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CHAPTER # 1

TELEGRAPHYOBJECTIVES: Upon completion of this chapter the students will be able to :

1. Understand the necessity, types and application of telegraph codes.

2. Compare the telegraph codes.

3. Identify types of telegraphy .

4. Working principle of machine telegraphy .5. Salient features and applications of teleprinter .

1.1 INTRODUCTIONTelecommunication is that branch of electrical engineering , which deals with the

transmission and reception of information between two distant stations. It means that for

telecommunication there basic things are required which are transmitter , receiver , and meaning

or sense can be derived

Telecommunication covers various methods used for communication of

information at a distant station . Telecommunication includes Telegraphy , Telephony facsimile,

Television Telemetering , Radar , Internet , etc . In all these systems the information are

interchanges in the form of electric signals. In this chapter only telegraphy has been described.

Telegraphy is that branch of telecommunication , which deals with interchange of information between two distant station in the form of code . It is a combination of two words ,

“Tele” means at a distance and “Graph” means code. Hence telegraphy is to write at a distance .

In the eatly days of civilization , there were various methods of transferring information

(messages) over long distances these methods went on improving with the passage of time andtoday we have the most modern methods of communication .

Telegraphy was the first electrical communication system that allowed the people to

communicate with each other beyond the limited ranges of voice and vision .

Telegraphy depends upon the transmission of electrical signals, which are arranged according to

some definite code. The information to be sent is first converted into some form of code for ease

of transmission and reception . Codes are symbols that represent units of information

understandable by both sender and receiver.

Transmitter Receiver

1.2 TELEGRAPH CODE:In telegraphy , messages or information to be interchanged between distant station are

first changed to be telegraphic codes in terms of current flowing and current not flowing . In some

telegraphic codes currents flows for short time or current flow for long time.

Telegraph codes can be classified into , (1) unequal length code and (11) equallength code.

In the unequal length code all the characters are represented by marks and spaces,

and the time of transmission for each code is not the same (equal) . There is no limit to the number of

characters combinations possible from this code. The most common unequal length code is the Morse

code .

In the equal length code each character consists of equal number of elements , each

Having the same duration , which is also the basic time unit of the system . Five – unit

Code is an equal length code and is most extensively used in machine telegraphy.



MORSE CODEIt is named in honour of Samuel F.B Morse, an American who invented the

Telegraph key with the help of which , telegraphic codes are made and transmitted .He also

Worked on how different characters, numbers etc are coded in telegraphy . The Morse

Code basically consists of two elements , the dot (.), called ” dit ”, which is shortest

Element .also called unit element and the dash(-), called “dash”, which is the longerElement . the dot is about is one-fourth the duration of a second. The dash is three times longer than the

dot. The relative durations of dots, dashes and other elements have been

Fixed by international agreement. Thus :

A dot length =1/4th

of a second.

A dash length = 3 dots length .

Spacing between elements of a character = 1 dot length .

Spacing between character of a word = 3 dots length.

Spacing between the words = 7 dots length

The international Morse Code for alphabets, numbers and punctuations are given in table

TABLE No 1.1 International Morse code for alphabets. Numbers and punctuations

Character Morse Code Character Morse Code Number Morse Code

A B

C

D

E

F

G

H

I

JK

L

M

N O

P

Q

R

S

T

U

V

WX

Y

Z

1 2

3

4

5

6

7

8

9

0

1.2.2 SINGLE AND DOUBLE CURRENT MORSE CODE SYSTEMThe Morse code system may be either single current or double current . In the

single current system single battery is used. The current on the line flows only in one

direction. There is current on the line when dot or dash is transmitted and no current

when neither dot or dash is transmitted. The advantage of single current system is low

power consumption. However, the flow of current is slower and thus is a disadvantage.

In the double current system two batteries of opposite polarities are used. There is

is always current on the line, which may be either positive or negative. For dot or dash the

current on the line is positive while for space it is negative. The double current system is

more sensitive and speedier than the single current system.The following illustration shows waveforms of current on the line for the words

“USE OF CODE

FIVE UNIT CODEIt is a binary code of equal length. Each character consists of five elements of

Equal time duration. In this system the maximum number of possible code combinations is 25 =32, which

means that this code is limited to 32 different code combinations. These



This 32 code combinations are insufficient to represent alphabet, number and different punctuations. In

order to cope with these difficulties, shift codes have been incorporated, which increase the number of

code combination to 64. Six combination are used for function codes, Thus we have only 58 code

combinations. This shows the limitation of the code to provide extra punctuation and computing codes.

The five-unit code uses letter shift and figure shift to increase the number of code combinations.

