[HN Gopher] Why does FM sound better than AM?
___________________________________________________________________
Why does FM sound better than AM?
Author : zdw
Score : 232 points
Date : 2024-10-13 22:32 UTC (1 days ago)
(HTM) web link (www.johndcook.com)
(TXT) w3m dump (www.johndcook.com)
| matrix2003 wrote:
| 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.
| crims0n wrote:
| Wow, that is pretty clever.
| pessimizer wrote:
| I'm stealing this.
| spacemanspiff01 wrote:
| This is the best explanation I have ever heard.
| userbinator wrote:
| It's not merely an analogy, just the same EM waves scaled up in
| frequency by a few orders of magnitude.
| mistercow wrote:
| 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 wrote:
| Right, but let's just consider the visual spectrum a single
| carrier/station
| mistercow wrote:
| 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 wrote:
| 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 wrote:
| It's not an analogy. This is precisely how it works.
| khazhoux wrote:
| Unless your car radio consists of a flashlight and a tree,
| this is an analogy.
| viraptor wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| > _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.
| dexwiz wrote:
| Rubber ducks and battleships both displace water in the
| same way.
| JumpCrisscross wrote:
| > _Rubber ducks and battleships both displace water in
| the same way_
|
| 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.
| aeonik wrote:
| But if you say that a battleship floats on the water in a
| similar way to a rubber duck floating in the water...
| it's actually not similar... they are the same. It's the
| same water and the same physics. The "only" appreciable
| difference is scale.
|
| 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.
| swores wrote:
| > _But if you say that a battleship floats on the water
| in a similar way to a rubber duck floating in the
| water... it 's actually not similar... they are the same.
| It's the same water and the same physics. The "only"
| appreciable difference is scale._
|
| 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.
| aksss wrote:
| Ducks and witches, on the other hand. . .
| almostgotcaught wrote:
| > Visible light and radio attenutate in meaningfully
| different ways. It's an analogy.
|
| 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".
| kuhsaft wrote:
| > 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".
|
| 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#Permitt
| ivi...
|
| 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/Quantum_electrodynamics
| asdefghyk wrote:
| RE "....Glass hardly attenuates visible light...." Clear
| glass blocks about 5% visible light
| kuhsaft wrote:
| Depends on the frequency of EM. Fiber optic
| communications use specific frequencies to minimize
| attenuation in cables.
|
| 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.
| Sesse__ wrote:
| Fiber optics also uses _exceptionally_ clear glass.
|
| 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).
| kuhsaft wrote:
| > Fiber optics also uses _exceptionally_ clear glass.
|
| Clear in certain wavelengths. Depends on the composition
| of the glass.
|
| https://en.wikipedia.org/wiki/Optical_fiber#/media/File:S
| i_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.
| Sesse__ wrote:
| It's exceptionally clear compared to e.g. window glass
| even in the visible spectrum of light. You can shine a
| red light source into a 10 kilometer standard G.657 fiber
| (optimized for 1310/1550nm, i.e. deep infrared) and it
| will still be visible just fine on the other end. If you
| did that with regular glass, it would hardly go ten
| meters.
| kuhsaft wrote:
| Oh yeah. I'm not saying otherwise. Someone replied "Clear
| glass blocks about 5% visible light". I guess "clear
| glass" is pretty subjective. At what level of attenuation
| would someone consider glass not clear? xD
| schoen wrote:
| What are the relative contributions of the total internal
| reflection property of the fiber optic cable and the
| particular low-attenuation material it's made of?
| almostgotcaught wrote:
| > There's the concept of permittivity
|
| > 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
| kuhsaft wrote:
| > It's amazing how condescending some people on here are
|
| 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.
| JumpCrisscross wrote:
| > _as I 'm aware Maxwell's equations don't have an
| asterisk on them that say "doesn't work below 1 GHz"_
|
| 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?
| acje wrote:
| Both are examples of communication by means of frequency
| modulated and amplitude modulated electromagnetic waves
| with distortion from a moving three. Also a good example
| that a large change in quantity is a change in kind.
| Probably a legit analogy imho.
| JumpCrisscross wrote:
| > _Probably a legit analogy imho_
|
| 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.
| treyd wrote:
| But RC boats and battleships both have propellers and
| rudders.
| wruza wrote:
| You could make Bob Ross a new wig from all the hairs
| split in this subthread.
| digitalsushi wrote:
| a lot of people here have a lot of passions, but
| sometimes the passions overlap and we rub shoulders. if
| someone had made a pokemon playing card metaphor we might
| be in the same general condition - but i think we're
| better behaved showing each other how smart we must be
| with radio waves instead of greymon
| swores wrote:
| It's not saying the violin IS "merely an analogy for how
| a double base works", just that a violin can be used as a
| simple analogy to somebody who understands how a violin
| works but doesn't know what a double bass is.
|
| 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.
| mistercow wrote:
| It is not literally the same. The colors you perceive on
| the screen in front of you demonstrate why. That banner
| above is not orange; it's red and green. You don't
| actually have the ability to distinguish a varying
| frequency between red-orange and yellow-orange, and two
| amplitude modulated "carriers" at red and green.
|
| That's the hallmark of an analogy. It gives the general
| idea, but breaks down if we interrogate it in too much
| detail.
| senorrib wrote:
| Both concepts are based on frequency and amplitude of
| waves (radio vs light).
| khazhoux wrote:
| That's what makes the analogy so clear.
| kelnos wrote:
| Essentially that is actually the case. Human-visible
| light and AM/FM radio waves are just different
| wavelengths along the EM spectrum.
|
| 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.
| khazhoux wrote:
| The explanation asks us to imagine shining a flashlight
| through a tree, first changing the flashlight brightness
| and then changing its color.
|
| 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.
| mordae wrote:
| Remove cover, locate LNA, modulate light correctly and
| voila... :-)
| Sesse__ wrote:
| It _is_ the wrong feel for a concept. The analogy breaks down
| because the color changes are way too wide in frequency (and
| thus too robust to noise) compared to what happens in a radio
| broadcast. If you changed the color from RGB(127, 0, 0) to
| RGB(126.999999, 0.000001, 0), the movement of that tree would
| actually start to make your strategy difficult.
|
| Going from red to orange is about 50 THz. Typical FM radio
| modulation width is 100 kHz.
| Workaccount2 wrote:
| The analogy isn't wrong, it's just that it is incomplete as
| given. You are bringing the sensitivity of the receiver
| into the equation. But it doesn't really break down the
| analogy, because the frequency shift in color is calibrated
| for the human eye's sensitivity. Calibrate it for a FM
| receiver and that tiny color shift becomes easily
| discernible. The tree leaves have no impact on frequency,
| just amplitude.
|
| 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.
| beala wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| 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.
| carlmr wrote:
| In the stained glass case, maybe you need to go digital
| where brightness and color don't matter, but only on-off
| state.
| ploynog wrote:
| You'd be surprised by the amount of brightness and color
| produced if you are turning things on-off sufficiently
| fast.
| ykonstant wrote:
| Related: lots of optical illusions.
| a-dub wrote:
| a better analogy for frequency domain interference would be
| something like the spinning flashing lights on a fire engine
| or utility truck occasionally shining colored light on your
| detector.
| mkehrt wrote:
| I think this is reasonable analogy, for FM interference,
| and it points out why FM is resilient to noise: the
| flashing lights have to be relatively bright (high
| amplitude) to interfere with your color based scheme.
| tejohnso wrote:
| 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 wrote:
| 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 wrote:
| A leaf blocking some light doesn't change the color of the
| light that passes through.
| JumpCrisscross wrote:
| > _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 wrote:
| > 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?
| arnarbi wrote:
| > What am I missing?
|
| 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.
| jmts wrote:
| FM works better because it is easier to detect the change in
| frequency independently of any change in the amplitude.
|
| 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.
| jareklupinski wrote:
| if the leaves are blowing back and forth between the
| transmitter and the receiver, they would introduce a doppler
| shift into the signal
|
| of course, you shouldnt be listening to radio during a
| tornado, but...
| evoke4908 wrote:
| The analogy is getting a bit tortured, so I'll try a more
| practical explanation.
|
| 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!
| bonzini wrote:
| > That's also why FM is in stereo!
|
| 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.
| wkjagt wrote:
| Could you make AM stereo by somehow using the two
| sidebands (on each side of the carrier) for left and
| right?
| Johnythree wrote:
| Yes, this is one of the proposed methods. It's known as
| "Independent Sideband".
|
| It works, but it is a fairly expensive method to
| implement.
| LocalH wrote:
| AM stereo does exist, however.
|
| https://en.wikipedia.org/wiki/AM_stereo
| Johnythree wrote:
| There were a number of successful AM stereo broadcasting
| methods proposed and trialed. These were completely
| compatible with conventional AM transmissions.
|
| 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".
| bonzini wrote:
| Of the methods listed on Wikipedia only ISB is a true AM.
| All the others use phase modulation for the difference
| signal, as an easy way to achieve compatibility with mono
| AM receivers; and PM is basically the integral of FM.
|
| 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".
| JumpCrisscross wrote:
| Let's switch the analogy to sound. Amplitude is loudness.
| Frequency is pitch. You are trying to discern two sources of
| sound. One is a constant pitch but variable volume. The other
| can always blast at max volume with variable pitch.
| therein wrote:
| Also harder to discern and then quantify the loudness of a
| sound or brightness of a light as a human modem but we are
| better and more certain of the color. We have different
| names for the ranges and everything.
| JumpCrisscross wrote:
| > _harder to discern and then quantify the loudness of a
| sound or brightness of a light as a human modem but we
| are better and more certain of the color_
|
| 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.
| ra wrote:
| 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 wrote:
| 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 wrote:
| How does the radio follow the frequency modulations if the
| radio cannot "see" at a specific direction?
| cbolton wrote:
| 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__ wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| I read a similar explanation on slashdot a few decades ago
| that's stuck with me.
