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Radios in tunnels
February 27, 2005 6:27 PM   Subscribe

Why, when driving through a tunnel, does my FM radio receive noise, and not silence, when it loses the signal?

I can understand static in other places, there's plenty of EM noise around, but shouldn't the tunnel shield any noise as well as the signal I'm trying to receive?
posted by Mwongozi to Science & Nature (17 answers total)
 
It's possible that the tunnel has a built-in antenna system and is trying to pipe your station in to you (but failing). The Caldecott tunnel, which I must take to visit my parents, has great radio and cell phone reception all the way through. Look for wires hanging from the ceiling.

Just a guess, really. Though I'm not sure that "silence" is equivalent with "no signal." A lack of a stable signal might simply leave you with background radio waves of some kind (and there must be lots of those pretty much everywhere in the universe). In order to tune in silence, it's possible you would have to have a strong signal of a flat wave.
posted by scarabic at 6:57 PM on February 27, 2005


If you tune a channel without a radio station, you'd hear static as well. Some radio's have "squelch" that stops the noise--many do not. When you drive into the tunnel, the radio signals can't reach your radio. This is why you get static.
posted by wflanagan at 7:11 PM on February 27, 2005


but shouldn't the tunnel shield any noise as well as the signal I'm trying to receive?

not really- there's the noise from the flourescent or sodium lights that are lighting the tunnel, noise from outside the tunnel, from the electronics in your car, and so on. radio waves travel a lot like light waves do- and if you can see the light at the end of the tunnel, it's likely that some radio broadband noise can get in.

if you put your car in a faraday cage, you would expect to hear the noise significantly attenuated, but again, there would be the internal circuitry of the car generating at least some noise.

also, radios that recieve "silence" on a channel with no signal are squelching the channel- no radio frequency is really silent.

great question!
posted by fake at 7:13 PM on February 27, 2005


A lot of radio noise comes from natural sources both local thermal sources and atmospheric noise which is strong and prevalent enough to get into tunnels where distant radio transmitters may not. There's also local man-made noise (power lines, automotive transmissions, etc) and even noise from our galaxy and universe.
posted by vacapinta at 7:19 PM on February 27, 2005


radio waves travel a lot like light waves do- and if you can see the light at the end of the tunnel, it's likely that some radio broadband noise can get in.

Thats a great way of putting it. Its only "dark" enough that you can't "see" the radio station you're trying to get but rarely is even a tunnel completely dark in this sense.

Even if you just had the radio in a Faraday cage, you'd still hear the noise from the receiver. In radio astronomy, our own telescopes had to be put under deep freeze so as to minimize this.
posted by vacapinta at 7:23 PM on February 27, 2005


It's been a few years since my fields and waves class, but I do believe that the vast majority of man-made static is AM, not FM, so unless your FM radio is really cheap, it should clip the amplitude modulations. It could be a fringe area affect, though.
posted by rfs at 7:32 PM on February 27, 2005


I'm not sure, but I think it has something to do with the fact that it's FM radio, not AM.

FM is frequency-modulated, meaning, roughly, that the actual sound signal isn't broadcast, a highly transformed signal is sent instead.

I don't know the specifics of FM, but it's common in these kinds of transformations for a flat modulated signal to be impossible to translate back into the original format. It's like trying to divide by 0. When a device that picks up the "flat" signal in real life, it really picks up on tiny, random perturbations in the signal, and blows them up to white noise.

'Course, this may not be the case for FM, just for things like Fourier transfoms. Can any EE's speak to this?
posted by maschnitz at 7:36 PM on February 27, 2005


Maschnitz: I'm not sure what you mean when you're talking about dividing by zero and fourier transforms, but FM can be seen as a kind of phase modulation as well (that is, the the phase of the signal is being varied, to a degree).

When you have "complete silence" that's being modulated as FM, I suppose you'd be sending just the carrier wave with no phase or frequency modulation, or, looking from the frequency domain perspective, a huge peak at the center frequency. If you look at a frequency vs. signal intensity plot on a spectrum analyzer, you'd see that there would be a large peak at the center frequency, smaller peaks at the harmonics of the center frequency, and tons of relatively small peaks at various frequencies (noise).

As vacapinta pointed out earlier, nose comes in all forms and even when there's "no signal" because tons of earth are shielding the signal from the radio station, there still would be random EM waves at a given frequency, and that's what you hear as noise.

