So, being an awkwardly old-fashioned guy for someone who grew up with computers and programming, I kept using the iPod my little brother got me for Christmas like 10 years ago. Had the moving hard drive and everything.
Finally, it conked out on me, so I bought an iPod nano.
I was surprised to find the thing can pick up radio. Then it says the headphones need to be plugged in for it to work.
Clearly they’re using the headphone wires as the antenna. But how can this be? Doesn’t the much stronger electric signal going up to the headphones mess with receiving the radio signals? Can a wire that’s conducting like 5 volts of eletcrity and .2 amps or whatever it is even pick up radio waves in the first place? Do the signals get picked up anyway, and just slightly modify electricity that’s going up/down the wire, and so the iPod just filters that out? I know there must be some kind of filter in general. Is it an electronic filter, or does it use the computer of the iPod to do it algorithmically?
One of the first things they teach you in EE school is how easy it is to make a radio receiver. Then you spend a lot of time learning how to make all of your circuits NOT be radio receivers. If you don’t very carefully design your iPod (or whatever else you are designing) then radio noise from the headphone wires can easily end up all over the place in your circuits. A good example of this is computer speakers. Most computer speakers these days (except for the really small and cheap ones) have an amplifier in the speakers and don’t rely on the (usually smaller) amplifier that is built into the sound card. Place a cell phone next to your speakers, and your speakers will “chirp” periodically, and will get very chirpy just before a call comes in (as the phone and the tower talk to each other, setting up the call). The reason the speakers chirp is that radio noise from the cell phone gets picked up by the speaker wires and gets coupled into the speaker’s circuitry, resulting in unwanted noise in the speakers.
It’s fairly easy to make a high pass circuit that only passes radio frequency energy into the radio receiver part of the iPod while rejecting all of the much lower audio frequency stuff that is going up to the headphones. It’s a bit of a challenge to make these kinds of circuits fit into the size of an iPod, but overall it’s not that difficult compared to other things these days.
Phone/DSL: Not the same thing, as those are both high power.
FM frequencies (~ 100 MHz) and audio (~ 10 kHz) are so far apart that they really don’t get in each other’s way. I’m also pretty sure they only use the ground wire as the antenna, further reducing the problem.
The radio needs to be tuned to a specific frequency to pick up a specific station, even if another station is only 200 kHz away. The audio 99.99 MHz away then really isn’t a problem for the filter, even though it may be quite strong.
What I never understood is how some two-way radio systems presumably send and receive at the same time using the same antenna. Or are they actually using time devision multiplexing, so the sending and receiving alternate?
OK, so here’s a related question I’ve always wondered about. I’ve seen those pictures where they show one fiber optic line, and it can transfer the data of a also-shown giant bundle of copper cables.
From what I understand about fiber optics, all data is transmitted by flashing on and off.
And what I was told is that it’s (apparently) easier to make a machine that can see a bajillion flashes per second and interpret them… well apparently this capability with these interpreters is so high as to replace the classic copper cables that carry/carried the audio signals.
What the hell did old phone lines carry? My understanding is it was just a typical AC audio signal from the phone… So I guess the same principle applied to phones, that is one wire can carry multiple signals? But apparently there’s a limit due to interference when you have too many signals? In that case, how did bigger cables help cary more signals? Or was that just a bunch of cables wrapped together?
Oh, so you are asking for the history of telephony in a paragraph?
Originally, there was only one conversion/line. That required a lot of switched circuits, and a lot of wires. Pretty quickly, it was discovered that there is a lot of dead air in voice, and if you chop the sound up fast enough, you can multiplex several conversions onto one wire. When things went digital, that became much easier, since now you could buffer the data, and do all kinds of neat compressions on it to reduce the amount of data that had to be transmitted.
To add to beowulff
Telephone lines carried information at the same frequency as the sound. That is why various tones (or when pushing a number, # or * two tones are used) were used to transfer information (in-band). It wasn’t until recent that the idea that copper could transmit frequencies outside of the aural range could be used to allow for out-of-band signaling and DSL.
Most radios that use the same antenna for transmit / receive use a special switch to switch the antenna from the power amplifier to the low-noise amplifier. This means that the transceiver can’t transmit and receive at the same time - which is generally not a problem, since most systems spend the majority of their time listening.
That is also why at one time you got “ghost conversations” on calls; especially long distance ones. You could be talking to someone and faintly hear someone else talking as well. You were getting overlap between your reassembled conversation and someone else’s.
Modern digital circuits have pretty much eliminated that now.
If the requirement is to receive a signal on an input frequency and rebroadcast it on a different output frequency (e.g. a radio repeater) then you would use a “Duplexer” http://en.wikipedia.org/wiki/Duplexer
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You don’t usually see them on things like handheld radios, but there’s a thing called an RF circulator that you can use to allow you to transmit and receive on the same antenna at the same time, and on the same frequencies.
Basically, the outgoing RF signal goes out through one port and to the antenna, and incoming signals from the antenna go to a third port instead of going to the first port (the first port being the one where the outgoing signal came from).
[hijack]
Yes, and a circulator is a non-reciprocal device. It allows the signals to propagate between the three terminals in a clockwise direction but not in a counterclockwise direction. On first blush, this seems to a physicist as a violation of the left/right (parity) symmetry of the laws of electromagnetism, but there is a trick. Can you guess what it is?
A magnetic field applied to a non-reciprocal material breaks the symmetry
I know that. I was just illustrating that one piece of wire was carrying two or more signals at the same time without them interfering with each other.
Only for FM, though, AM was handled by some sort of internal antenna. Or so the Walkman[sup][sup]TM[/sup][/sup] brand personal stereo documentation claimed.