With letter shift , the machine is set to print letters only and this condition remains unchanged, until figure

shift is received , where upon all combinations are printed as figures.The 5-unit code, also known as CCITT (consultative committee for International Telegraph and

Telephone) Code No.1, is shown in the table No. 1.2.

TABLE No. 1.2

S. No. Letter Case. Figure Case. No. of

CodeElements

1 2 3 4 5

1 A 1 1 0 0 0 0

2 B 8 0 0 1 1 0

3 C 9 1 0 1 1 0

4 D 0 1 1 1 1 0

5 E 2 0 1 0 0 0

6 F * 0 1 1 1 0

7 G 7 0 1 0 1 0

8 H + 1 1 0 1 0

9 I * 0 1 1 0 0

10 J 6 1 0 0 1 0

11 K ( 1 0 0 1 1

12 L = 1 1 0 1 1

13 M ) 0 1 0 1 1

14 N * 0 1 1 1 1

15 O 5 1 1 1 0 0

16 P % 1 1 1 1 1

17 Q / 1 0 1 1 1

18 R - 0 0 1 1 1

19 S . 0 0 1 0 1

20 T * 1 0 1 0 1

21 U 4 1 0 1 0 0

22 V ! 1 1 1 0 1

23 W ? 0 1 1 0 1

24 X , 0 1 0 0 1

25 Y 3 0 0 1 0 0

26 Z : 1 1 0 0 1

27 Carriage Return 1 1 0 0 0

28 Line Feed 1 0 0 0 1

29 Letter Shift 0 0 0 0 1

30 Figure Shift 0 0 0 1 0

31 Error 0 0 0 1 1

32 Instrument idle 0 0 0 0 0

0 = indicates space.



1. = indicates marks.

* = free for internal use by a country or administration.

1.2.4 SEVEN AND A HALF UNIT CODE (7.5 Unit Code)

In this system it is necessary to start the receiving mechanism before the code

Combination received and stop it after the code combination has been received. In order

to do this, the 5-unit code is preceded by a start element, which is always a space and

followed by a stop element , which is always a mark. With the addition of these two elements thecode becomes the start – stop code and is used in the teleprinter. This code is also called teleprinter

code . The duration of the start element is the as that of 5-unit code elements but the stop element is

1.5 time the duration of the 5-unit code elements, that,s why this code is called 7.5 unit code or

CCTTT code No. 2 and is shown in the table No. 1.3.

Table No .1.3.

S.No. Letter

Case

Figure

Case No. of

code

Elements

Start 1 2 3 4 5 Stop

1 A 1 0 1 0 0 0 0 1

2 B 8 0 0 0 1 1 0 1

3 C 9 0 1 0 1 1 0 1

4 D 0 0 1 1 1 1 0 1

5 E 2 0 0 1 0 0 0 1

6 F * 0 0 1 1 1 0 1

7 G 7 0 0 1 0 1 0 1

8 H + 0 1 1 0 1 0 1

9 I * 0 0 1 1 0 0 1

10 J 6 0 1 0 0 1 0 1

11 K ( 0 1 0 0 1 1 112 L = 0 1 1 0 1 1 1

13 M ) 0 0 1 0 1 1 1

14 N * 0 0 1 1 1 1 1

15 O 5 0 1 1 1 0 0 1

16 P % 0 1 1 1 1 1 1

17 Q / 0 1 0 1 1 1 1

18 R - 0 0 0 1 1 1 1

19 S . 0 0 0 1 0 1 1

20 T * 0 1 0 1 0 1 1

21 U 4 0 1 0 1 0 0 1

22 V ! 0 1 1 1 0 1 123 W ? 0 0 1 1 0 1 1

24 X , 0 0 1 0 0 1 1

25 Y 3 0 0 0 1 0 0 1

26 Z : 0 1 1 0 0 1 1

27 Carriage

Return

0 1 1 0 0 0 1

28 Line Feed 0 1 0 0 0 1 1



29 Letter

Shift

0 0 0 0 0 1 1

30 Figure

Shift

0 0 0 0 1 0 1

31 Error 0 0 0 0 1 1 1

32 InstrumentIdle 0 0 0 0 0 0 1

1. TYPES OF TELEGRAPHY

1. MORSE TELEGRAPHYMorse code telegraphy is a manual telegraphy . At the sending end Morse key is

Pressed and current flows to the line . At the receiving end this current passes through the

Pressed and current flows to the line .At the receiving end this current passes through the

windings of a telegraphy relay. The relay closes a circuit for the buzzer to operate.