| johndavid9991 wrote:
| I grew up listening to the radio and was always curious why FM
| indeed sounds cleaner than AM. My assumption before is that it's
| setup that way since FM is intended for music stations.
| timeon wrote:
| I had AM associated with international broadcast while FM with
| local one.
| dylan604 wrote:
| Before FM, all music stations were on AM.
| dmitrygr wrote:
| This is 100% nonsense. Phase noise exists too, not just amplitude
| noise.
|
| The answer is actually rather simple. AM stations are limited to
| 10KHz band width. FM gets 200KHz. More bandwidth allows
| representing a higher fidelity signal...
| basementcat wrote:
| It's more than that. FM is several times less spectrum
| efficient than AM and needs more bandwidth to transmit the same
| information.
|
| https://en.m.wikipedia.org/wiki/Carson_bandwidth_rule
| IX-103 wrote:
| You provided a citation, but it doesn't prove your point. In
| general frequency modulation is more efficient than amplitude
| modulation (but requires more complicated receivers). For
| example GMSK in the 2G standard GSM replaced the less
| efficient 1G AMPS system which used amplitude modulation.
| Stratoscope wrote:
| Yes, this is the right answer, although I would correct the
| numbers a bit.
|
| If we look only at the audio bandwidth, AM stations are limited
| to 5 kHz of audio spectrum. The 10 kHz figure comes from the
| fact that AM is double sideband modulation (as opposed to
| single sideband as used in ham radio and other radio services).
| So the broadcast signal uses twice the bandwidth of the audio.
|
| FM stations have 15 kHz of audio bandwidth, three times that of
| AM. They are able to do this because they transmit at a much
| higher frequency.
|
| The 200 kHz figure includes other things like stereo (two
| channels of audio), subcarriers for RDS data and such, and the
| "Carson bandwidth rule" that 'basementcat' mentioned.
|
| I am surprised that the article overlooked this simple and
| obvious explanation.
| nullc wrote:
| WFM in mono without RDS is still ~200kHz wide, the width
| isn't primarily a product of the extra signals, it's a
| product of the modulation index.
| crackalamoo wrote:
| Yes, phase noise exists, but I would think that in practice
| amplitude noise is greater.
|
| In physics, when a wave passes from one medium to another, its
| frequency is supposed to stay the same. Even if this isn't
| perfectly true in the real world, I would think amplitude is
| more likely to decrease due to obstacles, distance, and the
| medium absorbing some energy.
| IX-103 wrote:
| If the noise is white gaussian noise (AWGN) then the phase
| noise is essentially the same as the amplitude noise (by the
| properties of the Fourier Transform).
|
| Also, the information in AM is carried by the _relative_
| amplitude of the signal. Flat attenuation like you 're
| describing doesn't really distort the AM signal. What _does_
| impact both AM and FM is frequency selectivity. Imagine light
| traveling through a prism and being split by frequency. If
| there are obstacles in the way, some colors won 't pass
| through as well. The is can cause distortions in FM as the
| receiver loses lock on the signal. Am suffers from this too,
| but people are less likely to notice because they're used to
| these distortions -- these kind of effects happen with sound
| too.
|
| As other posters have mentioned, the reason FM sounds better
| is that it has more bandwidth for the signal.
| crackalamoo wrote:
| Very interesting, I'll have to look into AWGN and the
| Fourier transform. I guess in the trees blocking the
| flashlight example that's not at all AWGN.
|
| Although while we care about the relative amplitudes in AM,
| AWGN would make this harder to pick out if the signal is
| attenuated. Is the same idea true for frequencies? I don't
| see a direct parallel here.
| kragen wrote:
| You do get some frequency deviation from AWGN. The sum of
| equal-amplitude 100Hz and 120Hz sine waves is a 110Hz
| sine wave that "beats", which is to say, is amplitude-
| modulated, at 10 Hz (or 20Hz from a certain point of
| view). So, if you have a 120Hz signal and you add a 100Hz
| signal to it, you should expect that to deviate the
| frequency of the detected signal downwards. AWGN will
| have varying, random amounts of all frequencies in it,
| which will cause varying, random amounts of frequency
| deviation as they add to your signal.
|
| It's definitely easier to understand in the Fourier
| domain.
| 317070 wrote:
| But the two are the same thing. You took the fourier
| transform of white noise and find white noise, both real
| (amplitude noise) as complex (phase noise).
|
| You can think of it like this: the noise is not about the
| phase changing, it is about your ability to tell what the
| phase is. The noisier the signal gets, the harder time you
| will have to tell what the amplitude is, as well as what the
| phase is.
| kragen wrote:
| It's not _100%_ nonsense, though it 's true that phase noise
| does exist. FM radio can transmit silence, which gives it a
| better dynamic range, which is important for music. If your AM
| radio signal is 10dB stronger than the radio noise in the band,
| you'll get noise in the demodulated signal only 10dB quieter
| than the signal. Due to the so-called "capture effect"
| https://en.wikipedia.org/wiki/Capture_effect the effect on an
| FM-demodulated radio signal is _potentially_ much less--though
| it 's true that, with narrowband FM, it won't be.
|
| That's why commercial FM broadcasting uses a +-75kHz deviation
| even though it was originally only transmitting audio of
| <=20kHz. Adding all this extra bandwidth to an AM station
| wouldn't actually help, because beyond +-20kHz, you're only
| improving your radio station's ability to reproduce ultrasound.
| But it does help FM; it greatly reduces the amplitude of
| demodulated noise, because, even without a PLL, the frequency
| deviation caused by additive white noise increases much more
| slowly with bandwidth than the frequency deviation you can use
| for your signal. With a PLL, I think the frequency deviation
| caused by additive white noise basically doesn't increase at
| all with bandwidth. (I guess I should simulate this; it should
| be pretty easy.)
|
| Unfortunately neither Cook's article nor the flashlight analogy
| explains any of this.
| guidedlight wrote:
| No need to read the article. It's literally in the name.
|
| AM = Amplitude Modulation FM = Frequency Modulation
|
| Obviously environmental factors can affect the amplitude of a
| radio signal. But environmental factors are less likely to affect
| the frequency.
| chasil wrote:
| FM spends bandwidth to reduce noise.
| kelnos wrote:
| > _Obviously environmental factors can affect the amplitude of
| a radio signal. But environmental factors are less likely to
| affect the frequency._
|
| I don't think that's "obvious" to most people.
| zekica wrote:
| And it's not correct. Resulting frequency due to random noise
| is also changed, but in FM, noise is less perceivable. There
| is no such thing as "affecting amplitude" and "affecting
| frequency" - they are not separate concepts.
| davekeck wrote:
| I always assumed it was because FM station bandwidths (200kHz)
| are much wider than AM (10kHz). AM's 10 kHz chops off a lot of
| human-hearable frequencies.
| ndndjdjdn wrote:
| AM doesn't use the frequency for modulation though so it
| shouldn't matter.
| t-3 wrote:
| AM does use the frequency, it just doesn't need as much and
| uses it differently than FM. If it was all at a single
| frequency, there just be a single tone getting louder and
| softer.
| ndndjdjdn wrote:
| Thanks. I just learned that doing a rabbit hole about
| sidebands! Still getting my head around it.
| YZF wrote:
| Once you change the amplitude of a sine wave (modulate
| it) it's no longer a side wave. It spreads in the
| frequency domain. Take the fourier transform of that and
| you can see the frequency components.
| kragen wrote:
| When you amplitude-modulate a carrier wave with an audio
| signal, you spread it out into a bunch of sum and difference
| frequencies, as you can see if you use the trigonometric
| angle-sum formula to factor cos(85000*2 _pt_ ) * (2 +
| cos(440*2 _pt_ )), a 440-hertz flute being transmitted on
| 85-kilohertz AM. These so-called "sidebands" mean that the
| bandwidth of AM _does_ matter, and consequently, using a too-
| narrow bandpass filter on your AM radio station will result
| in low-pass filtering your demodulated audio signal.
| KK7NIL wrote:
| Other comments gave a nice explanation of why AM does need a
| bandwidth, but here's the information theory explanation: htt
| ps://en.m.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theo...
|
| TL;DR: the information one can reliably send through a noisy
| channel (C) is proportional to the bandwidth of that channel.
| Optimal_Persona wrote:
| Also the audio frequency bandwidth is narrower on AM, so fewer
| treble frequencies.
|
| TBH I think music from up to the late '60s (especially if
| originally released in mono) sounds really good, or at least more
| "era-appropriate" on AM radio. I remember my grandparents tuning
| in to easy-listening AM stations as I grew up in the '70s and
| '80s, to my ear Tennessee Ernie Ford's "16 Tons" or a classic
| Phil Spector "Wall of Sound" production sounds more "right"
| coming through the AM bands.
|
| And, in the age of cellphone speakers and compressed
| MP3/Bluetooth codecs - I'm not sure how much people actually care
| about audio quality.
| epcoa wrote:
| > And, in the age of cellphone speakers and compressed
| MP3/Bluetooth codecs - I'm not sure how much people actually
| care about audio quality
|
| Bizarre thing to say after waxing nostalgic about incredibly
| lo-fi bandwidth limited AM.
|
| This is also the age of $9 per month unlimited lossless 24/96
| streaming and $1000+ headphone amps.
| tacticus wrote:
| they have to justify their non newtonian vibration dampeners
| (blutak) and custom AC power filter used to play noisy vinyls
| kragen wrote:
| You can use literally any bandwidth with literally any form of
| radio-wave modulation.
| duped wrote:
| Look, I'm an audio snob and will talk shit about terrible
| design of BT headsets that halve bandwidth in duplex until the
| cows come home.
|
| But the reason that codecs have survived this long without
| substantial changes is because they're far and away good enough
| (*) for the vast majority of listeners. To the point where
| today, even trained listeners can't perceive a difference in
| audio quality between lossless and lossy encoded audio at high
| enough bit rates (which is 320kbps MP3, or comparable AAC which
| can be as low as 50% of that).
