Watch a classified FM radio training video from WW2
It is worth remembering that one of today’s most common technologies often confused the public when it was first launched. While fewer people listen to FM radio today, it had become a hot topic during its widespread deployment in the late 1930s and early 1940s. Short for frequency modulation, FM’s appeal over AM (amplitude modulation) was immediately obvious: clearer, less static sound, and more reliable transmissions.
While most audiences only cared about the enhanced radio experience, understanding the technical aspects of FM was crucial if it was to be relied upon in real life or death situations. The problem was so bad during World War II that the United States government funded a 30-minute classified educational video for its armed forces in 1944 called “Fundamentals of Frequency Modulation.”
Before FM, the military transmitted much of its communications via AM frequencies that were highly susceptible to static interference.
“Frequency modulation solves this problem, and that’s why we want you to know about it,” a square-jawed military actor informs early in the film.
“BASIC PRINCIPLES OF FREQUENCY MODULATION” 1944 U.S. WAR DEPARTMENT FILM RADIO FM 86794
To send messages with AM, an oscillator first generates a carrier wave that powers an amplifier. Audio from a microphone is then routed to a modulator as electrical energy before heading to the amplifier. From there, the energy is focused on the wave and transmitted to the receivers. The problem with AM is that information is transmitted based on its carrier strength, making it very susceptible to a range of electrical interference. As the film illustrates at the beginning, even something as simple as a nearby electric razor is enough to create static electricity on an AM radio.
“Frequency modulation, or FM, is a big step forward in radio. When you tune in, you hear what the studio mic hears,” proclaims the narrator.
With FM, a carrier wave uses frequency instead of its amplitude, so static electricity only affects the amplitude. This means that any interference simply changes the part of the signal that does not transmit the message.
This may seem a bit cheesy and outdated in 2026, but as BoingBoing recently pointed out, the film still provides an incredibly concise breakdown of radio waves, along with discriminators, limiters, modulators, and oscillators.
“When frequency modulation is used, you can be sure that electrical interference will not disrupt your messages,” promises the host of the training video.
Roger, sir.
The best new PopSci 2025 releases
