This remarkable YouTube video was used to train soldiers about how FM worked in an era where most had only heard of AM. It was made by the U.S. War Department back in 1944, to help soldiers see the advantage of the new band - and it had nothing to do with music. 

"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."

Hard to believe this film was once classified. Now anyone can see it. 

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