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Someone gave me an analogy some time ago that made a lot of sense. If you shine a flashlight through a tree blowing in the wind and vary the brightness to conv

by matrix2003 2y ago

Someone gave me an analogy some time ago that made a lot of sense. 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.

MrLeap 2y ago
I read a similar explanation on slashdot a few decades ago that's stuck with me.
crims0n 2y ago
Wow, that is pretty clever.
pessimizer 2y ago
I'm stealing this.
spacemanspiff01 2y ago
This is the best explanation I have ever heard.
userbinator 2y ago
It's not merely an analogy, just the same EM waves scaled up in frequency by a few orders of magnitude.
- mistercow 2y ago
  Except that color isn’t the same thing as wavelength when it comes to humans perceiving light, because our eyes only deal with the total energy within each of three overlapping bands. An FM receiver knows the difference between a single carrier varying in frequency, and two carriers of different frequencies varying in proportion. Our eyes don’t, hence the banner above appearing orange, even though it’s actually made of different proportions of red and green.
  - idunnoman1222 2y ago
    Right, but let’s just consider the visual spectrum a single carrier/station
    - mistercow 2y ago
      But that’s not how FM receivers generally work. They don’t just take a chunk of the spectrum and measure relative amplitudes within that window. Some very quick and dirty FM demodulators do something like that, but they have poor noise rejection, so the analogy fails. 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).
reader9274 2y ago
I always shy away from analogies because more often than not they give the wrong "feel" for a concept. But this is one of those rare exceptions.
- Filligree 2y ago
  It's not an analogy. This is precisely how it works.
  - khazhoux 2y ago
    Unless your car radio consists of a flashlight and a tree, this is an analogy.
    - viraptor 2y ago
      Well... It kind of does. The source of the radio station is a kind of flashlight, just on a different frequency. The tree is still a tree (and all the other objects)
      - cloudwalk9 2y ago
        More accurately a giant lightbulb, but emitting at 102.7 MHz (my favorite local radio station) rather than ~450 THz (my favorite color). Put visible light over a really long waveguide and modulate the colors, you invented fiber optic telecommunication.
    - llm_trw 2y ago
      The flashlight is the radio tower, the tree is the tree, and the radio in the car is your eyes. There is no analogy here, it is literally the same EM waves shifted up to where our eyes can see them. It's like saying that the violins is merely an analogy for how a double base works.
      - JumpCrisscross 2y ago
        > it is literally the same EM waves shifted up to where our eyes can see them Rubber ducks aren't battleships because they both float. Visible light and radio attenutate in meaningfully-different ways. It's an analogy.
beala 2y ago
This makes a lot of sense so long as your source of noise is something like a tree swaying in the wind, ie something that interferes with the amplitude. If instead the source of noise is uhhh a piece of stained glass swaying in the wind then blinking the flashlight is the better bet. I guess it just turns out radio interference is more like the tree. But why?
- abnry 2y ago
  In this analogy, the AM and FM signals you receive aren't usually experiencing interference, they are experiencing multipath effects which includes things like path loss, attenuation, reflections, and so on. This is driven by geometry. You also have gaussian noise that the receiver has to deal with. 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.
- bee_rider 2y ago
  The stained glass would change the amplitude of some light selectively. But because the FM radio works at different distances, I wonder if it must have some way of adjusting for different amplitudes anyway?
  - xeyownt 2y ago
    Yes, stained glass is like band filter, they let through a particular frequency range, while reducing those outside the range. Your FM receiver will still lock on the desired frequency as long as their is enough signal strength. It's kind of the same as listening to an emitter that is very far while being very close to another. Of course, it'll stop to work at some point depending on minimum signal-to-noise ratio.
- arnarbi 2y ago
  Stained glass won’t (I think) shift any frequencies. It will attenuate different frequencies differently, but it won’t make up new ones. 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.
tejohnso 2y ago
This seems great at first, but more so as an explanation of how AM and FM differ; one being by amplitude (brightness), and the other by frequency (color). 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?
- kelnos 2y ago
  I think the idea is that the leaves don't block the entire signal. They just partially obscure it sometimes. And even if leaves do sometimes block the entire signal, you're still going to do better with varying the color than the brightness.
- treis 2y ago
  A leaf blocking some light doesn't change the color of the light that passes through.
  - JumpCrisscross 2y ago
    > leaf blocking some light doesn't change the color of the light that passes through 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.
- irjustin 2y ago
  > Either way you're going to notice the change. 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?
ra 2y ago
That's not the real story. The RF environment is noisy, with naturally occuring static "sparks", but also with manmade RF noise. 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.
- cbolton 2y ago
  That's exactly what the parent comment described in the beautiful example where the AM noise is due to moving tree leafs affecting the intensity of transmitted light, and you can fix it by varying color, which means varying the frequency spectrum of the light.
  - ikekkdcjkfke 2y ago
    How does the radio follow the frequency modulations if the radio cannot "see" at a specific direction?
    - cbolton 2y ago
      In the example, the amplitude of the flashlight signal is distorted by the movement of the trees. The signal is never completely hidden. Not sure if that answers your question...
- Sesse__ 2y ago
  It's not that simple, though. The only way you can detect frequency is by measuring the amplitude (and then differentiate; except of course in an analog circuit, you don't do that exactly, you have some mechanism that tries to track the carrier wave smoothly instead), so amplitude noise will necessarily also become frequency noise. But generally white AM noise will be pushed upwards in the spectrum after FM demodulation, away from the area where you care. (You can also add a hard limiter, which amplifies this effect; even more noise high up, even less noise further down.)
squarefoot 2y ago
Good analogy, however if you move back and forth the transmitter or the receiver at enough speed, frequency (color) will vary as well, and that analogy could be used to explain Doppler effect, and why civilian airplanes use AM.
- vel0city 2y ago
  Civilian airplanes aren't using AM because of the Doppler effect. You're not accelerating that rapidly to make the Doppler effect that pronounced on the kind of radio being used in airplanes to the point they wouldn't be useful. Even if you're going hundreds of miles an hour the shift is going to be a few dozen Hz in drift. A cheap FM discriminator will be able to handle that without any problem. Doppler shift starts to matter when dealing with satellites, but not airplanes unless you're taking a SR-71 on a civilian stroll. 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.

