telephone lines

[l***@yahoo.it]

Hi ------
Just a simple question.....
How a telephone tx rx works???...I mean how 1 pair of conductor in
the
telephone can trasmitt and recive simultaneously audio
signals......????(if I understood well the pair trasmitt and at the
same time it recives signals)
How can a signle pair do this job????
many thanks
jac

[Grant Edwards]

Post by l***@yahoo.it
How a telephone tx rx works???...I mean how 1 pair of
conductor in the telephone can trasmitt and recive
simultaneously audio signals......????

[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.

Post by l***@yahoo.it
(if I understood well the pair trasmitt and at the same time
it recives signals)

Yes.

Post by l***@yahoo.it
How can a signle pair do this job????

Physics. :)

[Dana]

Post by Grant Edwards

Post by l***@yahoo.it
How a telephone tx rx works???...I mean how 1 pair of
conductor in the telephone can trasmitt and recive
simultaneously audio signals......????

[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.

Post by l***@yahoo.it
(if I understood well the pair trasmitt and at the same time
it recives signals)

Yes.

Post by l***@yahoo.it
How can a signle pair do this job????

Physics. :)

Great answer

Post by Grant Edwards
--
Grant Edwards grante Yow! FOOLED you!
Absorb
at EGO SHATTERING impulse
visi.com rays, polyester poltroon!!

[h***@bbs.cpcn.com]

Post by l***@yahoo.it
Hi ------
Just a simple question.....
How a telephone tx rx works???...I mean how 1 pair of conductor in
the
telephone can trasmitt and recive simultaneously audio
signals......????(if I understood well the pair trasmitt and at the
same time it recives signals)
How can a signle pair do this job????
many thanks
jac

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.

[Al Gillis]

Post by l***@yahoo.it
Hi ------
Just a simple question.....
How a telephone tx rx works???...I mean how 1 pair of conductor in
the
telephone can trasmitt and recive simultaneously audio
signals......????(if I understood well the pair trasmitt and at the
same time it recives signals)
How can a signle pair do this job????
many thanks
jac

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!

[Tom Lager]

Post by Al Gillis

Post by l***@yahoo.it
Hi ------
Just a simple question.....
How a telephone tx rx works???...I mean how 1 pair of conductor in
the
telephone can trasmitt and recive simultaneously audio
signals......????(if I understood well the pair trasmitt and at the
same time it recives signals)
How can a signle pair do this job????
many thanks
jac

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!

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.

[Grant Edwards]

Post by Tom Lager
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...

[h***@bbs.cpcn.com]

Post by Grant Edwards
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.

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?

[Grant Edwards]

Post by h***@bbs.cpcn.com

Post by Grant Edwards
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.

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?

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.

Post by h***@bbs.cpcn.com
Do they want us to talk loudly for more efficiency?

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.

[h***@bbs.cpcn.com]

Post by Tom Lager
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.

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.

[Floyd L. Davidson]

Post by h***@bbs.cpcn.com

Post by Tom Lager
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.

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

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.

Post by h***@bbs.cpcn.com
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.

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

[h***@bbs.cpcn.com]

Post by Floyd L. Davidson
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.

[ABLE_1]

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

Post by h***@bbs.cpcn.com
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.

[Reed]

Post by ABLE_1
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)??? :-)

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

[ABLE_1]

Post by Reed

Post by ABLE_1
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)??? :-)

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

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

[Floyd L. Davidson]

Post by ABLE_1

Post by Reed

Post by ABLE_1
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)??? :-)

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

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...

[ABLE_1]

Post by Floyd L. Davidson

Post by ABLE_1
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...
--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>

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

[Grant Edwards]

Post by ABLE_1

Post by Floyd L. Davidson
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.

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???? :-)

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

[Floyd L. Davidson]

Post by Grant Edwards

Post by ABLE_1

Post by Floyd L. Davidson
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.

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???? :-)

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.

[ABLE_1]

Post by Floyd L. Davidson
Exactly. An artificial ground pig.

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

Les

[Grant Edwards]

Post by Floyd L. Davidson

Post by Grant Edwards

Post by ABLE_1

Post by Floyd L. Davidson
The button in the tin can only makes noise if the can is
grounded and cannot also vibrate.

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???? :-)

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.

I think the usual techinical term is "virtual ground pig".

[Rich Piehl]

Post by Grant Edwards

Post by Floyd L. Davidson

Post by Grant Edwards

Post by ABLE_1

Post by Floyd L. Davidson
The button in the tin can only makes noise if the can is
grounded and cannot also vibrate.

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???? :-)

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.

I think the usual techinical term is "virtual ground pig".

And here all along I thought ground pig was called pork sausage. ;-P

Take care,
Rich

God bless the USA

[ABLE_1]

Post by Rich Piehl

Post by Grant Edwards

Post by Floyd L. Davidson

Post by Grant Edwards

Post by ABLE_1

Post by Floyd L. Davidson
The button in the tin can only makes noise if the can is
grounded and cannot also vibrate.

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???? :-)

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.

I think the usual techinical term is "virtual ground pig".

And here all along I thought ground pig was called pork sausage. ;-P

Me thinks that this off tangent topic has reached new limits and shall never
return.
;-0 LMAO

[h***@bbs.cpcn.com]

Post by ABLE_1
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)??? :-)

Well, in the two can comfiguration, only one person can talk at a time
since you have the switch the can from mouth to ear.

