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
There are two basic kinds. FM and AM radio. Those are Frequency Modulation and Amplitude modulation, but you’ve asked about music which means we’re talking about
Frequency Modulation.
Sound waves are wobbles in the air pressure. That’s it. The speakers that play your music are shaking like the surface of a drum, and they push the wobbles out into your room.
So for radio…. we wobble a frequency instead of wobbling a physical drum skin.
The radio is tuned to a frequency… say… 101.1FM (that’s just a number of wobbles per second), and then the broadcast radio wobbles the frequency between 101.0 and 101.2 and your radio picks up those wobbles and makes your speakers wobble the same way.
Boom. Sound.
it varies. AM is Amplitude Modulation (variation in the signal amplitude) FM is Frequency Modulation (variation in the frequency). These are both analog signals, and you just have to have some simple components to filter for a specific channel and then send the wave to the speaker, and the inverse for broadcasting.
There is also DAB+, which is the newer digital radio. Im not exactly sure how it works, but it requires a smarter radio to read the signal, and then turn it into sound. It probably uses both FM and AM in its inner working to send a digital sound file, but I couldnt find a good source on exactly how it works.
You know how you have a cat, and you pull the tail at one end and it yowls at the other end?
Radio is just like that, but without the cat.
in addition to what everyone said here, think about you playing a song through speakers. it gets sent “audio signal” through electrical wires, but it still carries the appropriate sound “information” along with it.
so to make that happen there’s something, the recording device, which translate sound waves into some electrical signal, and there’s another device, the speaker, which translates electrical signal to sound waves.
replace the wire carrying electrical signals with radio frequency waves traveling through the atmosphere.
radio frequency is huge, relatively to the whole electromagnetic spectrum, so signals carried under a certain frequency can be brought back to the original sound waves. but if you take an entirely different frequency and attempt to “decode” the radio frequency signal under a different “speaker” (which in this case would be a station with a different frequency “speaker”), it just ends up sounds like random noise, aka static.
These are all good responses regarding how radio waves are transmitted and generally how they carry information. I think one aspect that hasn’t been discussed as much yet though is the fact that there is no such thing as “separate” radio/TV/etc signals. When an antenna for example is receiving these electromagnetic waves, it’s not just tuning certain ones. It’s receiving everything, all at once. As you can imagine, this “superimposed” wave is incredibly complex, and would be impossible to do anything with if we couldn’t somehow filter just the signals we want to see.
This is where the radio itself comes in, and more specifically, what we call the RLC Circuit.
* R = Resistor
* L = Inductor
* C = Capacitor
These are the three primary passive components in electrical circuits. Almost everything related to electrical circuits (that doesn’t have some kind of semi-conductor in it) probably consists of one of these three components.
It is difficult to explain in a simple way, but the important thing to know about these components as they relate to electromagnetic signals is that they have a really cool ability to work together to create what we call a “filter” circuit. Using different values for each of them – and some very complicated math – you can figure out the exact size of each one that will tune any frequency (or range of frequencies) that you like.
So let’s say you’re receiving this very complicated signal, but you really only want to tune those EM waves around 650 kHz so you can listen to your favorite AM Sports station. Your radio has an adjustable RLC circuit where it can tune those components just right, so that anything above or below 650 kHz will not resonate in the circuit and will be eliminated completely. This filters out just the frequency you want, effectively restoring it to the original wave that was sent out in the first place.
Of course there is a lot more to it than this, but if you really want to dive into more detail, here’s a great video that really lays everything out from start to finish.
This takes me back to basic electronics theory.
AM radio is all about Amplitude Modulation. The frequency you tune your radio to (say 1,620 Kilohertz) changes in strength, the varying intensity of the signal is converted to audio information. An AM signal is susceptible to electrical noise from florescent lights, electric motors and lightning, but can be transmitted over great distances. So the audio is determined by how much the signals amplitude changes over time.
FM radio uses changes in the base frequency to transmit audio information. The frequency the radio station transmits on is the base frequency (an example would be 99.1 Megahertz), the station’s transmitter changes the base frequency just a little bit by slowing it down or speeding it up in response to the audio waveform. An FM radio receiver has a very accurate frequency generator inside you can adjust to the base frequency of the station you wish to listen to. The broadcast signal received is compared to the fixed frequency of the radio’s internal frequency generator, the broadcast signals frequency slows down or speeds up based upon the audio being transmitted. The frequency variances of this signal is converted back into an analog audio wavelength.
