Modulation technique used over the 2-wire (Tip & Ring) of POTS 6

[john00436]

Hi Gurus,

I have been surfing for some time but am not able to figure out as to which modulation technique is used for voice communication over the two wire i/f from CO (Tip & Ring).

There are places were I find Amplitude Modulation is being used with distinct carriers for Tx & Rx, is it true ????

Any links or docs which could address my concern would be very helpful....

Regds.
John

 

[Westi]

There is NO modulation on Tip/Ring lines it is just the good old analog signal that is around since Alexander Graham Bell and Phillip Reis.
There is a DC voltage on the line to give power to the Microphone and the AC voltage is send to ring the phone but then it is all old stuff happening.
OK Bell didn't do touch tone dialing, just in case someone want's to comment on that.

Joe W.

FHandw., ACA, ACS

If you can't be good, be good at it!

 

[ISDNman]

Correct.

It is baseband audio superimposed on a DC loop current.

Once it hits the CO it is G.711 mu-law coded PCM at 64 kbps

If you ask a better question you will get a better reply. Be specific so we can help!

 

[SYQUEST]

Are you talking about 2B1Q? It is a modulation technique used for a variety of telephony services from HDSL to loop carrier systems, and even some fiber/coax services, etc.

But regular T/R analogue service is just voice baseband as mentioned above.

....JIM....

 

[john00436]

Thanks for the replies......

1) What I am not able to digest is how can frequencies in the order of "180 Hz to 3.2 kHz" travel such long distances (CO to CPE) without any modulation, well DC loop current & differential signaling could help.

2) Plz. refer to the link below:

http://www.jkaudio.com/article_10.htm

Under heading "Hybrids and Telephones" it says:

The signal on a tip/ring pair is full duplex, balanced bi-directional audio.

How can a both the Tx & Rx frequencies co-exist on a single pair of copper.

The hybrid ckt. is probably the answer:

say Tx signal is: A.Sin(wt)
Rx Signal is: B.Sin(wt)

Hybrid 4 to 2 does:
A.Sin(wt) + B.Sin(wt) on Tip
A.Sin(wt) - B.Sin(wt) on Ring

Hybrid 2 to 4 does:
2.A.Sin(wt) on Tx
2.B.Sin(wt) on Rx

Am I correct ????

These are only my assumptions, plz validate....

Thanks in advance

John

 

[strmwalker]

john00436...the way it travels such distance is the local
carriers put line extenders and repeaters.

no problems only solutions

strmwalker

 

[ISDNman]

This is why telephony is referred to as a loop.

Consider a battery and 2 carbon microphones and two speakers all wired in series.

As either side talks the carbon mic varies its resistance and modify the amount of current flowing through the loop

That would be equivalent to how it works. This is exactly how you could create an intercom

That said, if there is a central office one end is a transformer/hybrid which then converts the 2 wire audio into 4 wire audio.

If you ask a better question you will get a better reply. Be specific so we can help!

 

[john00436]

What does the Hybrid ckt. exactly do to carry "both the Tx & Rx frequencies co-exist on a single pair of copper"

Is it this:

Tx signal is: A.Sin(wt)
Rx Signal is: B.Sin(wt)

Hybrid 4 to 2 does:
A.Sin(wt) + B.Sin(wt) on Tip
A.Sin(wt) - B.Sin(wt) on Ring

Hybrid 2 to 4 does:
2.A.Sin(wt) on Tx
2.B.Sin(wt) on Rx

Thanks in advance

John

 

[PERRYPJ]

Trust me there are limitation to how far it will go even with loop extenders or battery boosters.

OLD ROLMEN WORKING ON NORTELS AND AVAYA

 

[jerryreeve]

you use a hybrid to separate the transmit signal and the receive signal. this is done so that you can amplify either signal since amps only work one direction only.

