Communication system : A communication system is the set-up used in the transmission of information from one place to another . The present day communication systems are electrical , electronic or optical in nature.
A schematic model of an electrical communication system is shown in figure .
It consists of three major parts:
Transmitter , Communication channel and Receiver.
A transmitter transmits the information after modifying it to a form suitable for transmission. This modification is achieved by a process called Modulation.
The communication channel carries the modulated wave from the transmitter to the receiver. A receiver reconstructs the original message after propagation through the channel. This is achieved by a process, called ‘ Demodulation ’, which is the reverse of modulation.
Important Terms
(i) Information : Information itself is that which is conveyed. The amount of information contained in a message is measured in bits. The set or total number of messges, consists of individual messages-which may be distinguished from one another.
(ii)Transmitter : For sending information, the incoming signals are converted into electrical variations. A transmitter is required to process and possibly encode the incoming information so as to make it suitable for transmission and subsequent reception.
In a transmitter, the information modulates the carrier i.e. information is impressed on a high frequency carrier wave. The system may involve amplitude modulation, frequency modulation, pulse modulation or any combination of these.
(iii)Communication channel : the communication channel carries the modulated wave from the transmitter to the receiver. In case of telephony and telegraphy, communication channel is a transmission line. In optical communication, an optical fibre is the communication channel and in radio / TV broadcasting, communication channel is the free space itself.
(iv)Channel Noise : The term ‘channel’ usually refers to the frequency range allocated to a particular service or transmission.
Noise is unwanted energy, usually of random character, present in a transmission system, due to some known or unknown causes. Noise may distort the information at any point in a communications system. Obviously, noise will have its greatest effect, when the signal is weakest.
(v) Receiver : The most important function of receiver is demodulation and sometimes decoding as well. Both these processes are the reverse of the corresponding transmitter (modulator) processes.
Receivers range from a very simple crystal receiver with head phones, to a complex radar receiver. The output of a receiver may be fed to a loud speaker, video display unit, tele typewriter, TV picture tube, pen recorder or computer.
Note that the transmitter and receiver must be in agreement with the modulation and coding methods used.
The schematics of an arrangement for transmission and reception of a message signal are shown in figure :
Basic Forms of Communications:
The various forms of communications in common use are :
(i) Telegraphy
(ii) Radio broadcast
(iii) Television broadcast
(iv) Telephony
(v) Radar communications
(vi) Sonar communications
(vii) Electronic mail
(viii) Tele printing
(ix) Fax
(x) Mobile phones
On the basis of links between transmitter and receiver, the various forms of communication are :
(i) Cable communications
(ii) Ground wave communications
(iii) Sky wave communications
(iv) Satellite communications
(v) Optical communications
On the basis of the signal used, there are two forms of communication :
(i) Analog communications
(ii) Digital communications
Message Signals:
Message signals are electrical signals generated from the original information to be transmitted, using an appropriate transducer. A message signal is a single valued function of time that conveys the information.
These signals are of two types :
(i) Analog signals
(ii) Digital signals
An analog signal is that in which current or voltage value varies continuously with time .
Figure represents the simplest form of sinusoidal analog voltage signal, having single frequency.
Mathematically , we represent such a signal as
F(t) = A sin (ωt)
Such signals can have all sorts of values at different instants, but these values shall remain within the range of a maximum value (Vmax) and a minimum value (Vmin).
The sound produced by a vibrating tuning fork provides such a signal. A microphone converts pressure variations in air, produced by sound waves into corresponding current/voltage variations.
Similarly, a photocell converts variations in light intensity to corresponding current/voltage variations .
A digital signal has two levels of current or voltage, represented by 0 and 1. Digital signals are discrete signals as shown in figure .
Zero (0) of a digital signal refers to open circuit and (1) of a digital signal refers to close circuit. 0 is also referred to as ‘ No ’ or ‘ space ’ and 1 is referred to as ‘ Yes ’ or mark.
