NCERT Textbook: Communication Systems | Physics Class 12 - NEET PDF Download

 Page 1


15.1  INTRODUCTION
Communication is the act of transmission of information. Every living
creature in the world experiences the need to impart or receive information
almost continuously with others in the surrounding world. For
communication to be successful, it is essential that the sender and the
receiver understand a common language. Man has constantly made
endeavors to improve the quality of communication with other human
beings. Languages and methods used in communication have kept
evolving from prehistoric to modern times, to meet the growing demands
in terms of speed and complexity of information. It would be worthwhile
to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19
th
 and 20
th
 century in
the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander
Graham Bell. The pace of development seems to have increased
dramatically after the first half of the 20
th
 century. We can hope to see
many more accomplishments in the coming decades. The aim of this
chapter is to introduce the concepts of communication, namely the mode
of communication, the need for modulation, production and deduction
of amplitude modulation.
15.2  ELEMENTS OF A COMMUNICATION SYSTEM
Communication pervades all stages of life of all living creatures. Irrespective
of its nature, every communication system has three essential elements-
Chapter Fifteen
COMMUNICATION
SYSTEMS
© NCERT 
not to be republished
Page 2


15.1  INTRODUCTION
Communication is the act of transmission of information. Every living
creature in the world experiences the need to impart or receive information
almost continuously with others in the surrounding world. For
communication to be successful, it is essential that the sender and the
receiver understand a common language. Man has constantly made
endeavors to improve the quality of communication with other human
beings. Languages and methods used in communication have kept
evolving from prehistoric to modern times, to meet the growing demands
in terms of speed and complexity of information. It would be worthwhile
to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19
th
 and 20
th
 century in
the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander
Graham Bell. The pace of development seems to have increased
dramatically after the first half of the 20
th
 century. We can hope to see
many more accomplishments in the coming decades. The aim of this
chapter is to introduce the concepts of communication, namely the mode
of communication, the need for modulation, production and deduction
of amplitude modulation.
15.2  ELEMENTS OF A COMMUNICATION SYSTEM
Communication pervades all stages of life of all living creatures. Irrespective
of its nature, every communication system has three essential elements-
Chapter Fifteen
COMMUNICATION
SYSTEMS
© NCERT 
not to be republished
Physics
514
Around
1565 A.D.
1835
1876
1895
1936
1955
1968
1975
1989-91
Year Event Remarks
The reporting of the delivery of
a child by queen using drum
beats from a distant place to
King Akbar.
Invention of telegraph by
Samuel F.B. Morse and Sir
Charles Wheatstone
Telephone invented by
Alexander Graham Bell and
Antonio Meucci
Jagadis Chandra Bose and
Guglielmo Marconi
demonstrated wireless
telegraphy.
Television broadcast(John
Logi Baird)
First radio FAX transmitted
across continent.(Alexander
Bain)
ARPANET- the first internet
came into existence(J.C.R.
Licklider)
Fiber optics developed at Bell
Laboratories
Tim Berners-Lee invented the
World Wide Web.
It is believed that minister Birbal
experimented with the arrangement to
decide the number of drummers posted
between the place where the queen
stayed and the place where the king
stayed.
It resulted in tremendous growth of
messages through post offices and
reduced physical travel of messengers
considerably.
Perhaps the most widely used means of
communication in the history of
mankind.
It meant a giant leap – from an era of
communication using wires  to
communicating without using wires.
(wireless)
First television broadcast by BBC
The idea of FAX transmission was
patented by Alexander Bain in 1843.
ARPANET was a project undertaken by
the U.S. defence department. It allowed
file transfer from one computer to
another connected to the network.
Fiber optical systems are superior and
more economical compared to
traditional communication systems.
WWW may be regarded as the mammoth
encyclopedia of knowledge accessible to
everyone round the clock throughout the
year.
