The other day, I was driving to work when suddenly my radio started playing. I don’t typically listen to the radio, but the music was nice, so I let it keep playing. After the song finished, the radio announcer said, “You’re listening to 98.9* FM radio.”
And that’s when my brain broke a little.
Up until this point, I knew very little about how radio worked. All I knew was that somehow, sound waves turned into electromagnetic waves and back into sound waves, and my brain was okay with this limited understanding because it seemed intuitive. The whole process was just waves becoming waves becoming waves. All waves have frequency and amplitude, so taking that information from one wave and putting it on another wave didn’t seem so strange.
The only problem is that my intuition was wrong. When you read a radio station number, that station number is the frequency of the radio station. Hearing the announcer’s voice, I realized that if the radio station number is fixed, then the station has a fixed electromagnetic frequency, so the station has to encode sound frequency and amplitude without changing their electromagnetic frequency.
AM radio uses a very elegant process for encoding sound. They do this through amplitude modulation. Amplitude modulation imposes the shape of one wave onto the amplitude of a carrier wave. Ideally, the carrier wave has a much higher frequency than the broadcast wave. The slowest AM carrier waves oscillate over 540 thousand times per second, which is roughly 30 times faster than the highest frequencies audible to adult humans, so AM carrier waves are definitely going fast enough. By imposing the broadcast wave over the carrier wave, AM radio stations can broadcast full audio waves without changing frequency.
AM radio is great, but for the vast majority of people, the last time they listened to music on the radio, it was FM radio. Unlike AM radio, FM radio uses a fixed amplitude. This is where I really got confused. If the amplitude and frequency are constant, then how do you convey any information? It turns out, the frequency is not constant. FM—frequency modulation—radio stations vary the frequency of the carrier wave to encode information. A higher frequency corresponds to a peak, and a lower frequency corresponds to a trough. When you listen to 98.9, you’re not actually receiving 98.9 MHz radio waves. Instead, your radio is listening to all frequencies between 98.8 MHz and 90.0 MHz. The vast majority of the time that your screen says 98.9, you’re not actually listening to 98.9. You could be listening to 98.89 or 98.91, but 98.9 is incredibly rare. Radio stations are lying about their station number. They almost never send you radio waves at their channel’s frequency.
Realizing this, I felt betrayed. I didn’t listen to FM radio to begin with, but I certainly wasn’t going to start. Why number your station off a frequency you almost never broadcast? Radio is an incredible medium, but this burden is too much to bear. Listen to anything else, even a podcast exploring life through etymology, but beware the lies of FM radio.
*not a real radio station