The Morse telegraph system is a simple electric circuit consisting of the following

Components:

1.

A battery: This acts as a source of electrical energy in the circuit. 2. Morse key: It is a manual key and acts as a switch to open and close the circuit. It

is also called a transmitter . Thus causing pulses of current to flow on the line.

When the operator starts opening and closing of the key . the messages and

information are coded and sent in the communication line, the transmission of

information and encoding starts at the same time.

3 Communication line: Acts a medium for the transmission of information

between the two stations. The dot and dashes flows on the line in the form of electrical

energy and are carried instantly through the wires from the transmitter to the receiver.

4 Receiver: The receiver used is called a sounder, because it produces click like

Sound . The receiver consists of an electromagnet and a movable armature. When a

Pulse of current passes though the electromagnet, it attracts the armature and a

Click is produced . When the pulse stops, the armature returns to its normal position

With the help of a spring and again makes a click . The time between the clicks represents

A dot or a dash . These clicks are heard by an operator and are decode into messages

In the written form by him.

The Morse telegraph system is illustrated in the figure No.1.3. (A,B)

1.3.2. MACHINE TELEGRAPHYWith the passage of time , the telegraph traffic went on increasing as

more and more people benefited form it, so the manually operated telegraph circuits were

not able to meet this rapidly increasing demand. To cope with this difficulty , machine

telegraphy was invented . With the invention of teleprinter machine and multiplex

operation of the line, the traffic handling capacity of the line has increased to a large

extent.

A teleprinter is a telegraph transmitting and receiving machine operating on the start-stop

principle. It is equipped with a typewriter like keyboard and receive capable of printing

characters from signals received over a telegraph circuit . The teleprinter is also capable of

printing a local record of signals sent from the keyboard

The teleprinter is a mechanical device driven from an electric motor. Other main

Electrical units are:

1. transmitter contacts, controlled from the keyboard.



2. Receiving electromagnets, controlled by signals received from the line or the local

transmitter.

Recently electronic machine have been developed . These machines are microprocessor

controlled, have keyboard , processing circuits and printers.

2. CABLE TELEGRAPHYThe cable signaling can be done over cables and to day telegraph communication

Between any two cities can be made by submarine and landline cables.The transmission and reception over cables are unaffected by atmospheric noise,

ionospheric variation and various other factors. Also a greater amount of secrecy is available

with the cable system . The cable telegraphy use cable code, which is basically a Morse code.

Its distinctive features lie in the way of transmitting the code elements Cable code has the

following two types:

1: NORMAL CABLE CODE (NCC)

In this code, dot is represent by positive current, dash by negative current of

Equal duration and spacing between letters by no current . The difference in the dots and

dashes are, current does not flow for the entire duration of a dot or a dash but for 85% of

it. For the rest of 15% of the signal no current flows on the line.

Normal cable code is a three level code, means that there are three differentcondition on the line:

a. Marking (positive battery is connected to the line )

b. Spacing (negative battery is connected to earth and no current flows)

c. Earth (when the line is connected to earth and no current flows)

Dot = 85% M + 15% E

Dash = 85% S + 15%E

Space = 100% E

2. DOUBLE CURRENT CABLE CODE (DCCC)

In this type, the line is never earthed and there is always current on the line , may

Be positive or negative . Double current cable code is a two level system .

Dot =50% S + 50% M

Dash = 50% M + 50% S

Space = 100% S

1.4 TWO-WAY MORSE TELEGRAPH SYSTEM

A simple Morse telegraph circuit, to work in either direction is shown in the figure

No.1.4.

When both keys are in their normal positions, as shown in figure, the batteries are

disconnected and no current flows in the line. Thus no transmission and reception of signals takes

place. When the key at station A is pressed, the battery is connected and current from this batteryflows in the line. This current is received at station B and the sounder at this station is energized

and de-energised, due to the closing and opening of the key at station A, thus producing click like

sounder. The earth provides the return path for the current.

The same process will take place at station A when key at station B is pressed .

Thus , if station A transmits, station B receives and vice versa.

1.5 AUTOMATIC MORSE WORKING TRANSMITTER AND RECEIVER

In manual operation the telegraph circuit cannot be used efficiently, because the



Rate of transmission of information in the manual operation is low. To achieve high-speed

Transmission and to make the system efficient, the manually operated keys have to be

Replaced by mechanical transmitters. The mechanical transmitter transmits signals to the

Line at a higher speed than that transmitted manually. The operator first of all types the

information on an instrument called a keyboard perforator. The perforator punches holes

In a paper tape according to Morse code. The information is stored on the perforated tape in the

coded form. For transmission, this perforated tape is passed through an automatic transmitter,which reads these perforations and transmits corresponding electrical signals. Thus the signals are

transmitted with a maximum speed of 200 words per minute. An receiver receives these signal at

the receiving side. These signal actuate a relay and the relay in turn controls a re-perforator,

according to the signals received . The re-perforator produces and exact copy of the transmitted

tape at the receiving side. Thus the message is received and stored in the coded form on the tape .