TABLE 15.1  SOME MAJOR MILESTONES IN THE HISTORY OF COMMUNICATION
© NCERT 
not to be republished
Page 3


15.1  INTRODUCTION
Communication is the act of transmission of information. Every living
creature in the world experiences the need to impart or receive information
almost continuously with others in the surrounding world. For
communication to be successful, it is essential that the sender and the
receiver understand a common language. Man has constantly made
endeavors to improve the quality of communication with other human
beings. Languages and methods used in communication have kept
evolving from prehistoric to modern times, to meet the growing demands
in terms of speed and complexity of information. It would be worthwhile
to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19
th
 and 20
th
 century in
the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander
Graham Bell. The pace of development seems to have increased
dramatically after the first half of the 20
th
 century. We can hope to see
many more accomplishments in the coming decades. The aim of this
chapter is to introduce the concepts of communication, namely the mode
of communication, the need for modulation, production and deduction
of amplitude modulation.
15.2  ELEMENTS OF A COMMUNICATION SYSTEM
Communication pervades all stages of life of all living creatures. Irrespective
of its nature, every communication system has three essential elements-
Chapter Fifteen
COMMUNICATION
SYSTEMS
© NCERT 
not to be republished
Physics
514
Around
1565 A.D.
1835
1876
1895
1936
1955
1968
1975
1989-91
Year Event Remarks
The reporting of the delivery of
a child by queen using drum
beats from a distant place to
King Akbar.
Invention of telegraph by
Samuel F.B. Morse and Sir
Charles Wheatstone
Telephone invented by
Alexander Graham Bell and
Antonio Meucci
Jagadis Chandra Bose and
Guglielmo Marconi
demonstrated wireless
telegraphy.
Television broadcast(John
Logi Baird)
First radio FAX transmitted
across continent.(Alexander
Bain)
ARPANET- the first internet
came into existence(J.C.R.
Licklider)
Fiber optics developed at Bell
Laboratories
Tim Berners-Lee invented the
World Wide Web.
It is believed that minister Birbal
experimented with the arrangement to
decide the number of drummers posted
between the place where the queen
stayed and the place where the king
stayed.
It resulted in tremendous growth of
messages through post offices and
reduced physical travel of messengers
considerably.
Perhaps the most widely used means of
communication in the history of
mankind.
It meant a giant leap – from an era of
communication using wires  to
communicating without using wires.
(wireless)
First television broadcast by BBC
The idea of FAX transmission was
patented by Alexander Bain in 1843.
ARPANET was a project undertaken by
the U.S. defence department. It allowed
file transfer from one computer to
another connected to the network.
Fiber optical systems are superior and
more economical compared to
traditional communication systems.
WWW may be regarded as the mammoth
encyclopedia of knowledge accessible to
everyone round the clock throughout the
year.
TABLE 15.1  SOME MAJOR MILESTONES IN THE HISTORY OF COMMUNICATION
© NCERT 
not to be republished
Communication System
515
transmitter , medium/channel and receiver. The block diagram shown in
Fig. 15.1 depicts the general form of a communication system.
FIGURE 15.1 Block diagram of a generalised communication system.
In a communication system, the transmitter is located at one place,
the receiver is located at some other place (far or near) separate from the
transmitter and the channel is the physical medium that connects them.
Depending upon the type of communication system, a channel may be in
the form of wires or cables connecting the transmitter and the receiver or
it may be wireless. The purpose of the transmitter is to convert the message
signal produced by the source of information into a form suitable for
transmission through the channel. If the output of the information source
is a non-electrical signal like a voice signal, a transducer converts it to
electrical form before giving it as an input to the transmitter. When a
transmitted signal propagates along the channel it may get distorted due
to channel imperfection. Moreover, noise adds to the transmitted signal
and the receiver receives a corrupted version of the transmitted signal.
The receiver has the task of operating on the received signal. It reconstructs
a recognisable form of the original message signal for delivering it to the
user of information.
There are two basic modes of communication: point-to-point and
broadcast.
In point-to-point communication mode, communication takes place
over a link between a single transmitter and a receiver. Telephony is an
example of such a mode of communication. In contrast, in the broadcast
mode, there are a large number of receivers corresponding to a single
transmitter. Radio and television are examples of broadcast mode of
communication.
15.3 BASIC TERMINOLOGY USED IN ELECTRONIC
COMMUNICATION SYSTEMS
By now, we have become familiar with some terms like information source,
transmitter , receiver, channel, noise, etc. It would be easy to understand
the principles underlying any communication, if we get ourselves
acquainted with the following basic terminology.