|
| (*) what we don't talk about is the latency of the codec
| itself, where regardless of available compute resources is
| still atrocious outside of proprietary codecs. While a listener
| cannot perceive noticeable differences in fidelity, they can
| perceive the delay, and this is a problem that doesn't have
| good solutions outside of specialized equipment today, although
| OPUS (as a descendant of CELT) is pretty darn good for the
| cases that consumers care about. Professionals still spend
| oodles of money on the proprietary gear that have codecs that
| not even ffmpeg supports.
|
| I would go so far as to say there is no practical benefit to
| uncompressed audio today at all. Lossy is fine for all
| consumers, and lossless encoding is faster to decode and
| playback (as well as encode and write) while using less
| disk/bandwidth than uncompressed for archival purposes.
| pseudosaid wrote:
| its a big difference.
|
| The frequency range for AM radio is 540 to 1600 kHz
|
| vs
|
| 30hz-15khz
|
| Bass and fundamental frequencies really contribute to fidelity
| trq01758 wrote:
| Those codecs got better with time. Also notebook and little
| portable speakers, while they are unable to physically
| reproduce low frequencies are getting better at emulating
| those. Somebody cares.
|
| And here's (dunno if true as they write in the description -
| probably the very first stereo) studio turntable from 1958
| playing a record from 1988 through Youtube's compression. I did
| have a lousy vinyl deck with so so speakers when growing up and
| this impresses me a lot: https://youtu.be/PRty-
| _eBEpg?si=GsrctxRbkvT3xRAV
| kaoD wrote:
| What does "emulating low frequencies" mean?
| trq01758 wrote:
| It may be impossible for a little speaker to produce any
| sound at some low frequency, so manufacturers use "virtual
| pitch" psychoacoustic phenomenon by introducing harmonics
| above that frequency. There is no low bass, but there will
| be added harmonics that will be perceived by the listeners
| as low bass:
| https://sound.stackexchange.com/questions/37755/how-do-
| psych... Here's also a project and some info from Asahi
| devs on Macbook audio: https://github.com/AsahiLinux/asahi-
| audio
| Johnythree wrote:
| Is commonly known as "Bass Boost".
|
| As the OP has said, it cannot give louder bass, but
| simulates the bass harmonics.
| userbinator wrote:
| _TBH I think music from up to the late '60s (especially if
| originally released in mono) sounds really good, or at least
| more "era-appropriate" on AM radio._
|
| That music also sounds more era-appropriate coming from a vinyl
| record than a CD.
| Johnythree wrote:
| This is yet another myth:
|
| The "Woolyness" of AM broadcast (at least in America) is due to
| the stations purposefully tailoring their audio processing to
| suit typical cheap AM receivers. And this in turn is because
| designers of cheap AM receivers fit narrow filters instead of
| using noise reduction techniques, eg a good outside antenna.
|
| There was a period (in the rest of the world) where high
| quality AM receivers had a narrow/wide switch to give better
| audio response to stronger signals.
|
| The good news is that modern SDR receivers usually have
| selectable bandwidth on AM so as to derive the full transmitted
| audio. And many of these have AM stereo decoders as well.
|
| If you listen to a good quality AM broadcast (eg Gov AM
| stations in Australia) you will hear audio which are very hard
| to tell from FM audio.
|
| Go back and read the many high-quality AM tuner articles in the
| electronic hobby magazines from the past.
| massysett wrote:
| What I've never understood is how the FM receiver can lock on to
| the signal if its frequency is always changing. Doesn't the
| receiver need to lock on to something? If the answer is "it locks
| on to the amplitude, which doesn't change," well AM is bad
| because the amplitude is subject to interference, so wouldn't FM
| have the same problem?
| analog31 wrote:
| One possibility is a phase-locked loop. I don't know if there's
| anything better. It matches the frequency of a voltage
| controlled oscillator to the frequency of the incoming signal
| by detecting the phase mismatch. Then, the control voltage for
| the VCO becomes the audio signal.
| rnhmjoj wrote:
| I don't think radios use a PLL to demodulate the FM audio:
| the signal has a huge "pilot" tone at 19kHz that you can
| match to get the first part of the spectrum, mono audio (L+R
| channels), and at double that frequency you know you'll find
| the stereo part (L-R channels). Precise phase estimation is
| only necessary to decode the RDS digital data (station name,
| datetime, etc.).
| lrasinen wrote:
| They do.
|
| First of all, the pilot is only required for decoding
| stereo and RDS. Mono FM does not use a pilot, so obviously
| there had to be a way to detect FM before stereo came
| along. I linked to a few of the approaches in a sibling
| (cousin?) comment.
|
| Second, the pilot is embedded in the decoded FM audio. You
| need to demodulate FM to get to it in the first place. If
| you look at the waterfall display in an SDR receiver, it
| might seem like the signal is already present in the
| original radio frequencies (especially during silent
| periods), but it's there only indirectly.
|
| If you have silence in an FM transmission (say 96.6 MHz),
| the only audio component present is the 19 kHz pilot
| signal, which causes the FM radio signal frequency to vary
| between 96.6 MHz +- k*19 kHz (not sure what's the value for
| k, but it's not 1). The sine likes to spend most of the
| time near the extreme values of its range; plot a histogram
| of a sine wave and you'll see peaks on either end.
|
| The waterfall is basically a histogram over frequencies so
| it gets those peaks as streaks on both sides of the main
| carrier frequency (plus smaller ones for other components
| in the signal).
| jhallenworld wrote:
| The 19 KHz pilot tone is interesting: it's not filtered
| out and is often reproduced by the speakers if you have
| good ones. You can verify this by using an audio spectrum
| analyzer application on your phone.
|
| I've wondered if FM stereo drives pets nuts with its
| constant high-pitched tone.
| kmbfjr wrote:
| It is not "huge", it is no more than 10 percent and no less
| than 8 percent of the total modulation.
| Johnythree wrote:
| Most cheap FM receivers definitely use a PLL to detect FM.
| Giving automatic tuning is just a side benifit.
| jhallenworld wrote:
| Do you mean AFT (automatic fine tuning)? This has nothing
| to do with a PLL-based demodulator. Older receivers with
| ratio detectors or discriminators also had AFT- there is
| a low frequency feedback path from the demodulator to the
| local oscillator to make this work.
| jedimastert wrote:
| I'm actually studying for my general ham radio license right
| now! Most FM receivers use something called a "mixer" to
| modulate the frequency to a known constant, then they use a
| circuit called a "discriminator" or "quadrature", both of which
| are "detectors".
|
| Typically they're not measuring the frequency or phase itself,
| but rather the _change_ in frequency or phase.
|
| Edit: I should note that's only for analog circuits. DSP is
| also common.
| CodeBeater wrote:
| Most FM receivers nowadays rely on creating a signal of a
| specific frequency that interferes with the desired on-dial
| frequency, this is called an intermediate frequency. Then the
| actual audio signal is analogous to the changes on that IF.
|
| This technique is known as superheterodyne, and Technology
| Connections has a wonderful video explaining it better than I
| can.
| kragen wrote:
| Disclaimer: I don't really know any of this stuff, and I've
| never built a radio. I'm just repeating what I've read, or in
| some cases, simulated in software.
|
| The simplest answer is that you use a narrowband bandpass
| filter around the transmitting station's center frequency to
| eliminate the signals from other radio stations, just as you do
| for AM radio, and then you measure the frequency of the
| remaining signal instead of its amplitude. This works because
| the frequency deviations are small compared to the spacing
| between the frequencies on which different stations are
| transmitting. Downconverting to an intermediate frequency by
| mixing with a local oscillator, as CodeBeater correctly said
| most FM receivers do, doesn't really alter this fundamental
| principle, although it does alter the details. (Most current AM
| radios are _also_ superheterodyne designs.)
|
| Most current FM radios use a phase-locked loop, as analog31
| correctly said, which is sort of the same but sort of
| different; it gives better results. A PLL uses a much narrower
| bandpass filter which is centered on, not the nominal center
| frequency of the radio station, but the instantaneous,
| modulated frequency, which makes it much better at rejecting
| interference than the simpler approach. So the frequency band
| you're filtering down to gets swept back and forth in real
| time, thousands of times a second, to follow the FM signal.
|
| There's the question of how your PLL can initially achieve its
| lock if its passband is so narrow, of course. I don't know how
| mainstream FM radio does this, but it's not as hard a problem
| as you might think; because broadcast FM radio's frequency is
| always oscillating back and forth around its nominal center
| frequency, you can just wait for the audio signal to cross
| zero. Alternatively, you can sweep the PLL's local oscillator
| frequency over the band until you achieve a lock.
|
| I hope this is helpful!
| jasonjayr wrote:
| Having recently purchased a RTL-SDR and watched and learned
| about FM -- there is "pilot" frequency that doesn't change and
| is fixed relative to the tuner frequency. See this chart here:
|
| https://en.wikipedia.org/wiki/FM_broadcasting#/media/File:RD...
|
| Each radio station has 100khz of bandwidth centered on it's
| tuner frequency. in the, there are channel spacing rules that
| give some gaps +/- another 100khz of that. (That's why in the
| US, radio stations are typically on 'odd' decimals, ie 92.3
| mhz, 94.1 mhz, etc) That chart does not show HD radio
| frequencies, which due to those spacing rules, and more
| accurate transmitters, are on the +/- 100khz spaces along side
| the original analog 100khz. You can "see" the audio modulating
| the frequency on the spectrogram. But the OFDM digital signal
| on either side looks like a band of more intense noise. It's
| mind blowing to realize there's a signal in that!
| lrasinen wrote:
| The pilot is there for stereo decoding, it has nothing to do
| with the ability to tune to an FM station.
|
| https://wiki.analog.com/university/courses/electronics/elect.
| .. has some of the analog approaches collected.
| jasonjayr wrote:
| You're right -- after reading some of the peer responses, I
| realized that (I think...) my response is just how the
| Broadcast FM signal modulates the parts of the signal, and
| not how it actually 'locks on'. I'm still learning!
