THEORETICAL DISCUSSION

A telephone hybrid is the component at the ends of a subscriber line of the public switched

telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio

paths. The need for hybrids comes from the nature of analog plain old telephone service home or small

business telephone lines, where the two audio directions are combined on a single two-wire pair. Within

the telephone network, switching and transmission are almost always four-wire circuits with the two

signals being separated. Hybrids perform the necessary conversion. In older analog networks,

conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links.

In today’s digital systems, each speech direction must be processed and transported independently.

The fundamental principle is that of impedance matching. The send signal is applied to both the

telephone line and a ‘balancing network’ that is designed to have the same impedance as the line. The

receive signal is derived by subtracting the two, thus canceling the send audio. Early hybrids were made

with transformers configured as hybrid coils that had an extra winding that could be connected out of

phase. The name hybrid comes from these special mixed-winding transformers as shown below:

The need of using a two wire versus a four wire telephone interface are inherent. A four wire

circuit are considerably less noisy, have less cross talk and provide better isolation between the two

directions of transmission either operating on full duplex or half duplex mode. However, four wire

circuit requires less wires, less circuitry and less money than their four wire counterparts making it

better choice to connect the phone to a central office rather than four wire signalling.

DATA & RESULTS

Procedure 9: Describe how the SLIC routes the sine wave signal present on the telephone line.

o We can clearly see that the output at TP4 looks quite similar to the input at the telephone

in which we are measuring it on a single ended output. This means that the SLIC already

convert the transmitted signal from a balanced differential signalling to a single ended

signaling output.

Procedure 11: Describe how the SLIC routes the sine wave signal received at SLIC RXA output.

o The RXA output signal is coupled to the telephone line converted from a four wire signal

to a two wire differential signal.

Procedure 11: Briefly explain why the received sine wave signal is not routed to the SLIC

TXA(TP4), although it is present on the telephone line (TP3).

o It is because of the echo canceller of the SLIC where in we subtract the received signal to

the original SLIC TX signal to remove it at the output TXA.

Procedure 12: Observe the signals displayed on the Oscilloscope, describe what happens?

o We have a normal telephone conversation which is the superposition of input of AUX C

and AUX D.

Procedure 13: Describe what happens, briefly explain.

o We can see an echo recevied by the transmit side of the SLIC at TP4 from the received

signal since we turned off the echo canceller. It is then added to the transmitted signal of

the SLIC A

Procedure 15: Describe what happens, briefly explain.

o We can clearly see that an echo signal is added to the transmitted signal of the SLIC A in

which the AUX C output is added to the signal and then re-added on the output signal.

Procedure 16: Describe what happens. Briefly explain.

o An echo appears on the received signal of the telephone set A in which it is the sum of

the superposition of the transmitted signal from telephone set B plus a small portion of

the transmitted signal from telephone set A in which it is received as the output of the

telephone set A.

Procedure 18: Briefly explain what happens.

o We can hear an echo of our selves which reach a point where it is quite irritating.

Procedure 19: Briefly describe what happens.

o It is a normal telephone conversation without an echo and having a good quality.

OBSERVED OUTPUT

Figure 1: Auxiliary C Output, 400Hz sine wave Figure 2: Auxiliary D Output, 800Hz sine wave

Figure 3: Abnormally high value of shifted echo Figure 4: Echo produced on the transmit side, yellow trace

Figure 5: Echo cancelled Figure 6: Normal Output

Figure 7: Echo on telephone line A Figure 8: Received echo from telephone line B.

Figure 9: Normal telephone conversation with echo canceller

REFERECE

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

en.wikipedia.org wiki Telephone hybrid

Electronic Communication Systems by Wayne Tomasi; ch.17 pp736 - 739

EXPERIMENTAL DISCUSSION

In this experiment, we try to learn how the hybrid works and its essential function in the

telephone system. Basically, a hybrid converts two wire balanced telephone signal into a four wire

signal that can be used for signal processing. Since the rise of digital control in telephone signals, two

wire to four wire conversion becomes essential since digital circuit can easily accept single ended

signals rather than differential signaling. Another function of the hybrid is to cancel echo of the

transmitted and received signal.

As the first part of the experiment, we connect the labvolt telephony experimenter on the

computer and set up a connection between the experimenter and the host computer. Next, we connect

the oscilloscope probe in four test points with continous refresh rate in Telephone Set A. Then, we dial

the number of telephone line B and proceed to the next part of the experiment.

To be able to observe the hybrid circuit, we then connect auxillary outputs of the experimenter to

a small speaker and then place the mic of the handset to the auxiliary output. Adjusting the output of

auxilliary line C to the output of the TP 3. We the have the output of procedure 9 which looks like the

exact copy of the inputs signal. Which can be found at figure 1 of the observed outputs.