MrLeap 2y ago

I read a similar explanation on slashdot a few decades ago that's stuck with me.

crims0n 2y ago

Wow, that is pretty clever.

pessimizer 2y ago

I'm stealing this.

spacemanspiff01 2y ago

This is the best explanation I have ever heard.

userbinator 2y ago

It's not merely an analogy, just the same EM waves scaled up in frequency by a few orders of magnitude.

mistercow 2y ago
Except that color isn’t the same thing as wavelength when it comes to humans perceiving light, because our eyes only deal with the total energy within each of three overlapping bands. An FM receiver knows the difference between a single carrier varying in frequency, and two carriers of different frequencies varying in proportion. Our eyes don’t, hence the banner above appearing orange, even though it’s actually made of different proportions of red and green.
- idunnoman1222 2y ago
  Right, but let’s just consider the visual spectrum a single carrier/station
  - mistercow 2y ago
    But that’s not how FM receivers generally work. They don’t just take a chunk of the spectrum and measure relative amplitudes within that window. Some very quick and dirty FM demodulators do something like that, but they have poor noise rejection, so the analogy fails. 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).

reader9274 2y ago

I always shy away from analogies because more often than not they give the wrong "feel" for a concept. But this is one of those rare exceptions.

Filligree 2y ago
It's not an analogy. This is precisely how it works.
- khazhoux 2y ago
  Unless your car radio consists of a flashlight and a tree, this is an analogy.
  - viraptor 2y ago
    Well... It kind of does. The source of the radio station is a kind of flashlight, just on a different frequency. The tree is still a tree (and all the other objects)
    - cloudwalk9 2y ago
      More accurately a giant lightbulb, but emitting at 102.7 MHz (my favorite local radio station) rather than ~450 THz (my favorite color). Put visible light over a really long waveguide and modulate the colors, you invented fiber optic telecommunication.
  - llm_trw 2y ago
    The flashlight is the radio tower, the tree is the tree, and the radio in the car is your eyes. There is no analogy here, it is literally the same EM waves shifted up to where our eyes can see them. It's like saying that the violins is merely an analogy for how a double base works.
    - JumpCrisscross 2y ago
      > it is literally the same EM waves shifted up to where our eyes can see them Rubber ducks aren't battleships because they both float. Visible light and radio attenutate in meaningfully-different ways. It's an analogy.