Actually, as a kid I tried that and never could get it to work. Some
books suggested techniques for improvement (waxing the string?) but it
didn't help.

Then someone lent us two handsets and some wire which worked great off
of a No. 6 dry cell. (Do they still even make them?) Unfortunately,
we couldn't keep the handsets. We wanted to run a private line
between our two hours but our parents nixed that idea and there were
logistical issues.

[Of course today kids have their own private phone lines, cell phone,
and computer with Instant Messaging, so the novelty of two handsets
and a wire is rather dated. They also make walkie-talkies which I
presume are more powerful than the ones we had to play with. Ours
were "rated" at one mile but were good only for about 100 yards at
best.)

[ABLE_1]

Post by h***@bbs.cpcn.com

Post by ABLE_1
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)??? :-)

Well, in the two can comfiguration, only one person can talk at a time
since you have the switch the can from mouth to ear.

As the OP I was wondering how long it would take for someone to realize that
small bit of techno thing with the cans.

Good job Hancock, please move to the head of the class you win.

What you have won is the pride in knowing that you were the first to post
that the two cans and string are in fact simplex in nature. You have passed
and may now continue on with life.

Congrats!!!!

Les

[Wolfgang S. Rupprecht]

Post by ABLE_1
What you have won is the pride in knowing that you were the first to post
that the two cans and string are in fact simplex in nature. You have passed
and may now continue on with life.

What about the more advanced design using a string-hybrid? Tie a
second can and string near each end of the original string so that
each end has a Y arrangement. Full duplex. Some sidetone. Serious
neck injury trying to talk and listen while keeping both cans pointed
in the same direction. ;-)

-wolfgang

(And yes, this isn't really a hybrid. To be a real hybrid there would
have to be another string (or lever ??) between the two cans to
subtract most of the outgoing signal from microphone can so the
earphone can wouldn't get its full signal.)

[ABLE_1]

Post by Wolfgang S. Rupprecht
What about the more advanced design using a string-hybrid? Tie a
second can and string near each end of the original string so that
each end has a Y arrangement. Full duplex. Some sidetone. Serious
neck injury trying to talk and listen while keeping both cans pointed
in the same direction. ;-)
-wolfgang
(And yes, this isn't really a hybrid. To be a real hybrid there would
have to be another string (or lever ??) between the two cans to
subtract most of the outgoing signal from microphone can so the
earphone can wouldn't get its full signal.)
--

Wow!!! The creative juices are starting to flow.
So when can we expect prototype submissions????

Later,

Les/

[Floyd L. Davidson]

Post by ABLE_1

Post by h***@bbs.cpcn.com

Post by ABLE_1
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)??? :-)

Well, in the two can comfiguration, only one person can talk at a time
since you have the switch the can from mouth to ear.

As the OP I was wondering how long it would take for someone to realize that
small bit of techno thing with the cans.
Good job Hancock, please move to the head of the class you win.
What you have won is the pride in knowing that you were the first to post
that the two cans and string are in fact simplex in nature. You have passed
and may now continue on with life.
Congrats!!!!

That is all incorrect though.

The can/string system provides full duplex for anyone with good
ears. There is no technical requirement that the can be moved
to the ear to hear it's output. There is a technical requirement
that the can be in front of the speaker's mouth to transmit a voice
though.

Hence if it is limited to simplex there might be two cause,

1) operator error.
2) operator inadaquacy.

[ABLE_1]

Post by Floyd L. Davidson
That is all incorrect though.
The can/string system provides full duplex for anyone with good
ears. There is no technical requirement that the can be moved
to the ear to hear it's output. There is a technical requirement
that the can be in front of the speaker's mouth to transmit a voice
though.
Hence if it is limited to simplex there might be two cause,
1) operator error.
2) operator inadaquacy.

Well I guess that you may be right.
However with my hearing loss I will still consider it to be simplex at best.

Later,

Les

[Floyd L. Davidson]

Post by ABLE_1

Post by Floyd L. Davidson
That is all incorrect though.
The can/string system provides full duplex for anyone with good
ears. There is no technical requirement that the can be moved
to the ear to hear it's output. There is a technical requirement
that the can be in front of the speaker's mouth to transmit a voice
though.
Hence if it is limited to simplex there might be two cause,
1) operator error.
2) operator inadaquacy.

Well I guess that you may be right.
However with my hearing loss I will still consider it to be simplex at best.

Don't feel bad, with my hearing it only works if the other party
shouts, and we limit the length of string to about 5 feet...
;-)

[ABLE_1]

Post by Floyd L. Davidson
Don't feel bad, with my hearing it only works if the other party
shouts, and we limit the length of string to about 5 feet...
;-)

Huh???? What did you say?????????????????

Les

[Tomi Holger Engdahl]

Post by l***@yahoo.it
Hi ------
Just a simple question.....
How a telephone tx rx works???...I mean how 1 pair of conductor in
the
telephone can trasmitt and recive simultaneously audio
signals......????(if I understood well the pair trasmitt and at the
same time it recives signals)
How can a signle pair do this job????

It is described in this document:
http://www.epanorama.net/documents/telecom/teleinterface.html