© NCERT 
not to be republished
Page 4


15.1  INTRODUCTION
Communication is the act of transmission of information. Every living
creature in the world experiences the need to impart or receive information
almost continuously with others in the surrounding world. For
communication to be successful, it is essential that the sender and the
receiver understand a common language. Man has constantly made
endeavors to improve the quality of communication with other human
beings. Languages and methods used in communication have kept
evolving from prehistoric to modern times, to meet the growing demands
in terms of speed and complexity of information. It would be worthwhile
to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19
th
 and 20
th
 century in
the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander
Graham Bell. The pace of development seems to have increased
dramatically after the first half of the 20
th
 century. We can hope to see
many more accomplishments in the coming decades. The aim of this
chapter is to introduce the concepts of communication, namely the mode
of communication, the need for modulation, production and deduction
of amplitude modulation.
15.2  ELEMENTS OF A COMMUNICATION SYSTEM
Communication pervades all stages of life of all living creatures. Irrespective
of its nature, every communication system has three essential elements-
Chapter Fifteen
COMMUNICATION
SYSTEMS
© NCERT 
not to be republished
Physics
514
Around
1565 A.D.
1835
1876
1895
1936
1955
1968
1975
1989-91
Year Event Remarks
The reporting of the delivery of
a child by queen using drum
beats from a distant place to
King Akbar.
Invention of telegraph by
Samuel F.B. Morse and Sir
Charles Wheatstone
Telephone invented by
Alexander Graham Bell and
Antonio Meucci
Jagadis Chandra Bose and
Guglielmo Marconi
demonstrated wireless
telegraphy.
Television broadcast(John
Logi Baird)
First radio FAX transmitted
across continent.(Alexander
Bain)
ARPANET- the first internet
came into existence(J.C.R.
Licklider)
Fiber optics developed at Bell
Laboratories
Tim Berners-Lee invented the
World Wide Web.
It is believed that minister Birbal
experimented with the arrangement to
decide the number of drummers posted
between the place where the queen
stayed and the place where the king
stayed.
It resulted in tremendous growth of
messages through post offices and
reduced physical travel of messengers
considerably.
Perhaps the most widely used means of
communication in the history of
mankind.
It meant a giant leap – from an era of
communication using wires  to
communicating without using wires.
(wireless)
First television broadcast by BBC
The idea of FAX transmission was
patented by Alexander Bain in 1843.
ARPANET was a project undertaken by
the U.S. defence department. It allowed
file transfer from one computer to
another connected to the network.
Fiber optical systems are superior and
more economical compared to
traditional communication systems.
WWW may be regarded as the mammoth
encyclopedia of knowledge accessible to
everyone round the clock throughout the
year.
TABLE 15.1  SOME MAJOR MILESTONES IN THE HISTORY OF COMMUNICATION
© NCERT 
not to be republished
Communication System
515
transmitter , medium/channel and receiver. The block diagram shown in
Fig. 15.1 depicts the general form of a communication system.
FIGURE 15.1 Block diagram of a generalised communication system.
In a communication system, the transmitter is located at one place,
the receiver is located at some other place (far or near) separate from the
transmitter and the channel is the physical medium that connects them.
Depending upon the type of communication system, a channel may be in
the form of wires or cables connecting the transmitter and the receiver or
it may be wireless. The purpose of the transmitter is to convert the message
signal produced by the source of information into a form suitable for
transmission through the channel. If the output of the information source
is a non-electrical signal like a voice signal, a transducer converts it to
electrical form before giving it as an input to the transmitter. When a
transmitted signal propagates along the channel it may get distorted due
to channel imperfection. Moreover, noise adds to the transmitted signal
and the receiver receives a corrupted version of the transmitted signal.
The receiver has the task of operating on the received signal. It reconstructs
a recognisable form of the original message signal for delivering it to the
user of information.
There are two basic modes of communication: point-to-point and
broadcast.
In point-to-point communication mode, communication takes place
over a link between a single transmitter and a receiver. Telephony is an
example of such a mode of communication. In contrast, in the broadcast
mode, there are a large number of receivers corresponding to a single
transmitter. Radio and television are examples of broadcast mode of
communication.