This tape can also be used for further transmission if needed or it may be passed through a

printer, which encoded the coded message into the original form and printed on a paper.

1.6 FOUR CHANNEL MULTIPLEX SYSTEM

A Baudot system with four channels is also known as quadruple Baudot

operation. This system is an example of time division multiplexing (TDM) In the TDM severalchannels can be transmitted on a single line, one after the other. Each channel uses the line for a

certain time by turn. A four channel multiplex system is described below:

The Baudot system uses 5-unit code and the basic principle of four channels (one way) multiplex

system is shown in the figure No.1.5.

An outer ring has been decided into four quadrants. Each quadrant is further divided into five

equal segments. Thus there are 20 equal segments. These segments are insulated from one

another . Each segment is connected to a transmitter contact switch. Within the segmented ring

there is connected ring called line ring. The outer ring and the line ring are connected together by

conducting brushes mounted on a metallic arm, but insulated from it, and driven at a control

speed by motor . A similar arrangement is done at the receiving side, but here each segment isconnected to receiver selecting magnets. The speed of rotation and phase are controlled , so that

signals transmitted from any segment at one station always arrive at the corresponding receiver

segment at the receiving station.

At the transmitter, the five contact switches are connected at one side to the five segments

of a channel and at the other side to a battery source. At the receiver the five selecting magnets

ate connected to the five segments of a channel. All the four channels are similarly connected .

Depending upon the contact switching position current will flow on the line. The contacts of the

switches are set according to the Baudot code (5-unit code) combination. When the brush is

passing over segment 1 of cannel 1 and if the contact of switch 1 is closed , then current flows

on the line . This current operates a relay P at the receiving side . The current path will be battery

, contact of switch 1 , segment 1, brushes, line ring, line , relay P, earth, P1 contact of relay P

at the receiver, which is connected to battery , operate the corresponding receiver selectingmagnet of segment 1 . Similarly , when brushes pass over segment 2 to 5 each channel, current or

no current flows on the line. Thus any code combination can be sent on the line, by setting of

contacts, will be received when the moving contact of the receiving relay is connected to the

corresponding segment of the same channel . The receiver, s selecting magnet actuates a printer

that prints the characters.

The transmitting and receiving equipment must operate in synchronism, i.e., the speed of

rotation and phase of the brushes of the transmitter and receiver must be maintained.



In practice, two extra correcting segments in the outer ring are permanently connected to

the positive and negative battery . The second ring insulated from other rings has one segment

and is connected to a correcting magnet . If the transmitter and receiver are not synchronized,

then the mechanism is such that the correcting magnet gets energized causing the distributor to

come back to synchronism. The brushes at one station run faster for this purpose. The signal

transmitted will take some time to arrive at the receiver. So the brushes at the receiver should lag

the brushes at the transmitter, by an amount equal to the transmission delay time . The principleof phase correcting arrangement in multiplex system is shown in the figure No.1.6.

1.7 SALIENT FEATURES OF TELEPRINTER

The salient features of teleprinter are listed below:

a. A teleprinter is a telegraph transmitting-receiving machine.

b. It resembles a typewriter because it has a type writer like keyboard.

c. A teleprinter is an electromechanical device driven by electric motor. However,

recently electronic machines have been introduced, controlled by microprocessor.

d. Code use by teleprinter machine in the 5-unit code, operating on double current

basis.e. A teleprinter works on the start-stop principle.

f. A teleptinter when used as a transmitter, takes the code in the form of parallel

signals and send it as serial signals. Parallel signals means that all the signals are

available at the different terminals simultaneously. Serial signal means that thesignals are send one after the other in a given order of lower to higher or first to

last. So, we can say that the transmitter is simply a parallel-to – serial converter.

g. When use as a receiver, the signal are received in the serial form and converted

into parallel . Then a detector converts it into the character and the printer prints

it on the paper.

h. Every teleprinter has also the facility of local record.

i. The actual mechanical arrangement of a teleprinter machine is very complicated,

but a block diagram is shown in the figure No.1-7, which shows the different partsof the machine.

