explainer [1]

['Ellsworth Toohey']

Date: 2026-03-02  

The U.S. War Department made this training film in 1944 to explain frequency modulation to soldiers, and it opens with the perfect sales pitch: two GIs playing ping pong while listening to the radio. A third man fires up an electric razor, and the AM signal turns to mush. Someone switches to FM. Problem solved.

What follows is a 28-minute breakdown of why the military needed FM and how it works. AM radio was fine in peacetime but nearly useless on a battlefield — tank treads, generators, and thunderstorms all produced electrical interference that buried the signal in static. FM solved this by encoding information in the carrier wave's frequency rather than its amplitude, so interference affected only the part of the signal that didn't carry the message.

The film walks through oscillators, modulators, limiters, and discriminators with animated diagrams that put most modern explainers to shame. Originally classified, the film was released to the public in 1949. It's part of the Periscope Film archive.

Previously:

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[1] Url: https://boingboing.net/2026/03/02/the-war-departments-1944-guide-to-fm-radio-is-still-a-great-explainer.html

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