If you shine a flashlight through a tree blowing in the wind and vary the brightness to convey information, the signal can get distorted pretty easily.
However, if you have a constant brightness source and vary the color, it’s a lot easier to figure out what the source is trying to convey.
It's like saying that the violins is merely an analogy for how a double base works.
What I don't see is how it explains why one would work better than the other.
If the tree is blowing in the wind, and a leaf obstructs the entire signal, it doesn't matter whether it's a change in brightness, or a change in color. Either way, that information is lost by the blocked leaf. And if the entire signal is not lost, perhaps many leaves may have blocked the signal but some signal managed to get through, it doesn't matter whether the signal change was a change in brightness, or a change in color. Either way you're going to notice the change. So I don't see how this clarifies why FM is better. What am I missing?
I see from the article that "noise tends to be a an unwanted amplitude modulation, not a frequency modulation." In other words, the tree is providing an unwanted change in brightness. It never provides an unwanted change in color.
I guess the tree is able to dim the signal so much that it appears to be a deliberate signal change? Couldn't this be dealt with if you know the details of the tree's dimming ability?
Put visible light over a really long waveguide and modulate the colors, you invented fiber optic telecommunication.
You model this by taking your signal and convolving it with the channel vector. Usually the channel vector is a finite number of dirac deltas. Each delta is a different reflection. They are like echos. They can cause the signal to constructively and desconstructively interfere with itself.
I haven't seen the math, but I am guessing this doesn't do as much to the frequency of the signal compared to the amplitude.
And even if leaves do sometimes block the entire signal, you're still going to do better with varying the color than the brightness.
For this, it's better to stick to many leaves - the analogy holds up well here because when is the brightness change due to the number of leaves being in the way vs the source changing its brightness?
A flashlight beams out waves that we can see; a radio transmitter beams out waves we can't. The brightness of the beam of light is related to its amplitude, just like the signal content in AM radio is related to its amplitude. And the color of the beam of light is related to its frequency, just like the signal content in FM radio is related to its frequency.
The tree blowing in the wind will introduce its own amplitude (brightness) fluctuations. It will be hard for you to tell which amplitude changes are signal from the source and which are noise from the tree.
Edit: Looks like you answered yourself while I typed that, where you added:
> Couldn't this be dealt with if you know the details of the tree's dimming ability?
If the tree is moving, and you’re far enough away to resolve individual leaves (which is not unreasonable) then its “dimming ability” is constantly changing.
I'm unsure of what the correct terminology would be, but (for my linear algebra brain) you could say something like, for FM the noise dimension is orthogonal to the signal dimension, while for AM the noise and signal dimensions are the same. Therefore for FM any change in amplitude in the noise dimension should be mostly isolated from the signal dimension, while it is essentially impossible to tell what is noise and what is signal for AM - you could probably do some radio equivalent of a differential pair in order to detect noise and remove it, but then why would you bother when FM has improved noise rejection anyway.
of course, you shouldnt be listening to radio during a tornado, but...
So when the signal frequency changes, you’ll still see that change, but the light might get brighter or dimmer at the same time due to the stained glass. But you don’t care about the brightness to begin with.
An AM receiver is a machine that senses the amplitude at a specific em frequency. In this situation, noise and interference become random additions or subtractions to that amplitude. Draw a sine wave, then go over the line with vertical ticks or scribbles. Now imagine taking a random sampling of points and reconstructing the original wave perfectly (without a computer). Most of the information is just gone and you end up with a noisy output wave.
Now an FM receiver is one that measures frequency changes above and below a 'carrier' frequency. The amount of deviation away from center represents the amplitude of the sound signal being transmitted. In this setup, noise and interference are also random additions to the amplitude, but also at random frequencies. On average, interference happens evenly over the entire range of frequencies you're looking at. That means that the highest amplitude is still the same frequency away from center, it just has a slightly different amplitude.
Go back to that sine wave. You can't see the original signal behind all the noise, but you can still see how far apart the peaks are. You can still easily extract its frequency content.
FM uses the frequency dimension to transmit data because random noise can't really affect frequency. Noise mostly happens in the amplitude dimension across all frequencies at the same time.
FM is more robust because it uses two dimensions to encode information vs AM's single dimension. That's also why FM is in stereo!
Of course it does. Real-life objects aren't perfectly opaque or transparent. Similarly, radio waves aren't blocked or received: they're mangled and self-interacted in complex ways.
Rubber ducks aren't battleships because they both float. Visible light and radio attenutate in meaningfully-different ways. It's an analogy.
Fair enough, this might be a sensory artefact. In this case, however, nature had a point. Energy scales proportionally with frequency but exponentially with amplitude. Increasing amplitude delivers more bang than increasing frequency.
Comparing similar things is literally what an analogy is, the fact that in these two cases (radio/light and string instruments) the things being compared are very similar it doesn't make them the same thing, nor does it make it not an analogy.
The flashlight is an analogy for a radio transmitter. We all get that they work on same principle but just on different wavelengths. But regardless I can't shine the flashlight in my kitchen drawer at my radio and pick up a signal.
Lol news to me and my physics degree, Do tell because as far as I'm aware Maxwell's equations don't have an asterisk on them that say "doesn't work below 1 GHz".
Yes. Just like light and radio waves are both EM. A rubber duck remains an analogy for the buoyancy of a battleship. Not "literally the same" thing.
This static and RF noise is AM. It's impossible to filter it out from an AM signal, and so the background noise gets amplified with the signal.
Encoding the signal in a modulated frequency (FM) means we don't need to amplify the detected AM signal and it's associated background noise.
