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Cell phone=speed of light

  1. Apr 24, 2006 #1
    I thought a cell phone sent messages at the speed of light, but I was told that much of it goes through wires and not the atmosphere. How do wires effect the speed, and does it mostly go through wires?
     
  2. jcsd
  3. Apr 24, 2006 #2

    ZapperZ

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    You are forgetting that when you make a phone call, you are not sending your signal directly to another cell phone. You are sending your signal to a relay tower which then probably send your call to an exchange center, etc. before it goes to the phone you are dialing.

    Zz.
     
  4. Apr 24, 2006 #3
    As ZZ says, it has to go to relay stations along the way.

    Not many calls will actually go via a satellite unless they absolutely have to, they go to masts instead that boost and redistribute the signal. There may be some fibre optics involved (which do transmit at near lightspeed) but it's unlikely.

    The conduction speed through copper won't be anywhere near lightspeed. But if you're thinking about the delay you hear on the phone sometimes, a lot, lot more of that will be coming from the processing that's going on with your signal.

    Phones, mobiles in particular, use very complex multiplexing stratergies to stack as many phone calls as they possible can into the smallest bandwidth. For an example of multiplexing, you can hear from 20hz to ~20kHz, but the fundamental pitch of your voice, the note you hear when you talk to someone and that carries the 'intelligence' of the conversation if you like, is only a few kHz. So if a phone transmitted from 20Hz to 20kHz, you'd be 'wasting' a lot of the extra space in that range with frequencies you don't really need to hear to understand what's being said.

    Instead, your phone cuts your voice off over about 4kHz, which is why they're not very good at reproducing high pitched sounds and they sound so boxy, it's not the speaker or mic in the phone so much as the filter that's applied to it. Now you can take your 4kHz and someone elses and stack them on top of each other. So you might get 0 to 4kHz, then the other guy has his voice multipled twice so he gets 4.0000whatever to 8kHz, and the next guy 8.0000whatever to 12kHz and so on. That gives you 5 voices in the original band; 20,000 total / 4,000 each. Of coarse, that'd sound very weird to listen to, you'd be able to hear five different people and all but one would be speaking in a higher than normal pitched voice, with the last one so high you'd have trouble hearing him. So at the other end the bands are split away from each other into individual channels and divided by the same amounts they were multipled by to restore them to their original frequencies of 0 - 4kHz.

    This method of packing signals in is absolutely key to the telephone network, and almost all data communications actually, and they have lots of patents on mind blowingly complicated ways of working it all out. They not only do it by just mulitplying and dividing the frequency of your voice but by shunting your into certain time slots and all kinds of things. By the time your voice comes out the speaker on the other guy's phone, it will have been multiplied, divided, boxed up into packets, allocated packet transmit space, depacketed and all manner of weird stuff.

    Interestingly, while your fundamental voice may only be a few kHz in range, the harmonics of your voice go much higher. These are the notes that you don't have to think about making, they're just part of your voice, they're the signature or fingerprint of your voice. Girls have more of them higher up, which is why they usually sound different to guys. It's also why guys pretending to be girls don't sound very realistic, because it's very difficult to change the harmonic pattern of your voice in this way purposefully.

    Because your phone cuts pretty much all of these higher notes off, it tends to make everyone sound similar.

    Mobiles also have things like compression applied to them. When you're not talking but listening to someone else, if your phone is just transmitting background noise at your end it's wasting power and bandwidth that the network could slip someone else's voice into temporarily; unknown to you. Instead, your phone switches off it's transmit signal until a sound at your end gets loud enough for the phone to think there's something worth sending. That's why if you've ever been talking to someone on a mobile, if you're talking for a while it sounds as if they've hung up, the line is completely silent. Then when the other person speaks or there's a noise at their end you can hear noise again. That actually really annoys me because it can result in a kind of stuttering effect on the line as the phone stutters in and out of transmitting; the background noise is just on the boarder of being too low to bother transmittting but is repeatedly getting just high enough to transmit. When I'm talking to people on mobiles I quite often find myself asking if they're still there. :P
     
    Last edited: Apr 24, 2006
  5. Apr 24, 2006 #4
    Thanks eeka chu, but what about the curvature of the earth? I was recently calling between Columbus Ohio and Las Vegas, 3 hours time difference. Now that should be like 1/8 the diameter of the earth, which is around 2000 miles. Is it not true that some light follows the curvature of the earth? Because of this, some ships can be seen further on the horizon?

    I quote: Air gets thinner the higher you are from the surface of the earth or the sea. In normal conditions, the density of air decreases with increasing altitude. When light rays travel along the surface of the earth, the air below the light ray is therefore denser than the air above it. One of the typical properties of light is that it refracts towards the denser medium, and thus a ray moving along the surface of the earth is in fact constantly refracted slightly downward, following the earth's curve instead of escaping straight into space.[/I]http://virtual.finland.fi/netcomm/news/showarticle.asp?intNWSAID=25722
     
    Last edited: Apr 24, 2006
  6. Apr 25, 2006 #5


    Theoretically that should work but I'm not sure if it's something telephones try to exploit too much because it might change quite a bit depending on the weather.

    You can use the ionosphere, a charged layer high up in the atmosphere, to send radio transmissions. The layer acts like a big reflective blanket that you can reflect a radio signal off, around the curvature of the Earth, without the need for satellites. But the solar wind interacts with layer so, again, I think the phone companies would rather rely on fixed, guarantees of where the buffer points will be. They're trying to transmit hundreds of thousands / millions of separate channels that will all call in and complain if their conversation is cut off.

    The signal between the buffer towers will actually be travelling quite close to the speed of light. The only thing slowing it down will be gas molecules and water vapour in the atmosphere. It'd take about 6ms for the signal to travel 2000km and an echo usually has a time period of tens or hundreds of ms.

    I think before the curvature of the Earth, most towers need placing close together to keep the signal strength up, as it starts to fade as it passes through the atmosphere. The only exception is in really mountainous terrain, where mountains and things will get directly in the way of the signal. Then you need to put quite a few towers down.

    The microwave frequencies phones work at are quite direction specific and they don't like going round things, so you need as clear a line of sight as possible between the two points.
     
    Last edited: Apr 25, 2006
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