What is the difference between AM and FM radio and why is FM more popular?
In: Technology
Think of a flashing strobe light, AM is making it more or less bright. FM is making it blink faster or slower. AM has poorer quality of sound. This is especially true if you’re moving since it can be hard to tell if the light is dimmer because you’re further away or because it is intentionally getting weaker.
FM … No static at all.
– Steely Dan
If you want to make a secret code that only uses waves to communicate, there are only so many ways you could do that.
You could make it depends on how big the waves are, or how fast the wave is.
Amplitude modulation (AM)is a code which depends how big the wave is.
Frequency modulation (FM) is a code which depends on how fast the wave travels.
These codes have different pros and cons. FM is designed in a way that you can give more information at once, AM however, can go further and still be understood.
For our radio, FM is king because we want that extra information, because that means we can have nicer sounding music. The range isn’t much of an issue because you can just build more radio stations.
But it’s easy to imagine a situation where we would prefer a less nice sounding signal that can travel really far.
Humans can hear sound roughly between 20 and 20,000 Hz (Hz = one cycle per second, 20 Hz = 20 cycles per second). In this case, each “cycle” is the air being compressed and decompressed, which we understand as sound.
Radio waves can have a frequency higher than that. So, what if we encode the sound wave into a radio wave? Or in simpler terms: what if we “draw” the sound wave using radio waves?
There are (at least) two ways we can “draw” the sound wave: when the sound wave is in the “compressing” part, we make the radio wave more intense, and when it’s in the “decompressing” part we make it less intense. This would be AM radio.
The other way would be for the frequency to get higher when “compressing”, and lower when “decompressing”.
[Here’s a GIF explaining it.](https://upload.wikimedia.org/wikipedia/commons/a/a4/Amfm3-en-de.gif) The signal would be the sound wave, the higher the line the higher the pressure. Notice how AM basically makes the intensity of the signal follow the curvature of the sound wave while using the same frequency all the time, while in FM the intensity doesn’t change, but the new signal gets faster and slower.
AM radio is amplitude modulated
Meaning at the frequency, the singal is transmitted by different size waves. The radio can play those different sized waves as sound.
FM radio is frequency modulated
At that frequency, the signal is transmitted by pulses of waves. The radio can play those different length pulses as sound.
AM transmitters and receivers are cheaper and easier to build, and can travel much further distances, which is why AM is used for emergency communication.
FM transmitters and receivers are a bit more complicated, and can’t travel as far, but the sound quality is much better, which makes it better for music.
When I asked my dad this in the late 60’s, (I was around 5) he said the difference was AM stations played “American music”, and FM stations played “Got damn commanist hippy music”….
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