1.7.1. APPLICATION OF TELEPRINTER

The following is the application of the teleprinter machine

1. The telelprinter is basically used for sending and receiving of telegraph signals.

2. Teleprinting switching system is used in Telex (teleprinter exchange) which is a

convenient method of sending printed messages. An auto-telex service has the

advantage of communication as in telephone system and transfer of written record

as in telegraph system.3. Due to these two advantages, these services are used both for commercial and

industrial puprposes.

4. The telprinter can also be used for typing the local records.

SUMMARY

1. Communication is the transfer of information from one station to

another station .



2. Telegraphy is aprocess in which transmission and reception of

information take place in the written form .

3. The information is converted into electrical signals for ease of

transmission and reception .

4. Codes are symbols that represent units of information .

5. Code is a system of signs used for secrecy or economy in sending

cables.6. Telegraph codes are of equal and unequal length.

7. Morse code consists of dots, of short duration , and dashes, of long

duration .

8. Morse code system may be either single current or double current.

9. In the 5-unit code each character consists of 5 element of equal time

duration .

10. In the 7.5-unit code is preceded by a start pulse, which is a mark and is

followed by a stop pulse, which is a space.

11. Morse , machine and Cables are the types of telegraphy.

12. Machine telegraphy is based on the principle of start-stop.

13. The teleprinter is a telegraph transmitting and receiving machine, having

typewriter like keyboard.

CHAPTER # 2

TELEPHONE INSTRUMENTSOBJECTIVES: Upon completion of this chapter the student will be able to :

Understand various telephone instruments.

Understand automatic telephone system.To draw block diagram of an electromechanical telephone set.

Understand the working of transmitter, receiver, dialing and signaling.

Understand side tone and anti-side tone circuits.

2.1 INTRODUCTION

Telephony is that branch of telecommunication, which deals with the

transmission of speech at a distance. It differs from telegraph in the type of

electrical signal used for transmission of information . In telephone system sound

energy (speech) is converted into electrical energy at the transmitting side and this

electrical energy is sent toward the receiving end through a communication

medium where it is again converted into sound energy, to reproduce the original

speech.

In the telephone system a person having a telephone connection is calledsubscriber. Each subscriber is provided with a telephone set. The subscriber

telephone set consists of transmitter, a receiver, a bell, a dial and a speech circuit.

The transmitter converts the sound waves into electrical signals and requires d.c .

current for its operation . The receivers does the reverse function of a transmitter

and does not require any d.c current. The bell is used to draw the attention of the

called subscriber. The dial is used to produce the required pulses in accordance

with the number dialed. In this chapter we will discuss the various telephone

instruments in detail.



1. AUTOMATIC TELEPHONE SYSTEMthe telephone system is an arrangement of electrically interconnected

transmitters and receivers, by means of which speech is communicated between

two points, either close together or remote.

In the telephone system the two-way communication is carried by

either a two wire or a four-wire system. The two wire system are less costly. Ageneral description of the automatic telephone system is given below and is

illustrated with the help of a block diagram.

A two-wire line from the telephone set of the subscriber terminates in

the distribution point (D.P), which consists of ten pairs for connecting ten

subscriber. These ten pairs are extended in the form of underground cable to the

cabinet. The under cable runs from the cabinet to the exchange where it terminates

in the main distribution frame (M.D.F) . The M.D.F connect the external and

internal parts of the system. From the M.D.F every pair of the individual subscriber

is extended to the subscriber line circuit. (S.L.C) inside the switch room . The

S.L.C with the help of control set seizes the free line finder when the subscriber

lefts his handset. The line finder ( L.F ) seized the free outlet in the 1st

group

selector (G.S) and subscriber gets dial tone, which means that the system is nowready for dialing. When the subscriber dial the 1st digit, 1st G.S is followed by the

2nd G.S .”is followed by the 3rd G.S and so on up to final selector (F.S) the

number of G.S depends upon the number of digits. From the F.S the line is

extended to the S.L.C of the called subscribers. The loop is completed and thecalling subscriber receives ring back signal from the F.S while the called

subscriber is ring up.

The sketch shown in the figure No 2-1 is a very limited system. In modern

system many paths are available to allow connection of any given telephone to

another selected from the hundred of millions of telephone available throughout

the world. The transmission links in modern telephone systems may take several

different forms, such as overhead wire, buried cable, microwave links, fiber optics,

satellite links and so on. All these systems of transmission are shown in the figureNo. 2-2.

Figure No.22:- Different transmission links in modern telephone system,.