15.3 BASIC TERMINOLOGY USED IN ELECTRONIC
COMMUNICATION SYSTEMS
By now, we have become familiar with some terms like information source,
transmitter , receiver, channel, noise, etc. It would be easy to understand
the principles underlying any communication, if we get ourselves
acquainted with the following basic terminology.
© NCERT 
not to be republished
Physics
516
(i) Transducer: Any device that converts one form of
energy into another can be termed as a transducer.
In electronic communication systems, we usually
come across devices that have either their inputs
or outputs in the electrical form. An electrical
transducer may be defined as a device that converts
some physical variable (pressure, displacement,
force, temperature, etc) into corresponding
variations in the electrical signal at its output.
(ii) Signal: Information converted in electrical form
and suitable for transmission is called a signal.
Signals can be either analog or digital. Analog
signals are continuous variations of voltage or
current. They are essentially single-valued
functions of time. Sine wave is a fundamental
analog signal. All other analog signals can be fully
understood in terms of their sine wave components.
Sound and picture signals in TV are analog in
nature. Digital signals are those which can take
only discrete stepwise values. Binary system that
is extensively used in digital electronics employs
just two levels of a signal. ‘0’ corresponds to a low
level and ‘1’ corresponds to a high level of voltage/
current. There are several coding schemes useful
for digital communication. They employ suitable
combinations of number systems such as the
binary coded decimal (BCD)*. American Standard
Code for Information Interchange (ASCII)** is a
universally popular digital code to represent
numbers, letters and certain characters.
(iii) Noise: Noise refers to the unwanted signals that
tend to disturb the transmission and processing
of message signals in a communication system.
The source generating the noise may be located
inside or outside the system.
(iv) Transmitter: A transmitter processes the incoming
message signal so as to make it suitable for
transmission through a channel and subsequent
reception.
(v) Receiver: A receiver extracts the desired message
signals from the received signals at the channel
output.
(vi) Attenuation: The loss of strength of a signal while
propagating through a medium is known as
attenuation.
* In BCD, a digit is usually represented by four binary (0 or 1) bits. For example
the numbers 0, 1, 2, 3, 4 in the decimal system are written as 0000, 0001, 0010,
0011 and 0100. 1000 would represent eight.
** It is a character encoding in terms of numbers based on English alphabet since
the computer can only understand numbers.
JAGADIS CHANDRA BOSE (1858 – 1937)
Jagadis Chandra Bose
(1858 – 1937) He
developed an apparatus
for generating ultrashort
electro-magnetic waves
and studied their quasi-
optical properties. He
was said to be the first to
employ a semiconductor
like galena as a self-
recovering detector of
electromagnetic waves.
Bose published three
papers in the British
magazine, ‘The
Electrician’ of 27 Dec.
1895. His invention was
published in the
‘Proceedings of The Royal
Society’ on 27 April 1899
over two years before
Marconi’s first wireless
communication on 13
December 1901. Bose
also invented highly
sensitive instruments for
the detection of minute
responses by living
organisms to external
stimulii and established
parallelism between
animal and plant
tissues.
© NCERT 
not to be republished
Page 5


15.1  INTRODUCTION
Communication is the act of transmission of information. Every living
creature in the world experiences the need to impart or receive information
almost continuously with others in the surrounding world. For
communication to be successful, it is essential that the sender and the
receiver understand a common language. Man has constantly made
endeavors to improve the quality of communication with other human
beings. Languages and methods used in communication have kept
evolving from prehistoric to modern times, to meet the growing demands
in terms of speed and complexity of information. It would be worthwhile
to look at the major milestones in events that promoted developments in
communications, as presented in Table 15.1.
Modern communication has its roots in the 19
th
 and 20
th
 century in
the work of scientists like J.C. Bose, F.B. Morse, G. Marconi and Alexander
Graham Bell. The pace of development seems to have increased
dramatically after the first half of the 20
th
 century. We can hope to see
many more accomplishments in the coming decades. The aim of this
chapter is to introduce the concepts of communication, namely the mode
of communication, the need for modulation, production and deduction
of amplitude modulation.