Phones use 2 wire loops since that is the most economical to the phne companies (putting the copper cables in the ground or up on poles is expensive) but the transmission of signals over long distances is done by what is refered to as 4 wire operation (2 wires a tip and ring or a signal and ground are used for each direction)

I will try explaining how the 2/4 wire conversion worked with the older analog devices so you can understand the conversion that takes place inside of a hybrid chip on the newer solid state circuits.

the two wire loop is sent into a hybrid that is immediately fed into a transformer, primary coil is the 2 wire and two secondary coils for the TX and RX pairs, the result of this is that any signal fed into any of the pairs result in 1/2 of that signal being output by the other two pairs (normal signal loss) There is a feedback control that prevents the receive pair from sending the same signal back on the transmit pair. the Transmit pair is then fed into an amplifier to recover the signal loss of the transformer. the receive pair was amplified prior to entering the transformer to prevent loss thru the transformer. the end result is that you have one pair (Transmit) that only has the signal being sent and one pair (receive) that only has receive signa. the 2 wire pair is a mix of both.

with a circuit divided into 4 wire you can now use multiplexer equipment to place many circuits on one path be it fiber cable, microwave, satellite, etc.

you were asking earlier about the 180Hz-3.2Khz bandwidth. the intiligence of a voice conversation takes place between about 300Hz and 3KHZ, some people talk with higher or lower voices and there are harmonics in a normal converstaion that you hear when you are talking one-to-one but are not really needed to pass the intelligence of your conversations. you really notice this when you are listening to a radio station that puts someone you know that has called in on a telephone call on the air, the quality of the voice is changed but you still know what they are saying.


this reduction of the bandwidth is done to allow the maximum number of circuits to be multiplexed together on an available bandwidth. again going to methods used in the past a circuit would be modulated with a "carrier" that caused the circuit to shift in frequency upward by 4,8,16 etc KHZ 4 KHZ bandwidth shift was used to provide a "buffer" between circuits but still maximize the number of channels. The old microwave systems used groups of 12 voice channels for a total of 64KHZ per group. groups themselves were multiplexed again using Frequency Division Multiplexing into 5 group supergroups.

Newer diagital multiplexing still uses the basic 4KHZ bandwidth per channel but samples that 4KHZ bandwith 8000 times each second and assignes the voltage value at each of those 8000 samples a value from 0 to 255 and sends those sample values in a serial stream of 64,000 bits per second. at this point the circuit is in the digital world and multiplexing is just a simple case of taking smaller and smaller samples of each bit and streaming them with other sample streams (I am really simplifing this part since it wasen't part of your original questions). the receive circuit signal is taken at 64,000 bits per second and separated into 8 bit blocks, converted to a value of 0-255 and then a voltage at that value is sent out on the circuit for 1/8000th of a second, whereupon the next voltage sample is sent. this receive signal is sent into the hybrid receive amplifier-thru the transformer-and out the 2-wire pair of the hybrid.

OK I got a little further than the hybrid but take a look and I hope I have not thrown up too much confusion to you on this.

----------------------------
'Rule 29', "The enemy of my enemy is my enemy's enemy. No more. No less."
----------------------------
JerryReeve
Communication Systems Int'l
com-sys.com

 

[ISDNman]

John,

Do you understand my example of the intercom? That is how you get send and rcv on the same pair.

Separating it to 4 wires is the hard part!

You might look up the book "Understanding Telephone Electronics" a classic that has been revisewd many times.

For more on hybrids see:

http://www.telos-systems.com/techtalk/telephone_qa.pdf

If you ask a better question you will get a better reply. Be specific so we can help!

 

[john00436]

Thanks all of you for your help & support:

I now have a decent idea of what a Hybrid exactly does....

Tx1 |--->---| Rx2
| |
Rx1 |---<---| Tx2

Loop Current: Tx1 + Tx2

Rx1 hears: (Tx1 + Tx2) - Tx1 => Tx2

Rx2 hears: (Tx1 + Tx2) - Tx2 => Tx1

I hope this is correct....

Kindly validate.

Thanks again.......

Regds.
John

 

[jerryreeve]

not quite:

Transmit Tip <---<----) || (--\
Transmit Ring<---<----) || (   \
                        || (    -<---> Loop Tip
                        || (    -<---> Loop Ring  
Receive Tip----->---->) || (   /
Receive Ring---->---->) || (--/

The "(" in the drawing above is as close as I can come to the drawing the windings of a transformer schematic. the "||" is the core.

if a signal is received on the receive tip/ring
current is received from an amplifier or other equipment on the receive pair and is applied to the receive windings of the transformer, this current will induce electromagnetic force thru the core and create current on both the transmit and loop windings.

if a signal is received on the loop tip/ring
current is received from the loop on the loop pair and is applied to the loop windings of the transformer, this current will induce electromagnetic force thru the core and create current on both the transmit and receive windings. The current that shows up on the receive pair will go no further since the amplifier that would be connected to the receive pair is an output only and will not accept the input of the receive pair. the current that shows lup on the transmit pair is sent via that pair to an amplifier or other device to be sent on.



this is for a mostly used for a 2wire to 4 wire converter.


