telephone lines

[You need to get those sticky punctuation keys fixed.]

Pretty much the same way that two people can see and hear each other at the same time. Light can propogate both ways through space at the same time, sound can propogate both directions through air at the same time, and so can signals in a pair of wires.

Yes.

Physics. :)

In the very simplest sense, the transmitter, receiver, distant transmitter, distant receiver, are all wired in series with each other. Note that you hear your own voice as you talk.

If you had two handsets, a pair of wires, and a battery, you could make a simple connection and talk between the two handsets.

This is a great question! (And there have been a couple good answers as well).

When you think about how clever the early engineers were in designing the interfaces between the telephone set and the CO the scheme they came up with is pretty elegant/efficient. Notice that telephones provide full duplex voice communications and provide signaling in both directions, all over one pair of wires. Compare that to RS-232, a design from the middle 1960s, which took eight or nine wires to communicate in both directions and provide signaling!

snipped-for-privacy@aracnet.com (Al Gillis) wrote in :

Actually the key to the whole thing is a device contained in the telephone set called a hybrid network. This device takes what should be a four-wire circuit and allows full duplex transmission of voice on two wires.

The instantaneous voltage across the end of the pair is the sum of two signals: the inbound signal and the outbound signal.

What the hybrid network does is (more-or-less) take that combined signal and subtract the outbound signal. That leaves just the inbound signal which is routed to the earpiece. In reality it doesn't cancel out the outbound signal completely. That allows you to hear your own voice in the earpice. The outbound signal that is heard in the earpiece is called "sidetone". People will generally adjust the volume at which they are speaking in order to maintain the sidetone at an intelligible, comfortable level.

That means you can control how loudly somebody speaks into a phone by adjusting the sidetone level. If you turn down the sidetone gain, people will talk louder to compensate. Increasing sidetone gain will make people talk more softly. For some reason I don't understand, many/most mobile phones don't have any sidetone, so that will generally cause people speak a lot louder into a mobile phone than they would into a landline with a proper sidetone.

Back at the dawn of time when I designed audio circuitry for cellular phones, our phones had sidetone at proper levels. Talking on those phones was a much more "natural" experience. That was also back in the days of analog FM when the mobiles had 3W transmitters, rx diversity, and proper antennas.

Today's mobile phones sound like complete crap by comparison. Of course there is a slight size/weight difference...

You don't need a 'network' box to have communication, you can connect two plain headsets and a battery and talk fine.

The "hybrid network" does other functions. It improves sound quality and allows a ringer operated by A/C current to be connected across the line.

My cell phone does have sidetone, but it's slight. As you describe, my natural tendency is to talk louder when on the cell phone. Unfortunately, so does everyone else, as in stores, the movies, and on trains. Drives me nuts!

Would anyone know why cell phones have so little sidetone? Do they want us to talk loudly for more efficiency?

I've no idea. The first and second generation AMPs mobile phone designs on which I worked had sidetone that met the Bell/WECO specs for landline phones. It was pretty trivial to do.

I don't think it would make a measurable difference in efficiency. My guess is the designers just don't have a clue when it comes to basic telephony.

That is quite correct. The hybrid network merely provides a more _comfortable_ environment, and is unnecessary.

In modern telephones it might be replaced by Echo Canellation technology.

I would not agree that the sound quality is improved by the hybrid. It also is not required for the AC ringer.

Possibly confusion over terms.

My understanding of the defn of "hybrid network" was the box within the telephone set that contained (1) the anti-sidetone circuitry and (2) the capacitor to allow the A/C ringer to be online without tripping the D/C C.O. relay. It may also smooth out the sharp "pop" sound when the hookswitch is lifted.

Very interesting thread going here. Lot of good input.

However I was thinking.

If you use a two tin cans and a string set up, do you not have duplex communication on just one(1) wire(ahhh...... string)??? :-)

[your results may differ] [local laws may apply] [not valid for employees or family member] [any resemblance to actual events was purely by chance]

Just a little food for thought.

Les

Hmm...maybe "ground return" ...??

It is described in this document:

I don't think so..............................

If the person listening, and the can, are not "well grounded", they will vibrate right along with the string, and therefore won't hear a thing.

The button in the tin can only makes noise if the can is grounded and cannot also vibrate.

Hmmmm...

Maybe it is getting too warm in Barrow...............................

Hmmmmmmmm Soooooo then if Pigs could fly they would not be able to communicate on the two cans and string while in flight since they would not be grounded during the time of said communication........................ right???? :-)

Les

Nah, the pigs would have enough inertia to provde a "ground" for the vibrations in the string to work agains.

Exactly. An artificial ground pig.

Ohhhh yea, I saw one of them just last week. Strange looking thing.

Les