Like it picks up a signal and processes it and from that I can hear music. But is it just the variation in frequency? And if so, then how do stations work?
In: Technology
The others have given some great explanations about *what* FM modulation is, but I’d like to try to explain more of the *why* and the *how*. Also, my answer is a little long, so I carried over to a second comment to finish the explanation.
Sound waves, as should be obvious from the name, are waves that vibrate the air, and the speed at which the air vibrates is called the *frequency* of that vibration, and is measured in hertz, or Hz (where 1 Hz means it vibrates once per second, 10 Hz means it vibrates 10 times per second, and so on). Sound waves can exist in any frequency, but the human ear can only hear frequencies between 20 Hz to 20 kHz (=20,000 Hz).
When we want to transmit this sound, the first thing we might try is just send the sound waves over the air, but this encounters four problems :
1. Everyone can hear the sound being transmitted
2. The sound rapidly becomes weaker as you move away from the source of the sound, and you can barely hear it after a small distance
3. If you transmit multiple sounds at once, you hear all of them at once and you can’t make out any individual sound
4. You often have random noise in the surroundings that, while it may be weak, can distort the transmitted sound that you hear and make it harder to make out.
To solve problem #1, we convert the sound into something else that can’t be heard or sensed directly by humans. In this case, radio waves make the perfect medium, because we can’t sense them directly in most cases, and they transmit in all directions just like sound waves. It’s also possible to easily make electronic antennae that can easily pick up extremely tiny radio signals. So our first attempt is to directly convert a sound wave into the exact same radio wave and transmit it over the air. But then, we realise it still has problems #2, #3 and #4 from before. A radio wave that’s less than 100 kHz (=100,000 Hz) is easily absorbed by the atmosphere and rapidly loses its energy as it passes through the air. In addition, you still have the problem that if you try to send more than one signal at once, they all conflict with each other. You also suffer the problems of small amounts of radio noise that constantly exists all around us and even in the circuits themselves, that can distort the radio signal that you pick up.
The first solution to these problems is a process called AM, or amplitude modulation. In this process, you choose a very high radio signal frequency, called the carrier frequency, that’s normally a few hundreds to thousands of kilohertz. Let’s say we choose 550 kHz as this frequency. Then, we take our sound signal, and overlap it over the carrier frequency in such a way that the frequency of the carrier signal doesn’t change, but the strength (or *amplitude*) of the signal changes exactly in sync with the original sound signal. This is a process called *modulation*, and it’s very easy to create a circuit to both *modulate* the sound into a radio signal of that frequency, as well as to *demodulate* the radio signal to get back the original sound wave. This suddenly solves problems #2 and #3 quite nicely. When the radio signal has such a high frequency, it’s not easily absorbed by the atmosphere around us, so it can be picked up by a receiver radio that’s much further away from the transmitter station. Also, since the carrier frequency isn’t affected at all, we suddenly realize that we can pack in multiple signals at once, and give each one a different carrier frequency. So if one sound wave is transmitted through a carrier frequency of 550 kHz, another one is transmitted at 600 kHz, and another one is transmitted at 650 kHz, a single radio wave can contain all three frequencies at once, and the receiver can separate out these different signals by only tuning it’s demodulator circuit to the specific frequency you want. As long as two signals are transmitted with at least a 10 kHz difference in frequency, even the cheapest demodulator circuits can easily separate the signals, and this difference in frequency between stations is called the *bandwidth* of the signal.
However, as good as this solution is, it doesn’t completely solve problems #2 and #4. Because the modulation is performed on the strength (amplitude) of the carrier wave, and because this strength of the wave is what reduces as we move away from the source, the resulting signal that the demodulator reconstructs becomes lower and lower in volume as you move away from the source, and you need more circuits to push that volume back up to an audible level. But when you do that, the ambient radio noise in the atmosphere also gets amplified along with the original sound wave, and so the sound you hear from the AM signal gets more and more garbled and noisy.
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