15.2  ELEMENTS OF A COMMUNICATION SYSTEM
Communication pervades all stages of life of all living creatures. Irrespective
of its nature, every communication system has three essential elements-
Chapter Fifteen
COMMUNICATION
SYSTEMS
© NCERT 
not to be republished
Physics
514
Around
1565 A.D.
1835
1876
1895
1936
1955
1968
1975
1989-91
Year Event Remarks
The reporting of the delivery of
a child by queen using drum
beats from a distant place to
King Akbar.
Invention of telegraph by
Samuel F.B. Morse and Sir
Charles Wheatstone
Telephone invented by
Alexander Graham Bell and
Antonio Meucci
Jagadis Chandra Bose and
Guglielmo Marconi
demonstrated wireless
telegraphy.
Television broadcast(John
Logi Baird)
First radio FAX transmitted
across continent.(Alexander
Bain)
ARPANET- the first internet
came into existence(J.C.R.
Licklider)
Fiber optics developed at Bell
Laboratories
Tim Berners-Lee invented the
World Wide Web.
It is believed that minister Birbal
experimented with the arrangement to
decide the number of drummers posted
between the place where the queen
stayed and the place where the king
stayed.
It resulted in tremendous growth of
messages through post offices and
reduced physical travel of messengers
considerably.
Perhaps the most widely used means of
communication in the history of
mankind.
It meant a giant leap – from an era of
communication using wires  to
communicating without using wires.
(wireless)
First television broadcast by BBC
The idea of FAX transmission was
patented by Alexander Bain in 1843.
ARPANET was a project undertaken by
the U.S. defence department. It allowed
file transfer from one computer to
another connected to the network.
Fiber optical systems are superior and
more economical compared to
traditional communication systems.
WWW may be regarded as the mammoth
encyclopedia of knowledge accessible to
everyone round the clock throughout the
year.
TABLE 15.1  SOME MAJOR MILESTONES IN THE HISTORY OF COMMUNICATION
© NCERT 
not to be republished
Communication System
515
transmitter , medium/channel and receiver. The block diagram shown in
Fig. 15.1 depicts the general form of a communication system.
FIGURE 15.1 Block diagram of a generalised communication system.
In a communication system, the transmitter is located at one place,
the receiver is located at some other place (far or near) separate from the
transmitter and the channel is the physical medium that connects them.
Depending upon the type of communication system, a channel may be in
the form of wires or cables connecting the transmitter and the receiver or
it may be wireless. The purpose of the transmitter is to convert the message
signal produced by the source of information into a form suitable for
transmission through the channel. If the output of the information source
is a non-electrical signal like a voice signal, a transducer converts it to
electrical form before giving it as an input to the transmitter. When a
transmitted signal propagates along the channel it may get distorted due
to channel imperfection. Moreover, noise adds to the transmitted signal
and the receiver receives a corrupted version of the transmitted signal.
The receiver has the task of operating on the received signal. It reconstructs
a recognisable form of the original message signal for delivering it to the
user of information.
There are two basic modes of communication: point-to-point and
broadcast.
In point-to-point communication mode, communication takes place
over a link between a single transmitter and a receiver. Telephony is an
example of such a mode of communication. In contrast, in the broadcast
mode, there are a large number of receivers corresponding to a single
transmitter. Radio and television are examples of broadcast mode of
communication.
15.3 BASIC TERMINOLOGY USED IN ELECTRONIC
COMMUNICATION SYSTEMS
By now, we have become familiar with some terms like information source,
transmitter , receiver, channel, noise, etc. It would be easy to understand
the principles underlying any communication, if we get ourselves
acquainted with the following basic terminology.
© NCERT 
not to be republished
Physics
516
(i) Transducer: Any device that converts one form of
energy into another can be termed as a transducer.
In electronic communication systems, we usually
come across devices that have either their inputs
or outputs in the electrical form. An electrical
transducer may be defined as a device that converts
some physical variable (pressure, displacement,
force, temperature, etc) into corresponding
variations in the electrical signal at its output.
(ii) Signal: Information converted in electrical form
and suitable for transmission is called a signal.
Signals can be either analog or digital. Analog
signals are continuous variations of voltage or
current. They are essentially single-valued
functions of time. Sine wave is a fundamental
analog signal. All other analog signals can be fully
understood in terms of their sine wave components.