| lrasinen wrote:
| I got an RTL-SDR this summer and brushed up my DSP skills
| playing with FM signals. PLLs are marvellous beasts; you
| can do a slapdash job in "designing" one and it'll still
| probably lock on just fine. Might not be optimal but will
| still lock.
|
| Another fun one, when you have IQ samples, is the polar
| discriminator: calculate x[t] * x*[t-1] where x* is the
| complex conjugate, and take the angle with arctan. Feels
| a bit like magic ("is that all?") but is justified by the
| theory.
| rileymat2 wrote:
| It locks into the range where the number on your dial is the
| center of the range, then listens over the whole range.
|
| The range does not change.
| Johnythree wrote:
| Most FM receivers can lock on the carrier because they have a
| "Phase locked loop" to cancel any tuning errors.
|
| Many good AM receivers do exactly the same thing, especially
| those receivers which have "Synchronous Detectors" for AM.
|
| It's just that the circuitry involved is simple for FM, but
| rather more complex for AM.
| S_A_P wrote:
| Lightning is a great example of noise causing amplitude changes
| and not frequency changes. That's why during a thunderstorm am
| radio plays each strike between the station and you. The is
| usually not any indication of lightning strikes on FM.
| shsbdksn wrote:
| I always thought about it as I can arbitrarily amplify and
| saturate the FM signal without changing it.
| dumbo-octopus wrote:
| Better is in the ears of the beholder. Personally I prefer AM
| because I can hear multiple stations at once, and hear sources of
| wideband EM interference in my environment.
| bigfishrunning wrote:
| I think this article is focussed on broadcast radio, where
| crosstalk and em interference are both considered negative
| properties (not that they're not useful in other applications)
| bdjsiqoocwk wrote:
| I guess the crux here is the claim that "the effect of random
| noise is to amplitude modulate". Does anyone here understand why?
|
| Ps I don't think analogies are helpful.
| jmts wrote:
| AM reception is essentially the direct conversion of the
| strength (amplitude) of a given radio frequency into an audio
| signal. Any other noise present at the same frequency is added
| to the signal (superposition/interference) and therefore
| impacts the strength of that frequency at the receiver.
| Therefore it is impossible for the receiver to know whether the
| amplitude it received is just signal or is signal plus noise.
|
| The claim 'the effect of random noise is to amplitude modulate'
| is probably not 100% correct, because to my understanding it's
| not actually performing modulation (the modulation happens at
| the transmitter but the noise happens between the transmitter
| and receiver), but it is impacting the amplitude at a given
| frequency and to a receiver this is impossible to know whether
| said change in amplitude happened before modulation (signal) or
| after modulation (noise).
| elahieh wrote:
| It's certainly perceived that way. "Diff'rent Strokes" "Baseball
| Blues", 1985... Willis does mention stereo is part of the appeal.
|
| - Now, Dad, you gotta picture me cruising along in my Mercedes.
| Head held high. Rocking to the FM stereo. Waving to the chicks.
| Hey there, mama, looking good. Catch you later, baby.
|
| (making engine noises)
|
| - You can do all of that in a $4,000 car.
|
| (imitating brakes squealing)
|
| - Dad, for $4,000 I'll have to slouch way down in my seat so no
| one can see me. And turn on my AM radio. Wave at the chicks. Hi
| there, mama, you're looking quite adequate. Chug, chug, chug.
|
| - That's just fine, son, chug chug chug means that you won't be
| spending any of your days in traffic court.
|
| - Or any of my nights at a drive-in movie.
|
| - Willis, you don't want to date a girl who only likes you for
| your car.
|
| - Sure I do.
| Animats wrote:
| It's quite possible to have wideband AM radio. Some radio
| stations did it in the US before the FCC standardized bandwidth
| and started checking envelopes. Radio Caroline, the UK offshore
| pirate station (1964-1968), was wideband AM.
|
| Noise on AM can to some extent be overcome with power and a low
| modulation percentage. That's how analog broadcast TV worked.
| (Broadcast TV was AM video, FM audio.) The black level for the
| video signal was well above zero. A high black level allowed
| showing black areas without excessive noise. About 80% of the RF
| power went into the carrier because of that. Simple, but
| inefficient. The same trick can be done with AM audio radio,
| although it seems that's not done much.
| BoxOfRain wrote:
| Radio Caroline would be such a good HN topic in its own right.
| Peter Chicago's name in particular should be up there in hacker
| lore for some of the things he did to keep Caroline on the air.
| fanf2 wrote:
| Some more about FM at http://www.theradiohistorian.org/fm/fm.html
| -- https://news.ycombinator.com/item?id=41471355
|
| << FM signals were much more immune to interference than AM due
| to its "capture effect" - an interfering signal needed to be more
| than 50% the strength of the desired signal to cause audible
| interference, compared to 5% or less with AM. This characteristic
| would considerably reduce the required separation between
| stations occupying the same channel and allow more channel re-
| use, which compensated for its greater occupied bandwidth. And
| most importantly, because all natural and man-made static is
| amplitude modulated, FM proved to be amazingly noise-free.
| Armstrong improved its resistance to noise still further by
| incorporating a new receiver component - a limiter that stripped
| off the amplitude variations in the received signal before it
| reached the detector. He had finally solved the problem of static
| interference that had confounded radio experts since the
| beginnings of the art. >>
| asdefghyk wrote:
| because the FM system provides a wider audio bandwidth signal
| than the audio signal bandwidth provided by AM
| Johnythree wrote:
| It doesn't. In both it depends on what the stations engineer
| chooses to transmit.
| brudgers wrote:
| And because FM broadcast radio caps audio at 15KHz, a CD "sounds
| better" than FM...yes, back in the day am SM57 was often good
| enough.
| tzs wrote:
| ...if you are young enough. By somewhere around 25 to 30 most
| people won't be able to hear above 15 KHz.
|
| I'm a little surprised I've not seen audio equipment
| specifically for older people that just covers what they can
| hear.
| thadk wrote:
| This short piece reminds me of a thread about the The Hedgehog
| and the Fox essay (1953)
| https://twitter.com/strangeattracto/status/13506001425970544...
|
| > The idea of code switching between multiple traditions doesn't
| seem to occur to a person who is fixated on The One True
| Aesthetic.
| kazinator wrote:
| FM sounds better than AM partly because frequency is more durable
| than amplitude, but it's not the whole story.
|
| Frequency does not diminish with the inverse square law, as does
| the amplitude of a wave that is broadcast in all directions. This
| is because frequency is related to a count of events over time.
|
| Frequency from a source light years away is intact; we can look
| at frequency bands from a radiating celestial body and know which
| chemical elements there are, and also tell exactly how fast it is
| moving away from us from the red shift in that spectral pattern.
|
| Be all that as it may, AM should sound great when you are close
| to the radio tower, and have ideal reception with no multi-path
| reflections, and good signal/noise ratio.
|
| It still doesn't sound good, and that simply because of the
| bandwidth allocated to it is low. Furthermore, AM Stereo is a
| retrofit and crams two channels into one via phase modulation.
|
| AM stations are separated only by 10 kHz, as you can see on your
| AM tuner (which you likely have only in your car, if that). The
| bandwidth is directly related to the audio bandwidth because
| modulation produces side bands.
|
| For instance, if we modulate the amplitude of a 650 kHz carrier
| with a 1 kHz audio tone, we get side bands of 651 kHz and 649
| kHz. You see where this is going? We can only go up to 5 kHz
| before we bump into the next station, which also needs +/- 5 kHz
| for _its_ side bands.
|
| This 5 kHz limitation is why AM radio sounds like your speakers
| have a heavy woolen blanket over them. It's almost as bad as the
| bandwidth limitation as narrow band phone calls. Listening to AM
| music is almost as bad as listening to on-hold music over a
| narrow band codec like G.711.
|
| The kicker is that only one side band is needed to reconstruct
| the signal, so in theory AM stations could have 10 kHz bandwith.
| Unfortunately, SSB was not deployed for broadcast AM, even though
| it was already known at the dawn of radio.
|
| (https://en.wikipedia.org/wiki/Single-sideband_modulation has a
| note about why)
| brcmthrowaway wrote:
| Can we use this fact to enable faster than light communication?
| cj wrote:
| Not if measuring by relative speed.
| schoen wrote:
| Even though the frequency survives the long trip, any changes
| in that frequency are observed only after a delay
| corresponding to the speed of light.
|
| Someone once pointed out that shadows (which aren't objects
| with a mass and position) can move fast than light, at a
| sufficiently large distance from their origin. That is, the
| location of the border between the shadowed and unshadowed
| region can be changing faster than light speed. But that fact
| can't be used to communicate faster than light, because the
| changes in the location of the shadow's edge still take a
| comparatively enormous amount of time to propagate from their
| source to their destination. If you're creating the shadow,
| you can know that one galaxy will observe the shadow long
| before another galaxy does, but you can't use that knowledge
| to signal something to one galaxy or the other without
| waiting for the light (or lack of light) to travel all the
| way to that galaxy.
| YZF wrote:
| I think we had some fairly recent discussion on HN since I
| remember commenting.
|
| As you're saying, it's about bandwidth and signal to noise. Not
| something inherent to modulation.
| kazinator wrote:
| The modulation is important. FM is more robust against
| external noise than AM.
| Johnythree wrote:
| Only while signals are strong. On weak signals however, AM
| has a considerable benefit in intelligibility over FM.
| kazinator wrote:
| FM stays good as the signal weakens, and then kind of
| drops off a cliff almost.
| wkjagt wrote:
| It would also be harder to tune into a station that is SSB
| because there's no carrier to detect. If you're slightly too
| high or low, the audio will have a slightly higher or lower
| pitch. I'm just guessing but with modern radios that wouldn't
| be a problem, but when AM was still used a lot I think (analog)
| oscillators tended to drift a bit, and you would have to adjust
| your radio often to correct for the changing pitch.
