3.
COMMUNICATION SYSTEM
Basic communication system (concept & explanation with help of Block
diagram)
The communication system a system which describes the information exchange
between two points. The process of transmission and reception of information is called
communication. The major elements of communication are the Transmitter of
information, Channel or medium of communication and the Receiver of information.
Block Diagram of Communication Systems
The block diagram given below represents the flow of the signal from the source to the
destination.
Information Input Output
Transmitter Channel Receiver
Source Transducer Transducer
Sound picture speech Information in Information in
data etc. electrical form Noise
original form
The essential components of a communication system are information source, input
transducer, transmitter, communication channel, receiver and destination.
Information Source
>Message or information is the entity that is to be transmitted. It can be in the form of
audio, video, temperature, picture, pressure, etc.
Input Transducer
> A transducer is a device which converts one form of energy into another form.
> The message from the information source may or may not be electrical in nature. In a
case when the message produced by the information source is not electrical in nature, an
input transducer is used to convert it into a time-varying electrical signal.
> Example: microphone converts the information or massage which is in the form of sound
waves into corresponding electrical signal.
Transmitter
It is the arrangement that processes the message signal into a suitable form for
transmission and subsequently reception.
> Modulation is the main function of the transmitter. In modulation, the message signal is
superimposed upon the high-frequency carrier signal.
Channel
> A channel refers to a physical medium such as wire, cables, space through which the
signal is passed from transmitter to the receiver.
There are many channel impairments that affect the channel perfornance to apronounced
level. Noise, Attenuation and distortion to mention the major impairments.
Noise
Noise is an unwanted signal which tend to interfere with the required signal. Noise signal
is always random in character. Noise may interfere with signal at any point in a
communication system.
Receiver
> An arrangement that extracts the message or information from the transmitted signal at
the output end of the channel and reproduces it in a suitable form as the original message
signal is a receiver.
The main function of the receiver is to reproduce the message signal in electrical form
from the distorted received signal. This reproduction of the original signal is
accomplished by a process known as the demodulation or detection. Demodulation is the
reverse process of modulation carried out in transmitter.
Output transducer
> Output transducer convert an electrical message signal into its original form.
> Example- loudspeaker which works as a transducer i.e. converts the electrical signal in
the forn of original sound signal.
Concept of Modulation and Demodulation, Difference between them
> Modulation is the process by which some characteristics or parameters of the carrier
wave is changed according to the signal containing information. These characteristics can
be amplitude, frequency or phase. Basically, it is assumed that the information signal is a
low-frequency one and thus needs conversion from low to high in order to have a better
transmission.
> Demodulation is defined as extracting the original information-carrying signal from a
modulated carrier wave. A demodulator is an electronic circuit that is mainly used to
recover the information content from the modulated carrier wave.
Need for modulation:
The baseband signals can be transmitted directly, but the baseband transmission has
many limitations which can be overcome using mnodulation.
In the process of modulation, the baseband signal is translated i.e. shifted from low
frequency to high frequency.
This frequency shift is proportional to the frequency of carrier.
The modulation process has the following advantages:
Advantages of modulation:
-Reduction in the height of antenna:
When the transmission occurs over free pace, the antennae radiate the signal out and
receiver receives it. In order to operate efficiently., antennae need to be in order of the
magnitude of wavelength of the transmitted signal.
For the transmission of radio signals, the antenna height must be a multiple of (J4), Here
2 is the wavelength. A =c/f where c is velocity of light and fis the frequency of the signal
to be transmitted.
The minimum antenna height required to transmit a baseband signal of f=10 kHz is
calculated as follows:
Minimum antenna height= W4 =c/4f-(3x10/(4x10x10=7500metersi.e. 7.5 km
It is impossible to build antennae this big.
Now consider a modulated signal at f= IMHz.The minimum antenna height is given by,
Minimum antenna height= N4 = cl4f-(3x10/(4x1x10 )=75 meter
This length of the antenna can be b uilt easily and this example clearly shows us how
hugely the process of modulation is enabling communication systems.
-Avoids mixing of signals:
If the baseband sound signals are transmitted without using the modulation by more than
one transmitter, then all the signals will be the same frequency range i.e 0 to 20Khz.
Therefore, all the signals get mixed together and a receiver cannot separate them from
each other.
Hence, if each baseband sound signal is used to modulate a different carrier then they will
occupy different slots in the frequency domain (different channels). Thus, modulation
avoids mixing of signals.
-Increase the range of communication:
By using modulation to transmit the signals through space to long distances, it removed
the need for wires in the communication systems.
The technique of modulation helped to use wireless equipment in a bug way in their lives.
-Multiplexing is possible:
Multiplexing is a process in which two or more signals can be transmitted over the same
communication channel simultaneously. This is possible only with modulation.
The Multiplexing allows the same channel to be used by many signals.
-Improves quality of reception:
With frequency modulation (FM), and the digital communication techniques like PCM,
the effect of noise is reduced to a great extent. This improves quality of reception.
Differences between Modulation and Demodulation
1. The key difference between modulation and demodulation is that modulation is done at the
transmitter side while demodulation is done at the receiver side of a communication
system.
2. Both modulation and demodulation occur during data transmission but the two
processes are exactly reverse of each other. In modulation original message signal is mixed
with the carrier wave whose parameters are required to be changed. As against, in
demodulation the combination of carrier and message signal are separated from each other,
to have original information signal.
3. Modulation requires a modulator section for mixing of the two signals whereas
demodulation requires demodulator to recover the original signal. However, combined
equipment is used for the two known as Modem.
4. Modulation is done to convert the low-frequency signal into a high-frequency signal.
While at the time of demodulation, the low-frequency signal is achieved form high
frequency signal.
5. Modulation is basically done to transmit data longer distance whereas demodulation is
done to regain the original message signal.
6. Demodulation is somewhat a complex process when compared with modulation.
Different types of Modulation (AM, FM & PM) based on signal, carrier wave
and modulated wave (only concept, No mathematical Derivation)
There are three types of modulation, namely:
1.Amplitude modulation (AM)
In amplitude modulation, the amplitude of the carrier wave is varied in proportion to the
message signal, and the other factors like frequency and phase remain constant.
Baseband Signal (Modulating Signal)
E.
Time, t
Carrier Signal
+Time, t
Amplitude modulated (AM) Signal
(E +E)
E-E)
Time,t
Unmodulated
Carier Envelope
2.Frequency modulation (FM)
Frequency modulation (FM) varies the frequency of the carrier in proportion to the message or
data signal while maintaining other parameters constant.
Baseband Signal (Modulating Signal)
En
Frequency Modulated (FM) Signal
No
Frequency Maximum Minimum
Change Frequency Frequency
3.Phase modulation (PM)
In phase modulation, the carrier phase is varied in accordance with the data signal while
maintaining other parameters constant.
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Baseband Slgnal (Modulating Signal)
Modulating voltage
Unmodulated
Carrier Phase Modulated (PM) Signal
AAAAAMUNENo Phase
Change
Ca strotching
rPhase Lag)
Carier Compression
(Phase Lead)