Phew, enough from me.
posted by scalespace at 8:13 PM on February 27, 2005


It's been a few years since my fields and waves class, but I do believe that the vast majority of man-made static is AM, not FM, so unless your FM radio is really cheap, it should clip the amplitude modulations.

This is a misstatement. Static isn't AM or FM, it just is. It affects AM more than FM if it is additive because of the way amplitude modulation works. If the noise or channel somehow affects the phase of the signal then FM can just as easily be interrupted.

I don't know the specifics of FM, but it's common in these kinds of transformations for a flat modulated signal to be impossible to translate back into the original format. It's like trying to divide by 0. When a device that picks up the "flat" signal in real life, it really picks up on tiny, random perturbations in the signal, and blows them up to white noise.

I have no idea what you're trying to say here, but I think maybe you have the right idea. It's true that you can't perfectly reconstruct an FM signal, but it isn't anything like dividing by zero.

A basic AM transmission (double side-band suppressed carrier) has the formula m(t)*cos(omega_c*t) where m(t) is your message and omega_c is the carrier frequency. If you have a synchronous demodulator (that is, it's cos generator is exactly in sync with the transmitter) then to demodulate you simply multiply by cos(omega_c*t): m(t)*(cos(omega_c*t))^2 = 1/2*m(t) + 1/2*cos(2*omega_c*t), and then throw away the higher frequency component. The problem, of course, is that additive noise is demodulated right along with it.

A basic FM transmission has the formula cos(omega_c*t - alpha(t)) where alpha'(t) = m(t) (or in other words, alpha(t) = integral(m(tau), tau, 0, t)). The effect of this is that the message is carried in the instantaneous frequency of the signal, not the amplitude. So before demodulating FM you just do what is called hard-limiting: you look at the signal and if it is > 0 then you output a 1, and if it is < 0 then you output a -1, and then you take this square wave and lowpass filter it to smooth it out. It may be hard to see on AskMe, but if you draw it out you can easily tell that this operation preserves the phase while normalizing the amplitude. This is why FM modulation doesn't generally care about additive noise.

The problem comes when you take the Fourier Transform of an FM signal. Suppose your message is limited to B rad/sec (ie: for all frequencies above B it has zero content). Using AM DSB-SC it's easy to show that the FT has bandwidth 2B. So theoretically, if we can perfectly reconstruct the transmitted signal from the received.

Things are worse for FM though. It turns out that the FT of an FM signal is a Bessel function of the first kind. The most important thing to take away from this is that since J1(n) has infinite support we would need infinite bandwidth to reconstruct the transmitted signal! In practice though you can do with much less, but you still have a much larger bandwidth requirement (IIRC, 8 times that of AM on average).

Take a look at your radio dial. The AM frequencies are marked in kHz while FM is in MHz. I read on a website that the separation between AM bands in 10kHz while for FM it is 200kHz! Part of the reason the difference is so large is because FM has a larger bandwidth requirement, but also because FM transmitts in stereo.

This is all stuff off the top of my head, I could have some mistakes since I don't have my notes with me at the moment. As far as the original question goes, I agree with the other answers. A tunnel/bridge/overpass is far from a perfect faraday cage. You will get significant fringe effects at the edges distorting the signals, plus all the interference from both the surrounding man made and natural sources. What you are hearing is the frequency noise do to the environment, and the reason it is soft is because usually this noise is minimal compared to the FM signal.
posted by sbutler at 8:32 PM on February 27, 2005


sounds about right, sbutler :)
posted by scalespace at 8:44 PM on February 27, 2005


Radio signals are received at vastly different strengths depending on where you are. Part of the radio's function is to make the signal sound like it is at the same volume as the signal improves and gets worse. This could be implemented as an automatic gain control (AGC), but is probably much more complex (see below). As the received signal becomes very weak the signal to noise ratio gets worse (the noise gets louder relative to the signal). The radio is still trying to pick some signal out, so it is applying a lot of gain, which makes what might otherwise be very weak noise sound quite loud. This combined with the small noise sources in the tunnel, like leaked light, and thermal noise, make the static sound.