We then remove the output of auxiliary C and open the output of auxilliary D and see a voltage

waveform that looks like this. Figure 2 of the observed outputs. This signals are the signals that appears

on the SLIC. This signals is coupled on a single channel (two wire) outside the SLIC Hybrid and to the

subscriber loop. This is the received signal with respect with telephone line A. Since the signal is

channeled on a single wire, we can see that no echo or positive feedback or ringing appear on the

tramsmit side since it is lineary subtracted at the SLIC by creating a copy of received signal in 180

degrees out of phase with the signal and subtracted to the output signal producing zero echo. Since echo

cancellation works by subtracting a copy of the received signal to the transmitted signal, when the

received signal amplitude is increased substancially, we can see that it reappears in the transmitted

signal shown in figure 3 of the observed outputs.

For the next part of the experiment, we look at what will happen to the output waveform if we

remove/disable the echo cancellation of the SLIC. Thinking of thee output, we will expect a

supperposition of the outputs of auxilliary c and d in which c is 400hz and d is 800 hz. Removing the

echo cancellation on SLIC A, we can have the output shown in figure 4 of the observed outputs.

When we turn on echo cancellation, we can see that the echo output (yellow trace) amplitude

will dramatically decrease as shown in figure 5 of the observed outputs. Echo is a very eary problem in a

two wire to four wire conversion since we are coupling a the received signal line to the transmitted

signal line. Without echo cancellation, positive feedback will occur which may cause irritable noise to

the users on both sides similar to the what we hear on radio when a caller is calling the DJ with a loud

radio. We can see that echo cancellation is esential to be enable on both parties hybrid since when echo

cancellation is disabled with a party, it will still cause echo feedback and small echo noise in the party

where it is received since the hybrids echo cancellation is only enabled to remove echo from the four

wire interface.

CONCLUSION

In this experiment, the students were able to learn the function of the SLIC Hybrid circuit and its

marvelous job of converting a two wire system to a four wire system with an added bonus of cancelling

echo on the line. A two wire to four wire system is essential since connecting a telephone wire to a local

subscriber is cheaper if you are using a two wire differential signalling rather than a four wire which it

can cause double than the former. A four wire system is needed though since digital signalling and

proccessings used by current industry standards accepts single ended signals. To compensate for the pro

and cons of the two different colliding world, a hybrid was needed. Also, since a received signal might

be impose to a the transmitted signal of the SLIC in conversion, a very cleaver way of cancelling out the

echo signal by adding an inverting buffer at the receiver side of the hybrid and then lineary adding it up

to remove the “echo” was devised bringing more comfort to the users of today’s telephone.

GLOSSARY

1. Differential signaling - a method of transmitting information electrically with two

complementary signals sent on two paired wires, called a differential pair

2. Echo - a sound or series of sounds caused by the reflection of sound waves from a surface back

to the listener.

3. Hybrid - the component at the ends of a subscriber line of the public switched telephone network

(PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths.

4. Impedance matching - the practice of designing the input impedance of an electrical load (or the

output impedance of its corresponding signal source) to maximize the power transfer or

minimize reflections from the load.

5. Matching transformer - A balun is an electrical device that converts between a balanced signal

(two signals working against each other where ground is irrelevant) and an unbalanced signal (a

single signal working against ground or pseudo-ground). .

6. Oscilloscope - a device for viewing oscillations, as of electrical voltage or current, by a display

on the screen of a cathode ray tube.

7. Positive Feedback - the enhancement or amplification of an effect by its own influence on the

process that gives rise to it.

8. Ringing - is unwanted oscillation of a signal, particularly in the step response (the response to a

sudden change in input).

9. Single ended signaling - the simplest and most commonly used method of transmitting electrical

signals over wires. One wire carries a varying voltage that represents the signal, while the other

wire is connected to a reference voltage, usually ground.

10. SLIC - a modular electronic circuit on a printed circuit board that interfaces with a

telecommunications access network.

Review Questions

1. Why are the transmitted and received signals on the local loop referred to as balanced

signals?

a. Since it uses two wires and it measures the signal as the difference between the two

signals.

2. Briefly explain why 2W to 4W conversion is required in the analog line interface.

a. Since digital circuits used in today’s telephone required a single ended output to

communicate, 4W connection is needed but since 2W connection will provide lesser

amount of money spent to connect all other telephone set to the central office, a 2W

connection will be needed thus a conversion between the two is a must to enjoy the

two different worlds of the 2W and 4W circuit.

3. Why do the transmitted and received voice signals travel on a single pair of wires between

the telephone set and the analog line interface in the central office?

a. Since via the property of a wave in which it will not be destroyed even if it collides

with an opposing wave. The two waves will just pass through each other.

4. Why 2W/4W conversion is also referred to as hybrid function?

a. Since we are using two different kind of circuits and we are getting the best of the

two circuits combining it into one system which is why it is called a hybrid

circuit/function.

5. Describe the 2W/4W conversion performed by the SLIC in the analog line interface.

a. It uses a matching transformer also called a balun which converts a balanced signal to

an unbalanced signal similar to how an antenna balanced signal is converted to the

TV’s unbalanced signal requirements.