2. BLOCK DIAGRAM OF AN ELECTROMECHANICAL TELEPHONE SETThe block diagram of an electromechanical telephone set is shown in the

figure No.2.3

1. PARTS OF AN ELECTROMECHANICAL TELEPHONE SETAn electromechanical telephone set consists of the following parts:

1. Transmitter or Microphone

2. Receiver

3. Rotary or mechanical dial4. Hook switch or switch connection to the telephone system.

5. Anti-side tone circuit

6. Magneto-bell

2. TELEPHONE TRANSMITTERMicrophone is telephony is regarded as transmitter. It is a transducer, which converts sound

energy into electrical energy. There are different types of transmitter but carbon granules



transmitter is the most widely used in the handset of the modern telephony. We will discuss

the carbon granule transmitter only.

It is based on the principle that the resistance of carbon granules is inversely proportional

to pressure. The constructional details of the carbon transmitter, is illustrated in the figure

No.2-4

It is the property of carbon that is resistance varies with pressure. The carbon transmitter

does not produce any e.m.f but only change its resistance with the changing pressure.Carbon granules are placed between two electrodes in an insulated chamber. One

the electrode is fixed to the back of the chamber while the other electrode is attached with the

movable diaphragm. The two electrodes are connected with the battery. The transmitter offers

an electrical resistance to the flow of current , which is the resistance of the carbon granules.

When the diaphragm moves inward and outward, with sound pressure, the pressure on the

carbon granules also changes. Thus the resistance of the carbon granules also varies with the

changing pressure and hence the current flow between the two electrodes also varies. A

current variation, corresponding to the sound pressure is generated in the circuit.

The electrical equivalent circuit of the carbon transmitter is shown in the figure No.2-5

The current flowing between the electrodes can be calculated as:

R= Static resistance of the microphone (when there is no sound pressure)Then, I=E/R

Suppose the sound pressure striking diaphragm =p sin(wt)

The varying sound pressure will vary the resistance accordingly about R.

If the varying resistance is r sin (wt)Where r is change in the amplitude of instantaneous resistance.

Hence the total resistance will be = R + r sin (wt)

The plus, minus signs shows maximum and minimum pressure respectively.

Suppose the pressure is minimum, then:

The total resistance = R + r sin(wt)

Therefore, I = E/(R+ r sin(wt) (where is the instantaneous current )

I = E/R {1/1+(r/R) sin(wt) }

I = I { 1+(r/R) sin(wt) }-1

Let, m=r/R

I = I {1+m sin(wt)}-1

As r<<R, therefore, m<1

Expanding, {1+m sin(wt) }-1 by binomial theorem, we get :

I = 1+ {1 – m sin(wt) + m2 sin2 (wt) – m3 sin3 (wt) + - - - - - - - - - - - - } .

M sin(wt) = 1st harmonic (fundamental)

M2sin

2(wt) = 2

ndharmonic

M3sin

3(wt) = 3

rdharmonic

Hence it is clear that the 2nd and 3rd harmonic current are also produced in the transmitter. As the value of

m is very small, so neglecting m2 , m3 , etc. We get:

I = I {1+m sin(wt) } = 1+1.m.sin(wt)

This equation is similar to the equation of amplitude modulation . Hence we can say that thetransmitter acts as a modulator of dc current. The carbon granules transmitter can also be considered as an

a.c generator, producing an alternating current of I.m.sin(wt) . This current is the output of conversion of

sound energy into electrical energy. The dc current through the transmitter is essential and is known as

“energizing current”.’ Otherwise, the transmitter appears as dead.

Advantages :a. It is a cheap.

b. It is very sensitive and gives good services.

c. It does not need any amplification.



disadvantages:

a. It produces harmonics.

b. The change in resistance in non linear, thus resulting in amplitude distortion.

c. It produces background noise.

d. It frequency response is limited, so frequency distortion occurs.

3. TELEPHONE RECEIVERThe sound producer is telephony is called receiver. The receiver does the reverse

function of a transmitter. It is a device, which converts electrical energy into sound energy.

The constructional detail of the telephone receiver, is shown in the figure No.2-6

It consists of a diaphragm, permanent magnet and windings of electromagnet. When the

incoming signal current passes through the windings of an electromagnet, magnetic flux is

produced. The magnetic flux follows the magnetic path, which consist of the iron path of the

permanent magnet, the poles pieces, diaphragm and the two air gaps between the diaphragm

and the pole pieces. Thus a varying magnetic pull is produced, which causes the diaphragm tovibrate in accordance with the signal current received and hence produce the sound. The

permanent magnet is used to polarize the receiver the pull on the diaphragm depends upon the

magnetic flux density in the air gaps between the diaphragm and the pole pieces. Thus

maximum change will be produced when the current changes in the coil.The effects on the polarizing the receiver can be explained as follow:

Consider the figure No.2-7 , in which in electromagnetic and a diaphragm is shown.