Stereo FM is essentially two waves transmitted at the same time (it's common and difference instead of left and right, but that's math). Stereo AM would be possible, it was never done because two different AM transmissions have to be spaced further away than FM.
Did you really just pull out Maxwell's equations?
EM interacts with matter in different ways. Glass hardly attenuates visible light, but wood does. 2.4 Ghz can pass through walls better than 5Ghz.
There's the concept of permittivity wherein Maxwell's equations are defined in free space with vacuum permittivity.
https://en.wikipedia.org/wiki/Vacuum_permittivity#Permittivi...
To accurately model EM waves, you need more than just Maxwell's equations. You require material equations to model interactions of EM with media.
If you want to get really advanced, whereas Maxwell's equations are classical physics, there's Quantum electrodynamics (QED) which can model interactions of EM and matter.
https://en.wikipedia.org/wiki/Optical_fiber#Mechanisms_of_at...
Same with communications over coax. Obviously visible light doesn't transmit well over copper, but a spectrum of radio waves do, some better than others.
Going from red to orange is about 50 THz. Typical FM radio modulation width is 100 kHz.
If the ocean were as clear as your average long-distance fiber cable, you would see down to the bottom of the Mariana Trench (also in the range of visible light, AFAIK).
Clear in certain wavelengths. Depends on the composition of the glass.
https://en.wikipedia.org/wiki/Optical_fiber#/media/File:Si_Z...
Silica glass behaves differently from ZBLAN (fluorozirconate glass).
Which goes to show how complicated EM interactions with media can be. It's generally easier to just empirically measure attenuation through some medium and use the empirical measurements as a model.
For me, the people saying they are the literal same thing are the same type of people that gave me that "aha" moment that really helped solidify my understanding of RF.
It was pretty mind blowing when I Understood that AM is a change in brightness and FM was a change in color. We just can't see RF, but if we could, that's what it would be.
The conceptually simplest of course whas where the LSB and USB are used as separate channels.
Although most of the systems did work, they were not ultimately successful simply because insufficient stereo receivers reached the market.
Go search in Wikipedia on "AM Stereo".
It works, but it is a fairly expensive method to implement.
The reason the analogy is good is because it isn't even really an analogy, it is in fact a description of electromagnetic waves and a noise source.
That’s the hallmark of an analogy. It gives the general idea, but breaks down if we interrogate it in too much detail.
Doing the math, if you're going 200mph away from a station transmitting at say 121MHz, the drift frequency would be ~36Hz. Not going to be a problem.
And even then, your AM transmission still gets affected by Doppler shift as well.
Airplanes use AM because when two SSB transmissions happen at the same time you can actually hear both at the same time. If you're using FM it's either an incoherent mess or one transmitter drowns out the other.
> You require material equations to model interactions of EM with media
> Quantum electrodynamics (QED) which can model interactions of EM and matter.
It's amazing how condescending some people on here are; how could you possibly have missed literally in the first sentence of my response
> ... my physics degree
Wikipedia says that there was basically one station doing ISB stereo; which I guess is close enough to "nobody did it", but not quite "it was never done".
It’s a terrific analogy. OP is arguing that it isn’t an analogy but an identity. For what should be obvious reasons, it isn’t. And in this case, the difference between analogy and our best model of reality is material.
You don’t see how one being able to attenuate around a hill while another needs line of sight isn’t material to the way we use light and radio waves?
Proper receivers use a phase-locked loop to “lock on” to a carrier, rejecting any weaker interference on nearby frequencies.
In the analogy, suppose you’re decoding a signal from a flashlight over the entire color spectrum, but sunlight shines through the leaves of the tree, adding a slight green noise component to what you see, while the flashlight is actually red. You’ll erroneously interpret the signal as slightly yellow.
We don’t have anything like the PLL in our eyes, so the analogy breaks down here. In the equivalent scenario with an actual FM signal, that slight “green” component would not affect the received signal (or it would affect it to a much lesser extent).
You literally started your comment with "Lol news to me", then you used your degree as if it made you more knowledgeable than anyone else here. Take a look in the mirror?
> ... Do tell
I did?
The extra information isn't to condescend. It's for other people that want to know more about the science.
But battleship doesn't equal floating in water, floating in water is a property it has.
If you're saying "the way a battleship floats in water is like how a rubber duck floats in water" then it's not an analogy, it's as you say just describing two versions of the same thing.
But it is an analogy to directly compare the two objects, because "floating on water" is a property of the objects it's not the object you are comparing.
Wikipedia begins its page on analogies with this, sourced from The Oxford Companion to the English Language: "Analogy is a comparison or correspondence between two things (or two groups of things) because of a third element that they are considered to share."
Or Marriam-Webster: "a comparison of two otherwise unlike things based on resemblance of a particular aspect"
Apart from rubber ducks and battleships both having the "third element", or aspect, of "primarily used for floating on water", they are definitely two completely different things. Nobody could look at a rubber duck next to a warship and say "they seem to be the same thing".
The more closely related two things are the more useful and less stretched the analogy available, which is why the analogy about radio waves was so enlightening to so many people in this thread. But it's bang on as the definition of what an analogy is.
IIRC, when Corning Co. first started working on optical fibers, the best available glass would be good for sending signals about ten meters. What was improved was not the total internal reflection; it was the purity of the glass.
That's an oxymoron, really. Frequency and amplitude are closely interrelated concepts (e.g. see Heisenberg's uncertainty principle in the context of signal processing). Frequency-varying and amplitude-varying even more so!