Sound and picture signals in TV are analog in
nature. Digital signals are those which can take
only discrete stepwise values. Binary system that
is extensively used in digital electronics employs
just two levels of a signal. ‘0’ corresponds to a low
level and ‘1’ corresponds to a high level of voltage/
current. There are several coding schemes useful
for digital communication. They employ suitable
combinations of number systems such as the
binary coded decimal (BCD)*. American Standard
Code for Information Interchange (ASCII)** is a
universally popular digital code to represent
numbers, letters and certain characters.
(iii) Noise: Noise refers to the unwanted signals that
tend to disturb the transmission and processing
of message signals in a communication system.
The source generating the noise may be located
inside or outside the system.
(iv) Transmitter: A transmitter processes the incoming
message signal so as to make it suitable for
transmission through a channel and subsequent
reception.
(v) Receiver: A receiver extracts the desired message
signals from the received signals at the channel
output.
(vi) Attenuation: The loss of strength of a signal while
propagating through a medium is known as
attenuation.
* In BCD, a digit is usually represented by four binary (0 or 1) bits. For example
the numbers 0, 1, 2, 3, 4 in the decimal system are written as 0000, 0001, 0010,
0011 and 0100. 1000 would represent eight.
** It is a character encoding in terms of numbers based on English alphabet since
the computer can only understand numbers.
JAGADIS CHANDRA BOSE (1858 – 1937)
Jagadis Chandra Bose
(1858 – 1937) He
developed an apparatus
for generating ultrashort
electro-magnetic waves
and studied their quasi-
optical properties. He
was said to be the first to
employ a semiconductor
like galena as a self-
recovering detector of
electromagnetic waves.
Bose published three
papers in the British
magazine, ‘The
Electrician’ of 27 Dec.
1895. His invention was
published in the
‘Proceedings of The Royal
Society’ on 27 April 1899
over two years before
Marconi’s first wireless
communication on 13
December 1901. Bose
also invented highly
sensitive instruments for
the detection of minute
responses by living
organisms to external
stimulii and established
parallelism between
animal and plant
tissues.
© NCERT 
not to be republished
Communication System
517
(vii) Amplification : It is the process of increasing the amplitude (and
consequently the strength) of a signal using an electronic circuit
called the amplifier (reference Chapter 14). Amplification is
necessary to compensate for the attenuation of the signal in
communication systems. The energy needed for additional signal
strength is obtained from a DC power source. Amplification is
done at a place between the source and the destination wherever
signal strength becomes weaker than the required strength.
(viii) Range: It is the largest distance between a source and a destination
up to which the signal is received with sufficient strength.
(ix) Bandwidth: Bandwidth refers to the frequency range over which
an equipment operates or the portion of the spectrum occupied
by the signal.
(x) Modulation: The original low frequency message/information
signal cannot be transmitted to long distances because of
reasons given in Section 15.7. Therefore, at the transmitter,
information contained in the low frequency message signal is
superimposed on a high frequency wave, which acts as a carrier
of the information. This process is known as modulation. As
will be explained later, there are several types of modulation,
abbreviated as AM, FM and PM.
(xi) Demodulation: The process of retrieval of information from the
carrier wave at the receiver is termed demodulation. This is the
reverse process of modulation.
(xii) Repeater: A repeater is a combination of a receiver and a
transmitter. A repeater, picks up the signal from the transmitter,
amplifies and retransmits it to the receiver sometimes with a
change in carrier frequency. Repeaters are used to extend the
range of a communication system as shown in Fig. 15.2. A
communication satellite is essentially a repeater station in space.
FIGURE 15.2 Use of repeater station to increase the range of communication.
15.4  BANDWIDTH OF SIGNALS
In a communication system, the message signal can be voice, music,
picture or computer data. Each of these signals has different ranges of
frequencies. The type of communication system needed for a given signal
depends on the band of frequencies which is considered essential for the
communication process.
For speech signals, frequency range 300 Hz to 3100 Hz is considered
adequate. Therefore speech signal requires a bandwidth of 2800 Hz (3100 Hz
– 300 Hz) for commercial telephonic communication. To transmit music,
© NCERT 
not to be republished