beala 2y ago

This makes a lot of sense so long as your source of noise is something like a tree swaying in the wind, ie something that interferes with the amplitude. If instead the source of noise is uhhh a piece of stained glass swaying in the wind then blinking the flashlight is the better bet. I guess it just turns out radio interference is more like the tree. But why?

abnry 2y ago
In this analogy, the AM and FM signals you receive aren't usually experiencing interference, they are experiencing multipath effects which includes things like path loss, attenuation, reflections, and so on. This is driven by geometry. You also have gaussian noise that the receiver has to deal with. 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.
bee_rider 2y ago
The stained glass would change the amplitude of some light selectively. But because the FM radio works at different distances, I wonder if it must have some way of adjusting for different amplitudes anyway?
- xeyownt 2y ago
  Yes, stained glass is like band filter, they let through a particular frequency range, while reducing those outside the range. Your FM receiver will still lock on the desired frequency as long as their is enough signal strength. It's kind of the same as listening to an emitter that is very far while being very close to another. Of course, it'll stop to work at some point depending on minimum signal-to-noise ratio.
arnarbi 2y ago
Stained glass won’t (I think) shift any frequencies. It will attenuate different frequencies differently, but it won’t make up new ones. 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.

tejohnso 2y ago

This seems great at first, but more so as an explanation of how AM and FM differ; one being by amplitude (brightness), and the other by frequency (color). 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?

kelnos 2y ago
I think the idea is that the leaves don't block the entire signal. They just partially obscure it sometimes. And even if leaves do sometimes block the entire signal, you're still going to do better with varying the color than the brightness.
treis 2y ago
A leaf blocking some light doesn't change the color of the light that passes through.
- JumpCrisscross 2y ago
  > leaf blocking some light doesn't change the color of the light that passes through 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.
irjustin 2y ago
> Either way you're going to notice the change. 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?

ra 2y ago

That's not the real story. The RF environment is noisy, with naturally occuring static "sparks", but also with manmade RF noise. 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.

cbolton 2y ago
That's exactly what the parent comment described in the beautiful example where the AM noise is due to moving tree leafs affecting the intensity of transmitted light, and you can fix it by varying color, which means varying the frequency spectrum of the light.
- ikekkdcjkfke 2y ago
  How does the radio follow the frequency modulations if the radio cannot "see" at a specific direction?
  - cbolton 2y ago
    In the example, the amplitude of the flashlight signal is distorted by the movement of the trees. The signal is never completely hidden. Not sure if that answers your question...
Sesse__ 2y ago
It's not that simple, though. The only way you can detect frequency is by measuring the amplitude (and then differentiate; except of course in an analog circuit, you don't do that exactly, you have some mechanism that tries to track the carrier wave smoothly instead), so amplitude noise will necessarily also become frequency noise. But generally white AM noise will be pushed upwards in the spectrum after FM demodulation, away from the area where you care. (You can also add a hard limiter, which amplifies this effect; even more noise high up, even less noise further down.)

squarefoot 2y ago

Good analogy, however if you move back and forth the transmitter or the receiver at enough speed, frequency (color) will vary as well, and that analogy could be used to explain Doppler effect, and why civilian airplanes use AM.

vel0city 2y ago
Civilian airplanes aren't using AM because of the Doppler effect. You're not accelerating that rapidly to make the Doppler effect that pronounced on the kind of radio being used in airplanes to the point they wouldn't be useful. Even if you're going hundreds of miles an hour the shift is going to be a few dozen Hz in drift. A cheap FM discriminator will be able to handle that without any problem. Doppler shift starts to matter when dealing with satellites, but not airplanes unless you're taking a SR-71 on a civilian stroll. 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.