If Fm is the magnetic pull on the diaphragm. Then,

f m a B2

(where B = magnetic flux density)

if the speech current following in the windings is :

i= I sin(wt)

since, B a ithen Fm a i

2

Fm = k i2

(where k is a constant of

proportionality)

Fm = k { I sin (wt) }2

Fm =(1/2) k.12 (1 – cos2 wt)

As the magnetic pull is proportional to i2. The current is alternating, consisting of positive and

negative lobes, but its square is always positive hence causing in amplitude distortion.

Also, the frequency is twice that of the transmitting current. This difficulty is constant

static pull on the diaphragm.

Hence Fm a (B0 )2

Where B0 is static magnetic flux density.

As i = I sin(wt)

B = magnetic flux density due to electromagnet.

B = total flux density

Then, b = B0 + B



B = B0 + b1 sin(wt)

Therefore, Fm =k b2 = k {B0 + b1 sin(wt) }2

Fm = k { B02 + 2B0 b1 sin(wt) + b1

2 sin2 (wt) }

Fm = k { B02 + 2B0 b1 sin(wt) + (1/2)b1

2 (1 – Cos2 wt) }

Fm = k { B02

{ 1+2 b1 sin(wt) + (1/2).B02)b1

2(1 Cos2 wt) }

If (b1/B0 2 <<1Then the equation reduces to :

Fm = k B02 + 2kB0

2 b1 sin(wt)

This equation consist of :

1. A dc term : responsible for the high value of polarizing flux, thus increases the

intensity of the sound wave and decrease the second harmonic distortion, by keeping b1

/B0 value small.

2. Fundamental signal term : responsible for vibration of the diaphragm to produce

the sound waves.

3. The introduction of the permanent magnet in the receiver produces an output of

correct frequency and of greater magnitude. Thus increases the sensitivities of the

receiver.

4. ROTARY OR MECHANICAL DIALA dial is an impulse sending device. When the subscriber lifts his handset, the D.C.

loop is completed and steady current flows through the line provided by the exchange. The

impulsing cam of the dial break the circuit as many times as the number dialled, thus

producing pulses of current and sends it over the subscriber, s line to the exchange.The dial consists of finger plate with ten holes in it. These holes are equally spaced

around 2/3rd of the outer ring of the finger plate. The number 1 to 0 are written on a number

ring below the finger plate. These number can be seen through the ten holes of the finger

plate. There is a finger stop adjacent to the digit “0” . The rotation of the finger plate isclockwise against the tension of a spring that restore it to its normal position, after dialing a

digit. During the anti clockwise or reverse movement of the finger plate, the speed of the place

is kept constant with the help of a “Governor” the governor is a mechanical device and

consists of a number of weights on the spring. It moves along with the dial through gearassembly.

The dial also consists of an impulsing contact. This contact is normally closed. When the dial

operates, a cam open and closes this contact, thus producing pulses of current on the line. The

number of pulses produce corresponds to the number of times the contact closes. The

adjustment of the dial is such that it produced 10 pulses per second, whereas + 10%

variation is acceptable. The number of pulses generated by the dial corresponds to the digit

dialed. For example,.

By dialing digit 1, one pulses will be produced.

By dialing digit 2, two pulses will be produced.

By dialing digit 3, three pulses will be produced.

By dialing digit 0, ten pulses will be produced.The dial also consists of off-normal contacts. These contact are normally open or off. These

contacts remain close, when the dial move forward and then come back to rest position. There

are two advantages of these contacts, (1) when dialing, due to the interruption of current

clicks are produced and are not acceptable. The off-normal contacts are used to bypass the

speech circuit when current from the exchange through then and (11) when the contacts

closes, the impedance of the line reduces, hence the current increases. The constructional

detail of the rotary or mechanical dial is shown in the figure No. 2.8



Figure No. 2.8 : - Constructional details of rotary dial.

The mechanism of the dial is such that it has to provide the necessary pause .

Between two successive digits. This pause is known as inter-digit-pause is known as iner-

digit-pause and is necessary for the switching circuit in the exchange distinguish between the

digits dialed by the subscriber. This inter-digit-pause is provided by the space between the

first hole of the finger plate marked “1” and the finger stop. This extra distance produces two

extra pulses with every digit dialed. In one type of dial, the two extra pulses were suppressedby short circuiting the impulsing contacts for the last two pulses. While in the other type, the

first two pulses are not generated by the mechanical design of the cam, which open the

contacts.

