What do AM and FM stand for?

Don't lie, unless you have searched it up you wouldn't have the faintest clue what these two acronyms on your funky radio stand for.

I don't know about you, but ever since I was a child cruising along the worn Malaysian highway in my mother's Perodua Kembara, I had always wondered what the two acronyms 'AM' and 'FM' that were displayed on the clunky radio stood for. As the world wide web was still in its angsty teenage years back then, the thought of 'Googling' what they meant never crossed my mind (or maybe I was just too young back then to be exposed to the wonders of the Internet).

Fast forward to today, I still don't know what they mean - well until recently at least, hence this article. I used to assume that AM and FM had something to do with the daily morning and night cycle as they shared some similarity to the AM/PM pair (on a side note: what do AM and PM even stand for? According to Dictionary.com [ 1 ], they are abbreviations of the Latin phrases ante merīdiem and post merīdiem which means "before midday" and "after midday" respectively). However, once I started to read up about it I realized that I couldn't be anymore wrong in my assumption.

So, What Do AM And FM Actually Stand For?

The brief answer is that they stand for Amplitude Modulation and Frequency Modulation. To understand why they are named that way however would require us to dive into the intricacies of how radio stations transmit data wirelessly.

We call our little (sometimes ginormous) boxes of music 'radios' because they operate using radio waves, a low frequency section of the electromagnetic spectrum that is good for long range transmission as the fewer the number of oscillations the wave undergoes in a second, the lesser the amount of energy is expended into the surrounding environment and thus the wave can propagate further. You can draw similarities between these invisible radio waves and the ones that we see in any body of water in that the amplitude of a wave is defined as the distance of the peak to the center of the wave while the frequency of a wave can be determined by the number of repeating 'S' patterns in one second.

The Electromagnetic Spectrum [2]

Transmitting Data Wirelessly

Imagine you wanted to send a song from one device to another without the need of the Internet (yes, we're going old school). How can we do that? Well firstly, we'll need to find a way to transmit that data. Luckily for us, any audio recordings can conveniently be expressed as a transverse wave. With the help of a base carrier signal - the radio band frequency that we're transmitting in, we can impose our input signal from the audio recording onto the carrier wave itself via modulation. Modulation can be viewed as a way of encoding and storing data on another wave that can be transmitted while retaining information.

The difference between amplitude and frequency modulation is that AM changes the amplitude of the carrier signal to store information where loud sounds results in higher peaks while soft ones create smaller peaks. On the other hand, FM plays around with the frequency of the base wave to produce the same result.

Amplitude vs Frequency Modulation [3]

Once we have the modulated wave, we can transmit it for the world (or everyone close enough to the antenna) to receive. To accomplish this, towering radio antennas use electricity to move electrons about in the antenna. This 'jiggling' of electrons produces a wave due to their inherent particle-wave duality (future post idea?) which travels away at the speed of light. We can't see or hear these waves as their frequency is so low that our eyes and ears are not able to pick them up. As such, these invisible waves are able to propagate everywhere freely without annoyance till they run out of energy or hit an oncoming receiver antenna where the same process but in reverse occurs to decipher and play the transmission on our radios.

But Wait, Why Do We Need To Do This?

This whole modulation concept is pretty complex huh, you may wonder what's the point of going through so much trouble just to transmit a sound wirelessly from one place to another. Why can't we just use the original audio recording itself as the carrier signal and transmit that? Well, it all boils down to a childhood phrase that I'm sure we're all familiar with: "Sharing is caring". We 'connect' to our beloved radio stations by tuning it to the frequency of the station. I say connect in that sense as we aren't actually connecting to anything, we're just setting our receiving antenna to pick up any waves in that frequency. We do this as there is a limited range of frequency bands that radio stations are allowed to broadcast in.

Apart from radio broadcasting, radio waves are used in many other technologies today - some examples include television broadcasting, navigation, air-traffic control as well as cellular communication. As such, we have cleverly separated and assigned the radio wave spectrum into 'bands' so that everyone knows which frequency range they are allowed to operate their respective devices in. Currently, AM carriers use the 535 - 1605 kHz band while FM stations operate in the 88 - 108 MHz band. These bands are further divided into distinct frequencies which are used by individual stations to broadcast. As the radio station can only broadcast in one frequency, they must find a way to encode data into that one frequency wave and that's why modulation is used.

This method of splitting the spectrum opens up an amusing (and definitely illegal) possibility that if you have access to a large enough radio antenna, you can hijack every radio receiving equipment in your vicinity by broadcasting at every frequency. Imagine hearing Dance Monkey by Tones and I playing everywhere - now that's something you've never seen anybody do before.

Never Look At Your Radio The Same Way Again

Now that you know that the AM and FM on your radio stands for Amplitude and Frequency Modulation, and that electrical engineers devised a method to interpose two wave signals together before transmitting them in order to allow for many simultaneous transmissions; I hope that the next time you sneak a glance at your radio to switch it on - or off (or whatever you do with your radio), you'll be able to imagine the vast invisible sea of radio waves gliding along and the network of lanky antennas that work coherently together in order to let you enjoy the music you love orchestrated by your humble radio.


[ 1 ] https://www.dictionary.com/e/what-do-am-and-pm-stand-for/

[ 2 ] https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html

[ 3 ] https://www.taitradioacademy.com/topic/how-does-modulation-work-1-1/