i don’t understand when i google it so Whats the difference
In: Technology
The information can be either coded into the radio wave by altering the Amplitude of the wave of or the Frequency of the wave. The wavelengths of AM vs FM are also in different ranges to prevent overlap
AM – Amplitude Modulation. The audio is encoded in changes of the amplitude of the wave … like a on a vinyl record. Big waves means big sound. Long waves means low notes, short waves means high notes.
FM – Frequency Modulation. The audio is encoded in the changes of the frequency of the wave. This is harder to visualize . but Wikipedia’s article on Fourier Transform has a good graphic https://en.wikipedia.org/wiki/Fourier_transform#/media/File:Fourier_transform_time_and_frequency_domains_(small).gif
There is also PM – Phase modulation. Harder still to visualize. This is often used by satelites. Or at least used to be before CM …
CM – Code Modulation. Basically all “channels” are on the same stream of signals, but yo extract your message from the signal you need to apply a code key to filter what is for you (other way around actually, but you get the idea). This is often used in cellphones.
TM – Time Modulation. Think of this as “from >=00:00 until <00:10 everything will be for Alice to read, and from >=00:10 to <00:20 everything will be for Bob to read”. Kind fo boring, but this is basically how ethernet cables work.
There are many variations on the above themes.
Both start with a particular frequency of radio waves. Then they encode the information (music, voice, etc) into it.
AM (Amplitude Modulation) encodes the information by changing the intensity (amplitude) of the radio signal. So it will stay the same frequency, just get more or less intense (think of a sound that gets louder and softer).
FM (Frequency Modulation) encodes the information by changing the base frequency a little bit. (think of a sound that gets slightly higher and lower).
Your radio signal is a paved road with speed bumps. AM is how big each speedbump is. FM is how many speed bumps you hit in a mile.
Your radio is a car suspension that is tuned to handle bigger and smaller speedbumps, or it’s tuned to handle more or fewer speedbumps in a mile. When properly tuned for a particular size of speedbumps, your AM radio is able to interpret the sound of that road. When properly tuned to the number of speedbumps, your FM radio is able to interpret the sound of that road. When neither are properly aligned, the sound of the road is all over the place…(static).
ELI5:
AM is a flashlight that changes its intensity / brightness.
FM is a flashlight that changes its color.
The different brightness levels / colors is what encodes the music.
AM was around first. While it worked ok for listening in the house, it didn’t work well while driving due to it fading out depending on location and obstacles like trees, bridges, tunnels, etc.
Then FM was developed and it worked much better in cars because the signal didn’t fade out as much.
AM radio signals can skip around the planet due to atmospheric conditions, and so be heard many miles from the transmitter, sometime hundreds of miles from the transmitter, especially at night.
But FM pretty much has a 70 mile range so it works well as long as you are relatively close to the transmitter.
AM works best with a long antenna whereas FM works well with a short antenna making easier for most people to get a good signal, and works better in cars.
Imagine you and I held either end of a string, how can we “talk” through it?
Way No. 1: I shake my end of the string up and down. I can do really BIG SHAKES or *really small shakes* and the by alternating shakes you could understand them as 1’s and 0’s or dits and dahs (morse code), either way, I can send you a message this way if you know how to read it.
Way No. 2: I *pull* my end of the string away from you and then push it back, the string itself goes slack and tight and slack and tight. Similar to above, you could “read” the tension as a 1 and the slackness as a 0.
AM stands for “Amplitude Modulation” and it’s like way No. 1. The *shape* of the signal is changing and the radio can read that.
FM stands for “Frequency Modulation” and it’s like way No. 2. The *location* of the signal is changing and a the radio can read that too.
In practice it’s not just science jargon, there are some nuanced differences between the signals for the users.
FM signals are typically clearer and higher quality so they are good for music. They also penetrate things better so inside buildings or even some tunnels you can still get the signal clears day.
AM signals travel further and can reflect off the atmosphere to even bend over the horizon so they are good for important communications or long distance communications (like news, government signals, or rural radio stations).
Finally, in the world of digital IOT communications, FM talks *faster* than AM. So you imagined something like transmitting a book via radio waves, FM could send the whole book out before AM finished a paragraph. This is also important for things like military communications. If you’ve ever seen a sub movie, fast signals are blocked by water, so the subs trawl these long radio arrays for slow signals, but they are so slow the signals they get are limited to “go up”. The sub needs to surface and then up the real radio array to receive a full message because it would take hours to transmit even a short broadcast to the sllllloooooowwww under water array.
Are you familiar with old fashioned morse code? Someone taps a button and the person on the other end hears a series of long and short beeps? The signal being sent is the pattern of beeps which are called “dits” and “dahs”.
Imagine a similar setup where the first person instead broadcasts a continuous hum at some fixed tone. They then need to modify this hum to send the pattern of dits and dahs. They could make the hum quiet for a moment to signal dit, and loud to signal dah. This is amplitude modulation. Or they could make the hum lower pitch for a moment to signal dit and higher to signal dah. This is frequency modulation.
The hum in this example is the carrier radio frequency that you tune your radio to to hear a specific station, and the way we encode the signal onto it is the modulation techique.
If you want to transmit a tone, you need to encode it on a so called carrier wave.
If for examle a station is on 100 MHz, that means 100 MHz is the carrier frequency.
This is basically an electrical signal going from its minimum voltage to it maximum and back 100.000.000 times per second.
Now if you want to encode an audio tone on it you can do that in many ways, AM and FM being common.
In case of FM, which stands for Frequency Modulation, the encoding is done by slightly varying the carrier Frequency. For example by making it swing between 99.999.900 and 100.000.100 times per second, around that centre of 100.000.000, the quicker you let it swing between those two, the higher the tone being transmitted. If you make the swing bigger, for example to 100.000.200 the tone will be louder.
In case of AM, you don’t swing the Frequency of the carrier, that is kept constant. For example on 600 kHz, which means going from minimum voltage, to maximum and back 600.000 times per second. But now to put a tone on it, we will vary what exactly that minimum and maximum is. And that variation will be equal to the tone on being transmitted.
It’s all how the information is sent from one place to another. To keep it simple, if you were trying to send a text message to someone over AM radio, you would change the power levels to send your alphabet, with each power level representing a letter. Whoever is on the other end knows what levels equal what letters, so they measure the strength of the signal as it comes in and write down the letters accordingly. FM, on the other hand, changes the frequency on which you’re sending the signal in order to send the letters. The signal moves between those frequencies and whatever frequency it is on at whatever time is what letter is being sent.
Or, if you prefer to use a flashlight/torch, with AM each letter would have a different power settings, while FM would keep the same power level but each letter would be a different color.
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