Digital FM takes this one step further. The changes in frequency happen at a much faster rate but are still applied to the base frequency, but the changes are converted into a stream of ones and zeros instead of analog information. This data stream is the run through a digital processor that decodes the data stream and converts it back into audio information. When there is sufficient data available the signal can transmit additional information; like song titles, artist, lyrics and more.
There is a lot more going on then what I described, hopefully this will give you enough information for your next question.
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.
You know how, when you have 2 tuning forks, and you strike one and bring it close to the other, the other one starts to sound as well, especiallly if they are “tuned” to the same frequency?
The effect is called resonance, and it happens because the two metals are shaped just right and when it starts to vibrate, a slight feedback loop in the metal itself happens, and it amplifies the signal.
Well, that’s basically how radios work, but instead of physical resonance, it uses electrical resonance.
The same way that a sound wave can have many frequecies, but you voice won’t necessarily cause a tuning fork to vibrate, a radio will have a circuit that is “tuned” to a specific frequency.
Electrical resonance uses the antenna length that should be related to the wavelength of the frequency you want to capture, and a circuit that helps filter out unwanted frequencies, like a coil. Think of a coil as an electrical damper spring that absorbs certain frequencies but lets others through.
**So how do radios work?**
Lets talk about the first method of transmitting signals, Amplitude Modulation.
Audio frequencies 20Hz to 20kHz are low frequency, and as electrical signals, are easily absorbed. High frequency signals in the 100kHz above are much better for transmitting. So how do we transcribe an audio signal to a radio signal?
We start with the “carrier wave”, or a high frequency signal. Think of a constant sine wave at a constant frequency. Now we alter (or modulate) the amplitude or the height of the wave using the audio signal, and we get.. Amplitude Modulation. You encode the audio signal changes as changes in the carrier wave *amplitude*,
[https://en.wikipedia.org/wiki/Amplitude_modulation](https://en.wikipedia.org/wiki/Amplitude_modulation)
Now we have a signal that contains information (the audio, encoded in the height of the wave) but with a high frequency, that can be transmitted a long distance.
The simplest AM reciever uses a wire, a coil (to tune the circuit to the desired frequency) and a diode to extract the “height” of the original audio wave from the carrier wave. The signal still contains the carrier wave so use a capacitor that resists changes by storing some charge an releasing, effectively filtering out the carrier wave and restoring the audio signal:
[https://en.wikipedia.org/wiki/Crystal_detector](https://en.wikipedia.org/wiki/Crystal_detector)
**So how to stations work?**
Just like tuning forks, each station is transmitting at a different frequency. The signals mix all together in the “air”, but the antennas and tuned circuits resonate at the desired frequency, letting you tune into one radio station.
Things are never cut-and-dried of course and station frequencies must have separation so that their signals don’t overlap. This is because the act of modulation generates “side bands”, additional resonant frequencies, weaker than the main carrier frequency, but strong enough to cause interference.
Amplitude modulation has a flaw, in that by modulating the amplitude, you also modulate the power, which leads to signal degredataion. Also lower carrier frequencies can only encode so much information in the amplitude, so that something like high quality music can’t be transmitted effectively.
To fix this, Frequency Modulation was developed. Instead of changing the amplitude, they varied the frequency, based on the input signal. Imagine you have a transmitter operating at a certain carrier frequency, like 88.3MHz. You then have a knob that you twist based on the audio signal, and this changes the fequency of the carrier wave by a few kHz, like 88-89MHz. This is frequency modulation. You encode the audio signal changes as changes in the carrier wave *frequency*,
Whenever an electrically charged thing moves (more accurately, when it *accelerates*, i.e. changes its speed) it will momentarily generate a magnetic field. A little pulse of magnetism that spreads out like a 3D ripple through the fabric of space itself. That pulse, when it washes over other electrically charged things, will cause a tug on those things, forcing them to also move a little bit.