| Johnythree wrote:
| This is a myth. There is no reason that channel spacing need
| limit the modulation bandwidth. The only downside is that
| listeners to adjacent stations will hear a slight "monkey
| chatter" from the overlapping sidebands. In reality stations
| are never allocated adjacent frequencies within the same
| coverage area so this usually doesn't happen.
| kazinator wrote:
| Be that as it may, AM radio is obviously low-pass filtered.
| It might not be a brick wall at 5 kHz, but it sounds
| obviously muffled to someone who can't hear anywhere near up
| to 20 kHz. If I were to guess, based on years of experience
| of playing with EQs, I would say that it has next to no
| content beyond somewhere around 8 kHz.
| raverbashing wrote:
| There's actually one important factor that's missing:
|
| AM radio is limited in bandwidth. The audio is cutting off around
| 10kHz or such (that's why it kinda sounds like a telephone)
|
| > To allow room for more stations on the mediumwave broadcast
| band in the United States, in June 1989 the FCC adopted a
| National Radio Systems Committee (NRSC) standard that limited
| maximum transmitted audio bandwidth to 10.2 kHz, limiting
| occupied bandwidth to 20.4 kHz
|
| (from Wikipedia)
| usr1106 wrote:
| That's because AM has a bearer around 800 kHz, while FM a
| bearer around 100 MHz. I guess if doing AM at 100 MHz it
| wouldn't be a problem granting wider channels, too. But the
| problem that amplitude is more sensitive to noise wouldn't go
| away.
| drmpeg wrote:
| Here's some SDR generated AM with 15 kHz audio bandwidth. Also
| shows why SSB isn't used for music broadcast.
|
| https://www.w6rz.net/am.mp4
| Aloha wrote:
| Needed an emphasis curve applied to it.
| rob74 wrote:
| TL/DR: because it's _frequency modulated_ and not _amplitude
| modulated_ , which makes it less susceptible to perturbation.
| twwwt wrote:
| Define "sound better", please. As we all know this is something
| subjective - mostly everyone has individual preferences. I would
| have found it better if the title was "Why is frequency stable in
| spacetime and amplitude not (which can be verified so easily by
| listening to audio radio)".
| analog31 wrote:
| The most familiar definition, though not spelled out in the
| article, is audio _fidelity_ which is the degree to which the
| output reproduces the input. It 's fair to take this definition
| as a default, or implied by the article. In this specific case,
| frequency response and signal-to-noise are both fidelity
| measures.
|
| Also, some of the comments did a better job than the article of
| explaining things.
| pkolaczk wrote:
| I don't buy this explanation. The FM modulation uses a much
| higher bandwidth than AM. The distance between channels on FM
| radio is 200 kHz compared to only 9 kHz on AM. That's more than
| 20x more bandwidth for FM. On AM, no matter how deeply you
| modulate the carrier, the bandwidth will not exceed twice the
| bandwidth of the input signal. On FM, the deeper you modulate it,
| the wider the output spectrum will be, and it can easily exceed
| the bandwidth of the input signal.
|
| In addition to that, the whole FM band is much higher frequency,
| while I guess quite a lot of noise, especially burst noise caused
| by eg thunderstorms is relatively low frequency. So it's not
| picked up because it's out of band.
|
| Any noise that falls inside the channel does get picked up by the
| receiver regardless of modulation. However because the available
| bandwidth is so much higher than the real bandwidth of the useful
| signal, there is actually way more information redundancy in FM
| encoding, so this allows to remove random noise as it will likely
| cancel out.
|
| If I encoded the same signal onto 20 separate AM channels and
| then averaged the output from all of them (or better - use median
| filter) that would cancel most of random noise just as well.
|
| Also another thing with modulation might be that if there is any
| narrow-band non-white noise happening to fall inside the channel
| (eg a distant sender on colliding frequency), on AM it will be
| translated as-is to the audible band and you'll hear it as a
| single tone. On FM demodulation it will be spread across the
| whole output signal spectrum, so it will be perceived quieter and
| nicer by human ear, even if its total energy is the same. That's
| why AM does those funny sounds when tuning, but FM does not.
| arghwhat wrote:
| The wider channels is the source of the available audio
| fidelity, but wider channels make you _more_ exposed to noise,
| not less. A wider channel means listening to more noise
| sources, and having transmitter power stretched thinner for a
| much lower SNR.
|
| In other words, the noise rejection of FM is what enabled the
| use of wider channels and therefore better audio quality. An
| analog answer before digital error correction.
|
| In FM, the rejection is so strong that if you have two
| overlapping transmissions, you will only hear the stronger one
| assuming it is notably stronger. This in turn is why air
| traffic still use AM where you can hear both overlapping
| transmissions at once (possibly garbled if carrier wave was
| off), and react accordingly rather than being unaware that it
| happened.
|
| Technology moved on from both plain AM and plain FM a long time
| ago, and modern "digital" modulation schemes have different
| approach to interference rejection.
| kees99 wrote:
| > (...) use AM where you can hear both overlapping
| transmissions at once
|
| Yes. Assuming signal strengths for both are comparable. Say,
| within 20 dB of each other.
|
| > (possibly garbled if carrier wave was off)
|
| Nah. If both stations have sufficient energy fall into
| receiver's bandwidth window (IF filter for analog receiver),
| no garbling. If one of stations has carrier sufficiently off
| to fall entirely outside IF, only other will be audible.
|
| You are probably thinking about SSB, where two stations with
| carrier offset indeed produce weird sounding interference.
|
| https://en.wikipedia.org/wiki/Single-sideband_modulation
| tomfanning wrote:
| In SSB there is no carrier transmitted. Two SSB stations on
| top of each other sounds exactly like two microphones
| mixed.
| pkolaczk wrote:
| Interesting. However, if one of those stations runs on a
| slightly different frequency, I guess its output would be
| garbled, correct? Like I guess SSB receiver just shifts
| down the band by a constant?
| vel0city wrote:
| If it's kind of close it just sounds like someone talking
| in a slightly lower or higher pitch. It can still be
| pretty intelligible with the frequency slightly off.
| Eventually it gets very distorted though and you start
| losing a big part of the waveform entirely. Try listening
| in to some websdrs and you'll see what it's like.
| pkolaczk wrote:
| Shannon theorem disagrees with you. The wider the channel,
| the MORE noise you can tolerate when transmitting signal at a
| given data rate.
|
| In audio, the amount of information you need to transmit is
| naturally limited by the audio bandwidth (for FM truncated at
| about 15 kHz), so the useful signal bandwidth is fixed.
| Hence, if you transmit the same audio band over a broader
| channel of frequencies, you can tolerate more noise; or, for
| the same density of noise in the channel, you can get better
| SNR at the output. This is exactly what FM does. It uses the
| information multiplied in the most of that 200 kHz channel
| and projects it on 0-15 kHz band.
|
| While you are right that a wider channel captures more noise
| in total, noise does not add up the same way as useful
| signal, because it's random. Doubling the channel width only
| increases the amplitude of noise by sqrt(2).
|
| There is no "magic noise rejection" coming from different
| ways of modulating the signal if all other things are the
| same. You can't remove noise; you can't magically increase
| SNR. If anything, FM makes the noise more pleasant to listen
| to and perceivably quieter by spreading non random, irregular
| noise over the whole band so it sounds more like white noise.
|
| But it also allows to use wider channels, and increase the
| fidelity of the signal, including increasing SNR. But that's
| thanks to using significantly wider channels than audio.
|
| Also, it's not like FM can use wider channels because of
| better SNR. FM can use wider channels because of how this
| modulation works - the spectrum of FM signal can be
| arbitrarily wide, depending on the depth of modulation. AM
| cannot do that. It only shifts the audio band up (and mirrors
| on both sides of the carrier). It can't "spread it".
|
| Btw: this is a very similar phenomenon as when you average
| multiple shots of the same thing in photography, eg when
| photographing at night. By adding more frames (or using very
| long exposures) you obviously capture more total noise, but
| the amount of useful signal grows much faster because signal
| is correlated in time, but noise is not.
| CHY872 wrote:
| It's not immediately clear that Shannon's theorem is a good
| point of comparison here, since it's only recently that
| coding schemes have really approached the Shannon limits,
| and FM and AM do not use these.
|
| Even if one does assume a Shannon-perfect coding scheme, as
| the noise ratio gets greater the benefits of spreading a
| signal across a higher bandwidth fades. Furthermore, most
| coding schemes hit their maximum inefficiency as the signal
| to noise ratio decreases and messages start to be too
| garbled to be well decoded.
|
| I'd additionally note that folks get near the Shannon noise
| limit _through_ 'magic noise rejection' (aka turbo and ldpc
| codes). It's therefore not obvious that FM isn't gaining
| clarity due to a noise rejection mechanic. The 'capture
| effect' is well described as an interference reducing
| mechanism.
|
| Empirically, radio manufacturers who do produce
| sophisticated long range radio usually advertise a longer
| range when spreading available power across a narrower
| rather than wider bandwidth.
| some_ee_here wrote:
| You are applying Shannon theorem incorrectly. Both AM and
| FM modulations are nowhere even remotely close to using
| their bandwidth with 100% efficiency, due to technology
| costs, and the difference in modulation is crucial. The
| article is correct and the mathematical models of AM and FM
| are well understood since decades.
| pkolaczk wrote:
| Where did I say AM and FM are close to 100% efficiency?
|
| I was only replying to an obviously incorrect statement
| that by using more bandwidth you decrease SNR. If it were
| the case, Shannon theorem would not work.
|
| It doesn't matter how close to the limit your encoding
| is, whether it is 20% or 99% the relationship between the
| bandwidth, noise floor and how much data you can send
| stays the same - by increasing bandwidth you can usually
| send considerably more information even if your encoding
| is poor. Which in translates to either a wider useful
| bandwidth or lower noise floor or any combination of
| both.