----------------------------
'Rule 29', "The enemy of my enemy is my enemy's enemy. No more. No less."
----------------------------
JerryReeve
Communication Systems Int'l
com-sys.com

 

[john00436]

Hi jerryreeve,

Thanks....

Plz. refer to this link:

http://www.epanorama.net/circuits/teleinterface.html

"full operational amplifier based hybrid circuit diagram"

Looks like the same thing is happening either way....

Regds.
John

 

[jerryreeve]

Simple telecom hybrid circuits

Primary 600 ohm                 Secondary 600ohm centre-tapped
                                       (same as 150ohm+150ohm secondary)
             
           Tip   >------------/ II /------------< audio to telephone line
                              / II /              (low impedance output)
                              / II /
                              / II /
                              / II /
                              / II /------150ohm--Ground
                              / II /
                              / II /                      
                              / II /                     
                              / II /
         Ring   >-------------/ II /-------------> audio to mixer
                                                   (high impedance input)

take my earlier drawing:

Transmit Tip <---<----) || (--\
Transmit Ring<---<----) || (   \
                        || (    -<---> Loop Tip
                        || (    -<---> Loop Ring  
Receive Tip----->---->) || (   /
Receive Ring---->---->) || (--/

and look at both the transmit ring and receive ring are the same tap on the transformer and you have the same circuit. the 150 ohm center tap is both the transmit and receive ring (sometimes called a ground)

the basic rules still apply: the signal that is applied to any one set of the windings is reflected on all other sets of the windings. there lies the big problem with hybrids and the reason that the designers are so careful with them, you can get return signal (aka echo)

I don't want to get too far away from your original question (how can both tx and rx signals be present at the same time) like you said the hybrid is the answer, first remember how the tx and rx signals are presented to/from the hybrid transformer set. then realize that when you are talking on a phone and the person on the other end is also talking you hear both sides in your handset. you get the same thing with data/modem tones and in that case the modem knows enough to ignore(subtract) the signals that it is sending out and only get the signals it should be receiving.

what are you trying for in this line of questioning? setting up a radio interface or just basic understanding?




----------------------------
'Rule 29', "The enemy of my enemy is my enemy's enemy. No more. No less."
----------------------------
JerryReeve
Communication Systems Int'l
com-sys.com

 

[ISDNman]

Yeah, we seem to have drifted from the original question.

There is nothing at all special about having two audio signals on the same pair - the output of any audio mixer does this.

Exracting one without the other requires a hybrid.

But neither case has anything with special modulation techniques, it is just audio!

If you ask a better question you will get a better reply. Be specific so we can help!

 

[SYQUEST]

john00436,

In your original post you mention "places where I find Amplitude Modulation is being used with distinct carriers for Tx & Rx, is it true?"

Please tell us where or what these places or equipment are, that way I think we may be able to provide an answer for you.

I get the feeling we are talking around for what you are looking!

....JIM....

 

[john00436]

Hi SYQUEST,

It basicly applies to toll transmission lines (probably an old technique) were multiple voice channels are amplitude modulated & are multiplexed using FDM.

Ref. "Understanding Telephone Electronics Fourth Edition.pdf" pg. 32....

Link:

http://rapidshare.com/files/78159150/9679E41B-C9E0-4C72-ADCC-AB824A3DE713.rar

Password:

http://www.freebookspot.com

Regds.
John

 

[john00436]

Hi ISDNman,

I simply couldn't imagine 2 waves with the same frequency range to co-exist in a single loop without interfering....

That was the original quest....

AM was my own assumption, very sorrry.....

Regds.
John

 

[john00436]

Hi jerryreeve,

I think we are both saying the same thing in different ways.

My objective is mere "basic understanding".....

thankss....

Regds.
John