sbutler: A basic FM transmission has the formula cos(omega_c*t - alpha(t)) where alpha'(t) = m(t) (or in other words, alpha(t) = integral(m(tau), tau, 0, t)). The effect of this is that the message is carried in the instantaneous frequency of the signal, not the amplitude. So before demodulating FM you just do what is called hard-limiting: you look at the signal and if it is > 0 then you output a 1, and if it is < 0 then you output a -1, and then you take this square wave and lowpass filter it to smooth it out. it may be hard to see askme, but if you draw it out you can easily tell that this operation preserves the phase while normalizing the amplitude. this is why fm modulation doesn't generally care about additive noise. /em>

It could be said that this process is analogous to automatic gain control. There are probably other stages of processing that look to some extent like AGC too...

posted by Chuckles at 10:46 PM on February 27, 2005


Now that I re-read the first paragraph in my post above it may confuse some people with regard to the difference between AM and FM. In an FM receiver the AGC (like?) process(es) will occur between the antenna and the demodulator. This isn't directly related to the volume of the demodulated signal. So, here is a better way to state the answer:

In the tunnel the received signal has dropped below the noise level. Thus the antenna picks up the noise and demodulates it. Demodulated noise sounds like 'static'. There are many sources of noise in the tunnel, thermal noise from resistors, trace radioactivity in rocks, etc. Those noise sources aren't as strong as environmental noise out in the open, but when everything else is shielded they are enough.

So, squelch must just be a received signal strength threshold... If the received signal is weaker than the squelch setting the receiver mutes because there is no 'significant' signal. I don't know that, but it is the logical extension of the discussion...
posted by Chuckles at 11:56 PM on February 27, 2005


fm radios work by tracking a signal as it varies in frequency. when there's no signal to track then you get noise because the circuit "hunts around" picking up random noise. however its simple to build a circuit that checks when there's no signal and cuts the volume. so generally with fm radios, you either get good sound or nothing.

in other words, "silence" in an fm signal doesn't mean "no signal", but rather a signal at a single frequency. because "silence" and "no signal" are different things the radio can detect "no signal" unambiguously and kill the volume.

with am, however, "silence" and "no signal" are the same. so if you did the same trick - killing the volume when the signal gets low - you'd erase the quiet passages in music, whispering, etc. so am radios can't pull this same trick and, as a result, when there's no am signal you get to hear the noise.

that explains why you don't normall hear noise on fm, but do on am. but i'm as mystified as you about why you do when in tunnels. your radio must be picking up a signal in the right range of frequencies, but the signal is somehow messed up. i guess it's either that it's just a very noisy environment - so random signals are above the cutoff threshold - or that there's somehow a lot of multipath signals so you're hearing the radio station with lots of random delays mixed up, and the result is a mess.
posted by andrew cooke at 4:39 AM on February 28, 2005


I was a RF (radio frequency) engineer in a past life and although a lot of what was said is generally true, they would not be where I would have focused. My first guess as to the source of static would have less to due with the radio signals received (or lack thereof), and more to do with the radio receiver/amplifier.

The electronics in the radio itself generate additional noise and if the amplifier is picking it up and amplifying it, you'll hear this noise. It might be doing this because the receiver was "locked" on that channel before entering the tunnel.

This more technical explanation says it more accurately than I do (warning it gets a wee bit technical).
posted by forforf at 5:12 AM on February 28, 2005


did you link to the right place? that descibes the basics of noise, but not why a receiver would stay locked with no signal.
posted by andrew cooke at 5:53 AM on February 28, 2005


Well, I'll put in my guess.

The engineers only designed the squelch to turn on when tuning, and have the squelch circuit disabled a few seconds after you fiddle with the knob.

Try tuning one frequency right, then tune back to the station that was staticy. My bets are on that it will be squelched now.

Why would they do that? Probably because... the squelch circuit, once activated, isn't designed to deactivate when there's signal again, thereby making a station that disappears for a moment disappear until you re-tune.

But that's just my guess. I'm no radio engineer.
posted by shepd at 9:05 AM on February 28, 2005


Sorry, yes the link was just about radio noise. Should have been clearer on that. As for the receiver staying locked, it depends on the design of the receiver. While it's fairly easy to design a radio to find a decent radio station, its more tricky to design one that notices when the station goes away ... and even more tricky to do something intelligent when the station does go away (should it change radio stations due to an intermittent fade? Would a user mind the radio switching stations on its own?). Most radios I know of won't change and will stay on that frequency, even though the signal has gone (unless the user does something).
posted by forforf at 3:11 PM on February 28, 2005


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