|
| A trivial thought experiment to illustrate this: For any
| analog encoding, if I double the transmission bandwidth
| by encoding the same signal over 2 channels instead of
| one, I can average the output signal coming out the
| receivers and get better SNR than using one channel and
| one receiver. That works regardless of AM, FM or whatever
| fancy encoding you could use.
| bobmcnamara wrote:
| > A trivial thought experiment..
|
| That's not how this works. That's not how any of this
| works. Averaging a high SNR channel with a low SNR
| channel is likely to produce something less good than the
| high SNR channel. Could you get an improvement over the
| high SNR channel? Yes, and the limit of the improvement
| is related to the SNR of each and averaging the signals
| won't get you anywhere near that.
| pkolaczk wrote:
| Averaging two noisy signals increases SNR. That's not
| even a thought experiment, that's a reality. This is a
| technique used by probably all modern smartphone cameras
| to do night photos, as well as a common technique used by
| astrophotographers. Instead of taking one picture, you
| take a series of pictures and then align them and
| average. This improves SNR dramatically. A very long time
| ago we used this technique to get razor sharp, low noise
| pictures of the Moon at 3k x 3k resolution using... a
| cheap VGA internet camera:
| https://astronet.pl/wydarzenia/n2309/ Note that cameras
| at those times were barely capable of doing
| videoconferencing in artificial evening light - what you
| saw was mostly noise. Those sensors were really, really
| terrible.
|
| What you seem to be missing is the fact we're talking
| here about transferring the same fixed bandwidth signal
| over a wider channel, not transferring a wider bandwidth
| signal over a wider channel.
|
| // edit: just noticed someone else gave another nice
| application of this phenomenon: GPS
| bobmcnamara wrote:
| No, that's one time varying signal.
| bobmcnamara wrote:
| Let's take it to the limit:
|
| Signal0: infinite SNR. Signal1: anything less.
|
| I just don't see how the output of averaging these would
| improve over Signal0. I don't think it can.
| xvedejas wrote:
| They're thinking about when you sample from the same
| noise distribution, averaging gives an unbiased estimator
| of the mean. But when you know one SNR is higher than the
| other, maybe this doesn't hold? But maybe if you
| transform the distributions to look the same, thus taking
| a weighted average? I'm not sure.
| bobmcnamara wrote:
| > This is a technique used by probably all modern
| smartphone cameras to do night photos, as well as a
| common technique used by astrophotographers...
|
| I think this is a lot simpler because each of your pixels
| is assumed to have a single, correct DC value. This
| doesn't hold for a time varying signal like AM/FM.
| arghwhat wrote:
| Well, yes and no. It's a bit more complex than just taking
| the numbers from Shannon-Hartley, but I admit that my
| original description was at best lacking, so thank you for
| pointing that out.
|
| Shannon-Hartley describes that the theoretical information
| capacity of a signal given a bandwidth and an SNR.
| _Doubling bandwidth halves your SNR_ (received noise
| increases, received signal does not), in turn reducing the
| bits gained per unit of bandwidth. At very high SNR,
| doubling bandwidth almost doubles capacity, but as SNR goes
| down, the benefit of additional bandwidth levels off until
| bandwidth no longer has any effect.
|
| However, this provides the number achievable by a perfect
| modulation scheme using all available bandwidth and signal
| strength. AM and FM are both incredibly inefficient, and
| more importantly have very different reactions to noise -
| something Shannon-Hartley does not concern itself with.
|
| With truly random noise, FM and AM noise both scale based
| on noise amplitude as you say. In AM, all noise overlapping
| with the band is played back verbatim, whereas in FM only
| the noise causing frequency variations in the carrier wave
| have any effect on the signal, and end up with a non-linear
| response to noise.
|
| However, we do not deal with purely white noise, and FM has
| far superior handling of non-random noise. In order to have
| any effect, it need to either induce frequency shifts to
| the carrier wave, or have enough power to cause the
| interference to be captured instead. There's also the far
| higher power efficiency, as FM puts all its power into the
| signal, whereas traditional AM puts most of it into a
| useless carrier and wastes half the remaining power on the
| redundant sideband (yes, SSB is a thing). These were
| certainly also factors in FMs demise.
|
| A simpler means to remove bandwidth from the equation would
| be to compare with a narrow-band FM transmission, or by
| multiplying the input waveform for an AM transmitter by
| some factor to fill the bandwidth. I believe FM should
| still handily beat it at least above its threshold. I don't
| see anyone giving exact numbers of this though, so I guess
| it could be a fun SDR project for someone wanting to prove
| either point. :)
|
| (Neither AM nor FM is of anything but historic value at
| this point - their only redeeming quality is discrete
| circuit simplicity if you need to MacGyver one out of
| shoelace and bubblegum, but that's it.)
| zb wrote:
| > This in turn is why air traffic still use AM where you can
| hear both overlapping transmissions at once (possibly garbled
| if carrier wave was off), and react accordingly rather than
| being unaware that it happened.
|
| I'm not convinced this is the reason. The carrier wave is
| _always_ off by a little. While you're transmitting you hear
| nothing anyway. And when two parties are transmitting
| simultaneously, any third parties just hear very loud
| screeching. A 0.001% difference in carrier frequency would be
| more than enough to cause this effect in a VHF radio.
| Notably, this exact problem was a major contributing cause to
| the worst accident in aviation history. Using FM would have
| prevented it.
|
| https://archive.ph/2013.02.01-162840/http://www.salon.com/20.
| ..
| p_l wrote:
| AM is used for two reasons - simplicity of transceivers
|
| AND the fact that two simultaneous transmissions result in
| buzz instead of locking onto stronger signal. We _WANT_ to
| know that there 's a collision in transmission so that we
| know we need to retransmit. What would be the expected
| effect if two FM transmission on same channel were sent?
|
| Fixing the "glitch" would result in way more problems than
| it solves. Interestingly, aviation authorities do not blame
| collission behaviour of AM radio for Tenerife, but instead
| corrected crew management procedures and pushed greater
| radio phraseology standardisation.
| BigTuna wrote:
| >We WANT to know that there's a collision in transmission
| so that we know we need to retransmit
|
| Digital trunked public safety systems solved this problem
| decades ago. If you key up when the frequency is in use
| you get a distinct rejected tone. I'd think prevention is
| far preferable to sorting it out once everyone's finished
| walking on each other.
| p_l wrote:
| It also means you need to replace _everyones_ radio at
| the same time because everyone needs to hear everyone on
| the channel.
|
| Where new _additional_ technologies are possible, they
| have been applied (digital packet networks, like with
| CPLDC - Controller-Pilot Data Link Communications).
|
| Replacing A3E modulated VHF radio requires you replace it
| for literally everyone, because there are way more users
| at airport than you think.
| mindcrime wrote:
| > It also means you need to replace everyones radio at
| the same time because everyone needs to hear everyone on
| the channel.
|
| In the public safety context it's not uncommon to phase
| in new systems (like digital trunked systems)
| incrementally. You accomplish that by simulcasting the
| dispatch audio over both systems, and monitoring incoming
| audio from both systems.
|
| A common pattern for how this plays out would be
| something like this: all the fire departments and ems
| agencies in a given jurisdiction are dispatched using
| two-tone (eg, motorola) paging over a VHF frequency. New
| digital radios are introduced, and all the fire/ems
| personnel keep their existing pagers, and (some|most|all)
| are given the new digital radios. People without the new
| radios can still talk to dispatch using VHF. And of
| course systems can be configured to mirror audio around
| so that if one person is transmitting on VHF they can be
| heard on the digital system (usually on a channel in the
| 800mhz or 900mhz band). It's basically a fancy version of
| a repeater.
|
| Dispatches are then given out over the same old VHF
| channel AND the new digital channel. In theory you can
| eventually replace all the old pagers and radios and quit
| with the simulcast deal, but IME, sometimes things stay
| in "parallel" mode more or less indefinitely for whatever
| reason[1]. That said, to your original point, you
| typically do want to get at least _radios_ standardized
| as much as possible, even if you maintain the split for
| (paging|operational communications).
|
| To illustrate, two jurisdictions I'm familiar with:
| Orange County NC, and Brunswick County, NC. Both followed
| the path I talked about above: all VHF dispatch for
| fire/ems, then adopted the NC VIPER digital trunking
| system, but continue to page on VHF and simulcast the
| dispatch information over both channels. I'm not sure
| exactly when Orange County adopted VIPER but it's been
| quite some time and they're still doing both. FSM only
| knows if/when they'll ever completely abandon the old VHF
| system.
|
| [1]: and that reason is often as simple as "money".
| Plenty of volunteer fire departments in rural areas are
| skating by with barely enough money to keep their
| apparatus road-worthy. Replacing every hand-held and
| mobile radio they own in one fell swoop is often out of
| reach.
|
| [Source: was a firefighter and 911 dispatcher in a
| previous life]
| p_l wrote:
| You're perfectly correct _except_ one small thing.
|
| You're writing about experience in a _closed system_ - as
| far as I know all such dispatch systems for public safety
| etc are closed system where everyone who is ever going to
| be on the net is part of the system, and it might at most
| be a case of "we don't have money to replace every
| member's radio".
|
| In comparison, aviation radio is an _open system_ - not
| only you do not know who is going to communicate, the
| communication is also peer to peer, unlike many digital
| trunked systems which often depend at least on some level
| of cellular support system.
|
| The only "access control" on the airband VHF and HF comms
| is of legal variety, with explicit carve out that the
| person actually flying the aircraft is way less bound by
| legalities in case of emergencies, and everyone has to be
| able to talk with everyone, especially on one of the
| standard common channels.
|
| Examples from personal experience involved various
| combinations of small airfield ATZ, MiG-29, gliders, old
| ursus tractor (agricultural kind), busted up Opel Kadett,
| airliners, ultralights, small transport planes, private
| helicopters, and dunno who was responsible party but
| helicopter working as diplomatic flight.
|
| All on one small airfield. And every one of those had to
| communicate independent of each other with everyone else
| on that list.