You can think of the effect as something like taking two strong magnets, putting one on top of a table, and holding the other below the table with your hand. When you move the magnet below the table, the magnetism propagates through the table and causes the other one to move with it. You are causing that second magnet to move, but you’re doing it remotely. You’re putting your energy into the first magnet, and due to ripples of magnetism in space, it causes the other magnet to get tugged along.
What radio antennas are are essentially long tubes full of electrons, which are electrically charged things. When you send a signal with an antenna, what you are doing is sloshing the electrons in the antenna up and down in unison in a very specific way that encodes what you’re trying to transmit. All that sloshing in unison causes big ripples in the electromagnetic field, which spread out in all directions**.
Then, there are receivers that contain smaller antennas that can “feel” these ripples. The electrons in them start to slosh around in tune with the ripples, which can be detected as a signal and converted into things like sound.
So, tuning and frequencies. I think this is best explained through an analogy that’s easier to see.
Consider [this YouTube video demonstration](https://www.youtube.com/watch?v=uFlIbujTuIY). It’s about the way buildings of different heights react to earthquakes. There are three dowels with masses on the end that represent buildings at different heights all on a block that can be moved back and forth. When the guy shakes the block slowly, the tallest building will start to sway violently, while the two other buildings barely shake at all. When he starts shaking the block quickly, the shortest building sways violently, while the other two are largely unaffected. And when he shakes it with a frequency somewhere in between, the medium-sized building is the one that shakes violently.
The electrons inside wires have a somewhat similar behavior. When you wash over an antenna with electromagnetic ripples, only ripples at or close to a very specific frequency will actually cause the electrons inside to resonate in unison. Most others will cause the sloshing to be chaotic, causing it to destructively interfere and mostly cancel itself out, leading to no signal beyond some white noise.
A radio receiver, called a tuner, is built in such a way that this “magic frequency” that causes the electrons inside of it to slosh is variable. That’s what you’re doing when you “tune” your radio. You’re changing which frequency of ripples causes the receiving antenna to react. With this technology, you can bombard the tuner with radio signals on all frequencies, but only the frequency that it is sensitive to will cause the electrons in it to slosh around, meaning you can auto-filter out everything except a single radio station. All you have to do is set the sensitive frequency of your tuner to match the frequency the radio station you’re interested in is broadcasting on.
There are a couple major ways to encode your information into radio waves, and have them decoded on the other end. The two most popular methods for public radio broadcast are AM and FM.
AM, or amplitude modulation, is the easier of the two to understand. In a nutshell, the way AM works is the broadcast antenna will constantly “hum” on its designated frequency, but it will constantly change how “loud” it’s humming, getting “louder” and “softer” back and forth very quickly. You can use this rapidly changing “loudness” to encode useful information.
FM, or frequency modulation, is trickier. In this setup, the radio tower hums on its designated frequency just like before, but instead of changing how loud it hums, it instead will hum “off-pitch” from its regular frequency, just a little bit. This will cause the ripples coming off of the tower to be emitted faster or slower than usual. A radio trying to detect that frequency can notice those ripples arriving earlier or later than they should. Information is encoded by strategically creating early and late pulses in a very specific way that the radio on the other end can piece back together.
^(** It doesn’t go in all directions equally. It’s possible to build your antenna with just the right shape that ripples cancel themselves out in certain directions, thus creating antennas that can beam most of their energy in a specific direction.)
The most intuitive way to understand it is that radio waves are actually photons, they just don’t have the same frequency range as the visible light that you see. Different frequencies of photons interact with the world differently – like x-rays we use for … well … X-rays.
The range of frequencies we picked for radio transmission is the best range for that purpose. They penetrate most materials, they travel a far distance before becoming too spread out, they are harmless to organic life, and they happen to interact well with electromagnetic circuits so we can build antennas.
This range of frequencies interact with the electromagnetic circuit in your antenna to wobble those electrons to the same frequency. We use further circuitry to then modulate those wobbles into audio.
Ultimately, the radiowaves travel in much the same manner as the visible light you see all around you. Our eyes just evolved to interact with the range of frequencies that happen to interact with the world in such a way that our brains can interpret (or touch) distant objects visually.
Latest Answers