Chapter-1) COMMUNICATION
What is a Communication?
It is the basic process of exchanging information.
What is an Electronic Communication?
It is communication which uses electronic media to transmit the information or
message using computers, mobile, e-mail, telephone, video calling, FAX machine, etc.
Elements of communication system
Any electronic communication system can be represented in its basic form, as shown in
the figure:
The explanation of communication system blocks are as follows:
1) Information Source
The messages come from the various information sources. The two main sources of
information are:
a) Ideas emanating from the human brain
b) Changes in any physical environment
-The information may contain human voice, picture, code, data, music and their
combinations.
2) Transmitter
-Most of the times information is in non-electrical form.
-The transmitter is a collection of built-in electronic circuits such as Encoder, Transducer
and amplifier.
-The encoder circuit is a type of IC (Integrated Circuit) used to encode the information
with
special code.
-The transducer circuit is designed to convert the information into a signal suitable for
transmission over a given communication medium.
-The amplifier available in transmitter increases the strength of the signal.
-So, the built-in circuitry makes incoming information suitable for transmission and
subsequent reception.
3) Communication Channel
There are basically two types of communication channel:
a) Wire communication
This medium is simply a pair of wires that carry the signal from one end to another.
Example- Copper Wire, Fiber optic cable, Co-axial cable etc.
b) Wireless communication
It is communication system in which there is no requirement of physical wires
between transmitter and receiver to carry the signal, on the contrary the signal is
send through free space or air.
Example- Electromagnetic waveforms such as radio signal, microwaves, infrared etc.
4) Noise
It is random, undesirable electric or non-electrical energy that enter the communication
system via the medium and interferes with transmitted message.
-Noise is one of the serious problems of electronic communication. It cannot be
completely eliminated.
-Normally there are two types of noises:
a) Man–made noise
It is the noise produced by electric ignition system of cars, electric motors, fluorescent
lights etc.
b) Natural noise
This noise includes noise produced in nature e.g. lighting during rainy season or noise
due to radiations produced by the sun and the other stars.
5) Receiver
A receiver is a collection of electronic circuits consists of Decoder, Transducer and
Amplifier designed to convert the signal back to the original information.
Types of Information Signals
Normally, there are two types of information signals.
1) Analog Signal
A signal that can vary in a continuous manner is called as analog signal.
Examples-Sine wave tone, Voice & Video signals
Figure – Sine Wave
2) Digital Signal
A signal that only takes discrete values (0 and 1) is called as digital signal.
Example-Computer Generated Signals, Logic Gate Circuit Generated Signals
Figure – Digital Pulse
Method of Signal Transmission
Baseband Signals
The original information signals (Analog or Digital) are referred as Baseband signals.
Baseband Transmission
Putting the original signal directly into the medium is referred as Baseband
transmission.
Example of Analog Signal:
The common example is : Intercom Telephone system used for local calls. Here the
voice signal (which is an analog signal by nature), converted into electrical form, is
placed on the wires and transmitted over some distance to the receiver.
Example of Digital Signal:
In LAN (Local Area Network) of computers, the digital signals are applied directly to
coaxial cables for transmission to another computer.
Note- There are many instances when the baseband signals are incompatible for direct
transmission over the medium for long distance.
For Example, voice signals cannot travel longer distances in air, the signals gets
attenuated rapidly.
Hence for transmission of baseband signals by radio, modulation technique has to be
used.
Modulation
Definition
It is the process in which baseband signal (such as voice, video etc.) modifies some
characteristics (for example-amplitude, frequency, phase etc.) of higher-frequency
signal called the carrier signal.
OR
It is the process of varying one or more properties of a high frequency periodic
waveform, called the carrier signal, with respect to a modulating signal.
What is a carrier signal?
-It is a high frequency signal.
Need of Modulation
Modulation is extremely necessary in communication system due to following
reasons:
1) Antenna Height
-In order to transmit a radio wave effectively, the Height (Length) of antenna is
approximately equal to the (1/4th of) wavelength of radio wave.
Velocity (V )
Wavelength (l) =
Frequency (F)
8
3∗10 meters
=
Frequency (F)
-As the audio frequencies range from 20Hz-20 kHz, therefore if they are
transmitted directly into space, the length of antenna required would extremely
large.
-For instance, to radiate an audio signal of frequency 20kHz directly into space. We
would need an antenna of height is 3750 meters.
-This is too long antenna to be constructed practically. For this reason, it is
impractical to radiate audio signal directly into space.
Solution-Use the carrier of 1000 KHz to carry the signal, so we need antenna length
of 75 meters only and this size can be easily constructed.