|
| The only time we do "rebroadcast" is when we end up
| having to do a manual relay due to distance, which is
| also one of the rare cases where comms might switch over
| to a more modern system, because someone could ask ATC
| over VHF to pass something over CPDLC to airliner or
| using HF, and vice versa.
|
| The poor A3E modulation on VHF airband is the lingua
| franca, the lowest common denominator, which allows
| random aircraft from anywhere in the world talk to
| another random aircraft, as well as ground.
| arghwhat wrote:
| > AM is used for two reasons - simplicity of transceivers
|
| That is not a factor anymore. Capable wideband
| transcievers like the ones in Baofengs and similar
| supporting multiple types of modulation cost cents.
| p_l wrote:
| There's cost in simultaneous replacement for huge portion
| of the fleet.
|
| Don't devolve into simplism, consider that you need to
| replace the radio for everyone sharing the same space,
| and that there might be way more planes sharing that
| space than you think.
| nradov wrote:
| And who would pay for the Supplementary Type Certificate
| for every single aircraft model out there, including many
| that were built by manufacturers that no longer exist? I
| don't think you understand how this stuff actually works.
| rlpb wrote:
| That article makes out as if transmission blocking leads to
| a safety problem if a transmission gets lost. It doesn't.
| What that article misses is that aviation radio
| communications _require_ readback and verification of the
| readback, in safety critical instructions such as "cleared
| for take-off". Not just for radio transmission blocking
| reasons, but also to detect mistakes in mishearing
| instructions.
|
| https://en.wikipedia.org/wiki/Tenerife_airport_disaster#Com
| m... tells a more accurate story: the root cause was that
| the captain assumed they were cleared for take-off without
| actually hearing their own callsign and the word "cleared".
|
| Since then, the word "take-off" is avoided in any other
| type of communication (eg. you might hear "report ready for
| departure" but never "report ready for take-off"), and
| every pilot knows never to assume that a clearance has been
| given unless they hear those exact words together with
| their callsign.
| zsellera wrote:
| What you "more bandwidth more noise" people miss is the
| difference in randomness: the noise is random while the
| signal is not.
|
| In case of gaussian noise, double the bandwidth means 1.41x
| more noise. For signal, double the bandwidth double the
| signal.
| kabouseng wrote:
| Noise is not gaussian.
| analogwzrd wrote:
| Where are you getting 1.41x? What you'd really like to
| increase is the SNR. As you open up the bandwidth, the
| amount of energy you can collect in your band increases,
| but there's no way to collect the energy from only the
| signal and not collect the energy from noise. So as you
| increase your bandwidth, your SNR stays the same.
|
| Not _all_ noise is gaussian. And the fact that the noise is
| random while the signal is not, is useful when you can
| average and drop your noise floor. But you need multiple
| measurements to do that.
| arghwhat wrote:
| 1.41x is sqrt(2), which suggests that they meant noise
| amplitude rather than noise power.
| pkolaczk wrote:
| Noise power increases twice but signal power increases
| 4x. Noise amplitude increases sqrt(2) times, signal
| amplitude increases 2x.
| CamperBob2 wrote:
| _The wider channels is the source of the available audio
| fidelity, but wider channels make you more exposed to noise,
| not less._
|
| From a signal:noise perspective, what matters is the ratio of
| bandwidth available in the transmission channel to the
| bandwidth of the content you are trying to send. Consider
| GPS, for instance, where the use of a 2 MHz channel to send
| 50 bps data provides an SNR advantage that would otherwise be
| achievable only through witchcraft.
|
| FM has strong noise immunity advantages -- notably AM
| rejection and the capture effect -- but they don't provide
| additional sound quality by themselves. That's where the
| bandwidth helps. An FM channel that is only as wide as an AM
| channel would sound pretty awful.
| arghwhat wrote:
| > FM has strong noise immunity advantages -- notably AM
| rejection and the capture effect -- but they don't provide
| additional sound quality by themselves. That's where the
| bandwidth helps. An FM channel that is only as wide as an
| AM channel would sound pretty awful.
|
| Comparing such low-modulation factor FM with traditional AM
| would be an interesting experiment.
|
| It certainly wouldn't sound good, but I'm not sure it would
| sound worse than traditional AM at the same SNR. The NFM
| use-cases I'm familiar with tend to cap audio bandwidth, so
| they're not fair comparisons.
| CamperBob2 wrote:
| I'm mostly imagining what music would sound like via NBFM
| on a VHF amateur or public-safety radio channel. It's not
| an appealing thought... the words "toll quality" come to
| mind.
|
| But hey, no static at all...
| https://www.youtube.com/watch?v=HV3zWSawJiw
| akira2501 wrote:
| FM has 15kHz of bandwidth per stereo channel or an effective
| 30kHz sampling rate. The rest of the space is used for
| supplemental signals, including, the "pilot carrier" that is
| used to generate the "stereo image." There is space for three
| more full bandwidth mono channels on the end of an FM
| broadcast. One of them is often used for RBDS.
|
| FM signals receive AM interference but heterodynes exclude them
| effectively. The cost is vulnerability to multipath reception
| in highly signal reflective environments and capture/wandering
| effects when two signals of similar strength are present.
|
| AM _can_ sound pretty good. Most AM transmitter sites are
| poorly maintained, combined with other stations into one
| antenna system (something you can do on AM with a phasor), and
| are typically just simulcasts of FM content or satellite
| delivered content. There's no real care put into it. On a well
| maintained, tuned, and properly programmed station, mono
| content on AM sounds quite pleasant.
|
| That's not even getting into "cost saving" measures that AM
| operators employ that completely compromise their signals. Or
| what Nielsen has convinced them to inject into their signals to
| register modern "ratings points" from the "portable people
| meter" system.
|
| Guess where I used to work.
| adrian_b wrote:
| FM has 15 kHz of bandwidth available for the audio signal,
| which is much higher than what had been previously
| standardized for the AM channels and which is an important
| reason for the perceived high fidelity.
|
| The modulated signal that is transmitted on the air has a
| much higher bandwidth. How much higher may differ between
| various broadcasting standards, but it can be e.g. 10 times
| or 20 times higher.
|
| The ratio between the bandwidth of the transmitted radio
| signal and the bandwidth of the audio signal is what is
| relevant for the noise rejection properties of FM
| broadcasting.
|
| When the bandwidth available for transmission is limited, FM
| is not an optimal kind of modulation from the point of view
| of resistance to noise, phase modulation (QPSK) is better
| (and optimum), so that is what is used for digital
| communications limited by noise.
| akira2501 wrote:
| > The ratio between the bandwidth of the transmitted radio
| signal and the bandwidth of the audio signal is what is
| relevant for the noise rejection properties of FM
| broadcasting.
|
| Yes. FM radio is "narrow band" which gives it additional
| noise rejection properties; however, it's measured against
| the total available signal content not merely the audio
| portion of the content.
|
| So, your pilot wave and RBDS and any additional carriers,
| if present, reduce this facility.
|
| > FM is not an optimal kind of modulation from the point of
| view of resistance to noise, phase modulation (QPSK) is
| better
|
| FM receivers often move and are often in highly reflective
| environments. FM is far better suited to this than plain
| PM.
| tboerstad wrote:
| Thanks for the interesting info!
|
| My guess would be iHeartMedia
| Anotheroneagain wrote:
| Neither is true. 9kHz, with two sidebands, means that the
| transmitted audio is limited to 4.5kHz, which is way too low to
| sound good. It was this, and not the noise, that made it sound
| much worse.
| adrian_b wrote:
| While one reason for limiting the audio bandwidth to 4.5 kHz
| was to allow a great enough number of channels in the long
| wave and medium wave bands, the second reason was to be able
| to reject the high frequency noise by low-pass filtering.
|
| So there were two reasons for the low audio fidelity of AM
| broadcasting, and noise was one of them, with the contention
| between multiple broadcasters for the narrow available bands
| being the other.
| giantrobot wrote:
| > with the contention between multiple broadcasters for the
| narrow available bands being the other.
|
| A non-obvious aspect of medium and long wave AM broadcast
| is depending on weather/atmospheric conditions a signal can
| propagate much further than its output power would suggest.
| This means a distant station on the same channel as a near
| station may end up in contention at certain times of day or
| random conditions. Solar flare? Suddenly stations a hundred
| miles away are overpowering local stations or just adding a
| lot of noise.
|
| Medium and long wave is also susceptible to local EM
| sources like switching power supplies and electric motors.
| So you can get the double whammy of local noice and distant
| stations adding additional interference to local stations.
| Johnythree wrote:
| There is no reason that the channel spacing need limit the
| sideband bandwidth.
|
| The only downside to this is that listeners on adjacent
| stations hear a slight "monkey chatter" from the overlapping
| sidebands.
|
| This is one of many reasons why station frequencies are never
| allocated close to stations which are physically close.
|
| You only need glance at the waterfall display on a good SDR
| receiver to see that the actual audio bandwidth is often much
| wider than the channel spacing implies.
| fredgrott wrote:
| for fun, try using a square wave amplifier to shift the wave:
|
| -for AM you get sound effects such as chip monks
|
| -for FM what do you get?
| xd1936 wrote:
| Going back to first principles, modulating the frequency
| instead of the amplitude inherently makes the system less
| lossy. Imagine you were communicating with someone miles away
| on a hilltop, and they had a lot of data to convey. Would you
| find it easier to distinguish signal vs. noise if the light was
| increasing and decreasing rapidly in brightness (AM) or color
| (FM)?
| pkolaczk wrote:
| If you AM modulate the carrier f0 by a single tone f1, you
| get a spectrum with only three tones: f0 - f1, f0, f0 + f1.
| If you FM modulate it, well the thing gets much more complex
| - depending on the depth of modulation you can get a much
| wider spectrum than from f0-f1 to f0+f1. So it is hard to
| compare. Your FM modulated signal may indeed be more
| resilient to noise but will require wider channel to be
| properly transmitted.
|
| I haven't seen a convincing explanation if FM would be really
| that better than AM if both were given exactly the same
| channel width.
