two way communication on two wires

[onix]

Has anyone ever noticed that only two electrical lines are used for two-way simultaneous communication over an analog phone line? If one of them is reference, i.e. ground, how do they get incoming & outgoing over the other line?

 

[Mark R]

Essentially the whole thing is one series circuit.

The exchange provides a power supply (about 40 Volts) and connects the two phone lines together in a loop. The microphones act as variable resistors, and the speakers respond to the flucuations in the current flow. This way both parties can speak, and both hear the same thing. You even get what you said echoed back into your ear so you don't shout.

here's a link

 

[onix]

So you're saying that when I talk, I hear myself just as loudly as the person talking to me, or perhaps even louder?

 

[Mark R]

With primitive phones, yes.

Modern phones now have filters and amplifiers to adjust the volume of the echo of your voice (called sidetone) to a more appropriate level.

 

[Loki726]

Its also interesting to note that you can put several different signals on the same line as long as they are sent at different frequencies and filtered out appropriately. ie DSL

 

[onix]

Originally posted by: Loki726
Its also interesting to note that you can put several different signals on the same line as long as they are sent at different frequencies and filtered out appropriately. ie DSL

True, but I was referring to ANALOG signals, and electronics available during Alexander Graham Bell's time. I wonder if they had op-amps back then, of if they needed vacuum tubes for the amplifier/filter circuits. J/K regarding op-amps...

 

[Loki726]

You should still be able to put multiple analog signals on the same line with no ill effects. However digital systems will generally have better quality. I'm pretty sure that filtering systems have existed for almost a hundred years, look at AM/FM radio.

I think op-amps were invented around 1950 though.

 

[harrkev]

Originally posted by: onix
So you're saying that when I talk, I hear myself just as loudly as the person talking to me, or perhaps even louder?

No. There is a way to separate them using a special transformer called a "hybrid," and it does a pretty good job. But what you say is true for the original phone system.

**EDIT**
Howstuffworks call this a "duplex coil."

 

[shortylickens]

Since I work in communications, I think I'll shed a little light on this subject.

We these items called Multiplexers. They split signals up and mix and match them with different inputs and outputs.
One millisecond you are recieving, next you are sending. (It could be 2 milliseconds or 1/2 a millisecond or whatever). This can be done with digital or analog signals, although most technology has been pushing for better digital multiplexers.
This makes it seem as if you were talking and listening at the same time. (Full Duplex)
In reality to get full duplex you would need two seperate comm circuits.
Like in radio you would need to have a reciever and transmitter each with their own freq.
The guy on the other end would have the same thing but reversed. You would also need two antennas. Its much simpler to just have the thing always in recieve mode except when talking (Half Duplex). Then you just need a little mike discipline.

Multiplexers allow us to have really complex comm circuits running on just one microwave transceiver, to give one example.
In fact the FRC-173 is a microwave transceiver, multiplexer and router all in a single unit.

 

[The Boston Dangler]

Originally posted by: Loki726
Its also interesting to note that you can put several different signals on the same line as long as they are sent at different frequencies and filtered out appropriately. ie DSL

It's not just phones and cable tv. The FCC frequency allocation chart in my office covers up to 300 GHz. This is over-the-air. Like 100 different services, all these carriers at the same time.

Sadly, most of this was given away freely to major industial players, courtesy of a very corporate-friendly FCC. Huge companies make out of control profits from this bandwidth, and what did the US taxpayers get for their property? Well, I got a $55 cell phone bill every month.

Any wired service can run any frequencies they want to, as long as it stays inside the wire.

 

[TuxDave]

You can achieve full duplex on a single wire. You just need your reciever to cancel out the signal that you placed on the wire for transmission.

 

[Loki726]

anyone know if you can do full duplex on a single TDM line? I know you can put multiple signals on a single line, but can you have them go both ways?

 

[Colt45]

Originally posted by: The Boston Dangler

Originally posted by: Loki726
Its also interesting to note that you can put several different signals on the same line as long as they are sent at different frequencies and filtered out appropriately. ie DSL

It's not just phones and cable tv. The FCC frequency allocation chart in my office covers up to 300 GHz. This is over-the-air. Like 100 different services, all these carriers at the same time.

Sadly, most of this was given away freely to major industial players, courtesy of a very corporate-friendly FCC. Huge companies make out of control profits from this bandwidth, and what did the US taxpayers get for their property? Well, I got a $55 cell phone bill every month.

Any wired service can run any frequencies they want to, as long as it stays inside the wire.

Here's the chart mentioned, if anyone is interested.
http://www.ntia.doc.gov/osmhome/allochrt.pdf

 

[harrkev]

Originally posted by: shortylickens
Since I work in communications, I think I'll shed a little light on this subject.

We these items called Multiplexers. They split signals up and mix and match them with different inputs and outputs.
One millisecond you are recieving, next you are sending. (It could be 2 milliseconds or 1/2 a millisecond or whatever). This can be done with digital or analog signals, although most technology has been pushing for better digital multiplexers.
This makes it seem as if you were talking and listening at the same time. (Full Duplex)
In reality to get full duplex you would need two seperate comm circuits.
Like in radio you would need to have a reciever and transmitter each with their own freq.
The guy on the other end would have the same thing but reversed. You would also need two antennas. Its much simpler to just have the thing always in recieve mode except when talking (Half Duplex). Then you just need a little mike discipline.

Multiplexers allow us to have really complex comm circuits running on just one microwave transceiver, to give one example.
In fact the FRC-173 is a microwave transceiver, multiplexer and router all in a single unit.

I did not know that Alexander Grahm Bell invented the multiplexer.

For an analog POTS line, this is a load of BS. The original poster is talking about the "last mile" which is simply an analog loop.

 

[SuperTool]

Basically, the receiver at one end subtracts what the transmitter at that end is sending.
You have basically two transmitters. One sending driving the output down the wire, and second driving the same output on the reference voltage of the receiver. That way, the transmitted signal becomes common mode to both the reference and the input of the receiver, and is filtered out, and the only thing coming out of the receiver is what is remaining, which is what was sent from the other end.