The opening and closing of electrical contacts by a cam that revolves with the dial and

engages with a joint are shown in the figure No.2.9. from (a) to (h), for dialing the digit 5.

Figure No. 2.9: From (a) to (h) for dialing digit.5

SIGNALLINGTo control the switching mechanism in the exchange from the subscriber, s telephone

set, control signals are needed. These signal are transmitted as information on the same wire

lines. The switching mechanism in the exchange is provided with suitable equipment that

takes signaling information transmitted by the subscriber, s telephone set and sends back thesignaling information to the set. These signaling are : (1) DC signaling, (11) tone signaling,

and (111) common channel signaling .

DC signaling is based on the changes in the DC loop. The state of the signal

indicates” NO-HOOK: When the telephone is not in use, the handset in on the cradle and operates a

switch an no current flow in the non-hook condition

OFF-HOOK: Removing the handset from the cradle operates the switch, which complete the

DC loop of the subscriber circuit and current flows in the circuit.

This change from the open circuit to close circuit represents the seize signal. If the

telephone is being called, the change represents the accept signal. In both cases the

replacement of the handset at the end of a call is the clear signal.’ Clear forward if it is the

calling terminal and clear back if it is the called ternDIAL PULSES: To interrupt the DC path of the subscriber , s loop for a specific number of

short periods to show the number dialed . This is called loop disconnect signaling. The loop is

disconnected as many times as the digit dialed by the subscriber.

STATES OF INTERCONNECTION : DC signaling, called reverse battery signaling, is used

between exchanges to indicate the state of the switched condition.

In tone signaling different tones are used for both control and states indications. The

tones may be single frequency or combination of frequencies. These are analog signals that

are either continuous or ON and OFF tones are different rates. The call progress tones are sent

by the exchange to the calling party to inform him about the states of the call .” these are listedin the table No.2.1.

Table NO.2.1

Tone Frequency (Hz) ON time (sec) OFF time (sec)Dial 350 + 440 Continuous

Busy 480 + 620 0.5 0.5

Ring back, Normal 440 + 480 2 4

Ring back, PBX 440 + 480 1 3

Congestion (Local 480 + 620 0.2 0.3

Record (Local) 480 + 620 0.3 0.2

Receiver off-hook 1400+2060+2450+2600 0.1 0.1



No such number 200 to 400 Continuous frequency

modulated at 1

Hz rate.

The common channel signaling is a special method of signaling which uses no-

voice circuit for control and addressing purposes. Computer manages the system. This will be

discussed later in chapter # 3.

1. SIDE TONEWhen a speak in front of the telephone transmitter some portion of our own voice is fed

back to the associated receiver, we hear our own voice in the receiver as a feedback. With the

help of this feedback we are able to the adjust the volume of our voice according to different

situations. When the level of this feedback is high, we lower our voice and when it is low, we

speak louder. This feedback voice is called side tone.

Side tone may be defined as the reproduction of sound in the receiver picked up from the

associated transmitter. Or the amount of voice power coupled from the transmitter to the receiver

of the same telephone.

Too much side tone and complete absence of it is undesirable. In the former case, the

speaker will keep his level of volume of voice very low and hence will affect the out put of thetransmitter. In the latter case, the telephone will appear dead to the subscriber and this is a very

uncomfortable feeling.

The desirable amount of side tone is that which we have in our daily. free air

conversation with each other across the table.

1. ANTISIDE TONE CIRCUITTo control the level of the side tone to the desirable amount anti-side tone induction

coil (A.S.T.I.C) is used in the subscriber, s telephone set. The anti-side tone induction

coil serves the following.”

a. Control the level of the side tone to the desirable amount.

b. Ensures that no dc current flows through the receiver.

c. Matching the Impedance of the transmitter and receiver with that of the line.A simple circuit arrangement for the reduction of side tone is shown in the figure

No.2-8

The principle of the arrangement can be explained as follow:

Consider the figure No 2.10 (A) in this case the transmitter is transmitting. If ZB = ZL

and L1 = L2 then the transmitter current divides equally in L1 and L2 . The magnetic

field produced by these two windings will be equal and opposite and hence cancel

each other, s effect, thus no emf is induced in L3 in the side tone is completely

eliminated. Practically L1 and L2 are not exactly equal and hence some emf in induce

in L3, thus the receiver receives apportion of a transmitter current.In figure No2-10(B) the receiving current passes through L1 and L2, the magnetic

field produced are in the same direction and reinforced each other effect. An emf isinduced in the receiver circuit and current flows through the receiver.



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