| fguerraz wrote:
| > noise tends to be a an unwanted amplitude modulation, not a
| frequency modulation
|
| said someone who didn't understand anything about signal
| processing.
|
| Been debunked so many times:
| https://physics.stackexchange.com/questions/94198/why-does-n...
| tsurba wrote:
| The article kinda sucks as it does not really answer the question
| it poses. Why "noise tends to be a an unwanted amplitude
| modulation, not a frequency modulation"?
|
| Is it due to naturally occurring background noise being low
| frequency high amplitude, showing up as AM? Could the situation
| change if humans keep generating more high-frequency noise? Or is
| it just that high frequencies do not travel as far so there will
| always be relatively little?
| wruza wrote:
| Because noise is in line with AM and perpendicular to FM? Let's
| read if that's still so.
|
| _Armstrong reasoned that the effect of random noise is primarily
| to amplitude-modulate the carrier without consistently producing
| frequency derivations._
|
| ...It doesn't talk much about the noise physics, but basically
| yes.
| getnormality wrote:
| Deeper explanations for those who aren't satisfied:
|
| https://physics.stackexchange.com/questions/94198/why-does-n...
|
| https://ham.stackexchange.com/questions/6312/why-are-fm-radi...
| myflash13 wrote:
| As a non radio engineer, reading this thread on HN is so
| fascinating because there are so many heated conflicting
| explanations for a common phenomenon in a well-established
| technology. I thought this simple question would be settled
| already. If such disagreement is possible even in an established
| "hard" science like this, then no wonder some people think
| everything is subjective.
| _fizz_buzz_ wrote:
| This question is of course settled. However, it has more than
| one aspect and I am pretty sure there are also a lot of rather
| amateur people chiming in here ...
|
| I am an EE, but in power electronics and not an RF engineer so
| I am a little bit hesitant to comment too much on it, but my
| understanding is that it mostly breaks down to two aspects:
|
| 1) noise interferes more with amplitude 2) the fidelity of the
| modulating signal in FM is higher (more bandwidth)
| wellbehaved wrote:
| HN is not a meritocracy, anyone with an opinion can just chime
| in and then get upvoted by people who don't really understand
| what's going on.
| josefritzishere wrote:
| Back in 2005-ish a Clearchannel enginer, Littlejohn proposed
| narrowing bandwidth of Am stations to improve the signal to noise
| ratio. They implemented a 5 Khz bandwidth. Back in the 70s we
| were all blasting 12 kHz. While that's probably OK-ish for talk,
| it's dreadful for music. https://www.radioworld.com/columns-and-
| views/the-5-khz-am-re...
| Aloha wrote:
| 12 kHz AM on a good receiver sounds absolutely fantastic.
|
| I have a Royal 51/810 (one of each) that I use as a
| travel/bathroom radio, ironically, both have fantastic AM
| performance, and.. lacking FM - the IF/Audio bandwidth appears
| twice as wide on AM, and FM just sounds like crunchy - probably
| needs caps in the audio section, but its so tightly packed, and
| has a PCB with the heaviest plating I've ever seen - which
| means it needs work I cannot easily do.
| josefritzishere wrote:
| Is it packed too tight for a solder sucker to get in there?
| ginko wrote:
| Kinda funny to see people argue AM vs FM when much of the world
| has already switched to DAB.
| spease wrote:
| I'm confused how this is even a question.
|
| With AM, anything that causes a variation in the intensity of the
| signal will introduce noise.
|
| With FM, anything that causes a variation in the timing of the
| signal will introduce noise.
|
| Unless you're traveling at relativistic speeds, operating a time
| dilation device, or colocated with a black hole, you usually
| aren't going to see the rate that time flows at vary.
|
| Thus if you can make the amplitude of your signal irrelevant past
| a certain threshold and embed all the information into the time
| domain, the only thing introducing interference should be other
| EM sources that happen to be on the same channel.
| taeric wrote:
| I was surprised this wasn't leaned on more explicitly for the
| explanation.
|
| I think this is largely held in the assumptions that go into
| saying most noise will be amplitude modulation?
|
| Edit: Reminds me of the banal but vital insight that digital
| uses repeaters to gain distance, whereas analog uses
| amplifiers. Makes it very easy to consider why/how digital took
| over.
| mumer101 wrote:
| https://www.pbs.org/wgbh/aso/tryit/radio/radiorelayer.html
| _fizz_buzz_ wrote:
| FM usually has higher fidelity than AM even if no noise is
| introduced.
| MBCook wrote:
| Right. Isn't FM just flat out higher bandwidth? So unless
| it's wasting that somehow it's just going to carry more
| information. And for audio, that means sounding better.
| cruffle_duffle wrote:
| The use of FM doesn't inherently imply higher bandwidth.
| For example, those consumer-grade FRS/GMRS radios you get
| from Costco use narrowband FM, which typically occupies
| about 12.5 kHz per channel. This is much narrower compared
| to the 200 kHz bandwidth used by FM broadcast radio.
|
| FM is simply a method of modulating the carrier signal by
| varying its frequency. The actual bandwidth depends on
| factors like frequency deviation and modulation, so FM can
| range from narrow to wide bandwidth depending on the
| application.
| aidenn0 wrote:
| That is true for uncorrelated broad-band noise.
|
| Correlated noise (e.g. multipath interference) and narrow-band
| noise (e.g. another FM transmitter) can both affect FM pretty
| badly.
| polishdude20 wrote:
| Speaking of multipath interference, why is it that we almost
| never hear the effect of that? Like, aren't these waves
| almost constantly bouncing off of other things and being
| reflected? How are we not always hearing echos all the time?
| adrian_b wrote:
| The effect is easily noticeable only for high frequencies,
| where the wavelengths are no bigger than a few meters.
|
| For the lower frequencies used by AM broadcasting, where
| the wavelengths are up to hundreds of meters or kilometers,
| and you use small antennas for reception, it is unlikely to
| have problems caused by multipath propagation (because the
| waves will go around obstacles instead of being reflected;
| only for the higher frequencies of the shortwave range you
| can have multipath reception of signals reflected by the
| ionosphere, but the objects that are around the receiver
| still do not cause problems).
|
| When there is multipath propagation, you would not hear
| echos, because the time difference between the different
| paths is too small, due to the high speed of the radio
| waves. What you get is interference between the multiple
| signals, which can reduce too much the strength of the
| received signal. When the signal is reflected on some paths
| by mobile objects, or when the receiver itself is moving,
| the received combined signal will have an amplitude that
| varies in time, with intervals when the signal cannot be
| received (i.e. fading).
| vel0city wrote:
| The other poster is correct but I feel there's still a
| simpler answer when you see the units at play. You don't
| constantly hear reflections as echos mostly because of the
| speed of light and the inverse square law.
|
| Let's think about how far the echo has to come from to have
| even a half second delay. At the speed of light, that half
| second is 93,141mi. So it would have to reflect off
| something half that distance, ~46,500mi. Not a lot of good
| reflectors pointed at me 46,500mi away.
|
| So then think of the inverse-square law on that. How weak
| of a signal is that going to be travelling those 93,141mi?
| Are you set up to even notice that from the noise? It's 11
| _billion times_ weaker than the original signal, assuming
| your reflector _perfectly_ reflects the source signal and
| you 're in outer space.
|
| So obviously whatever "echo" we experience, it's not going
| to be something in the realm of humans directly detecting
| it. The shift that is possible to really mess with the
| signal at distance you'll actually receive reflections at
| are only going to shift the timing in a very small way,
| usually by being a slightly different phase. This means
| you'll get constructive and destructive interference from
| the same signal at slightly different phases, but not
| really a noticeable "echo".
| aidenn0 wrote:
| Other people answered your question, but if you ever saw
| shadowing artifacts on an analog UHF TV station, that was
| probably multipath. Note that lines on an NTSC TV are
| scanned at over 15kHz, so this is a very small time
| difference.
| nixass wrote:
| > I'm confused how this is even a question.
|
| It sure is, why would everyone or anyone be aware of AM/FM
| differences? Even if one is tech savvy it doesn't mean this
| would be something trivial to understand at glance
| geocrasher wrote:
| Bandwidth.
| ml_comms_eng wrote:
| There are many differences that explain why one can be better.
| All else being equal:
|
| - bandwidth of modulated signal: it is better to spread the
| signal over a large bandwidth => N _log(1+snr) > log(1+N_snr).
| the bandwidth used by the FM signal is larger
|
| - wasted energy on the DC signal: AM signal is A + s(t) where A >
| abs(s(t)) to make sure the sent signal is always positive. A (DC)
| does not carry information so the effective signal to noise ratio
| of a DC-less signal should be higher (phase/frequency modulation,
| signalling that can detect the negative part...)
|
| - filtering of baseband signal => if you filter too much the
| original signal, you lose information even before transmission.
| Voice is usually filtered and 4KHz, but music needs more. FM has
| more margin (more allocated bandwidth) so can have less stringent
| filters
|
| - tolerance to fading: the wireless channel is not AWGN, it is
| frequency dependent due to multipath. While radio signals are
| relatively narrow, signal modulated in frequency are more robust
| to fading (OFDM...)
| aidenn0 wrote:
| One thing this seems to let out is that in FM, the signal is
| being broadcast with constant power. A 1MW FM station is always
| sending 1MW of signal; with AM the signal power varies; if you
| broadcast in AM powerful enough to mask the noise during periods
| of typical signal level, then you would still hear noise during
| quieter than typical periods.
| gpderetta wrote:
| Yet many (if not all?) digital wireless protocols use some form
| QAM.
| jdthedisciple wrote:
| Presumably because there's very little additive white noise
| inside copper wires
| ryanmcbride wrote:
| The way I had this explained to me when I was in highschool was
| something like:
|
| Imagine someone is shining a flashlight at you through some
| trees. It's a lot easier to tell what color it is, than how
| bright it is.
___________________________________________________________________
(page generated 2024-10-14 23:02 UTC)