Both 0 and 1 are called bits. A group of bits is called a binary word or a byte. A byte made of 2 bits can give four code combinations : 00, 01, 10, 11.
In general, number of code combinations = N = 2x, where x is the number of bits in a byte. Thus a popular 8 bit byte can give code combinations N = 28 = 256. 
The output of a digital computer is an example of a digital signal.
An analog signal can be converted into a digital signal by sampling it in time, quantizing and coding it.
The digital communication is far more advantageous than the analog communication. This is because in the former, the receiver has to recognize whether a pulse is present or not in any prescribed time interval. Further, a large number of digital signals can be sent through a single channel only.
Modulation :
The audio frequency signals (20 Hz to 20 kHz) cannot be transmitted as such over long distances. This is primarily because :
(i) energy carried by low frequency audio waves is too small (E = hν)
(ii) for efficient radiation and reception, the transmitting and receiving antennas should have heights comparable to a quarter wavelength of the frequency used. For 15 kHz frequency, the height of the antennas has to be about 5000 metre, which size is unthinkable.
(iii) audio frequency range being so small , there would be so much of overlapping and confusion.
To overcome these difficulties, we superimpose the audio frequency signals (called the modulating signal) on a high frequency sine wave, called the carrier wave. This process is called modulation. The resultant modulated wave is then transmitted.
In the process of modulation, some characteristic of high frequency sine wave (called the carrier wave) is varied in accordance with the instantaneous value of the modulating signal (i.e. the audio frequency signal).
Mathematically, we represent a Carrier wave by the equation :
e = E sin (ωt + φ)
where E is maximum voltage, e is the instantaneous voltage at time t; ω is angular frequency and φ is the phase w.r.t. some reference.
Any of the three characteristics/parameters (E , ω , φ) of the carrier wave may be varied by the modulating signal giving rise to the respective amplitude modulation; frequency modulation and phase modulation.
Amplitude Modulation :
Amplitude modulation (AM) is defined as a system of modulation in which the amplitude of high frequency (HF) carrier wave is made proportional to the instantaneous amplitude of the audio frequency (AF) modulating voltage.
Let the carrier voltage (vc) and modulating voltage (vm) be represented by
vc = Vc sinωct … (1)
vm = Vm sinωmt …(2)
Thus in amplitude modulation , max. amplitude Vc of unmodulated carrier is made proportional to the instantaneous modulating voltage Vmsinωmt .
Figure shows an amplitude modulated wave for one cycle of the modulating sine wave. The max. positive amplitude of AM wave is given by
A = Vc + Vm sinωmt … (3)
This is called top envelope .
The maximum negative amplitude of AM was given by
− A = −(Vc + Vm sinωmt) … (4)
This is called bottom envelope.
The modulated wave extends between these two limiting envelopes, and its frequency is equal to the unmodulated carrier frequency.
Vm = (Vmax − Vmin)/2
and Vc = Vmax − Vm = Vmax − (Vmax − Vmin)/2
Depth or index of modulation (m) :
m = Vm/Vc = (Vmax − Vmin)/(Vmax + Vmin)
The modulation index (m) is a number lying between 0 and 1 . Often , m is expressed in percentage and is called the percentage modulation.
Frequency Modulation :
Frequency modulation (FM) is defined as a system , in which the frequency of carrier wave is varied in accordance with the instantaneous value of the modulating voltage. However, the amplitude of the carrier wave is kept constant.
The general equation of an unmodulated carrier wave may be written as :
x = A sin (ωt +φ)
where, x = instantaneous value of voltage or current at any time t,
A = maximum voltage/current , called the amplitude,
ω = angular frequency in radian/s
φ = phase angle
When frequency of the carrier is made to vary in accordance with another signal (normally of a lower frequency) we obtain frequency modulated waves.
Comparison of Frequency Modulation & Amplitude Modulation
Following are some of the advantages of frequency modulation :
(i) The amplitude of FM wave is constant, whatever be the modulation index. Therefore, transmitted power is constant, unlike in the AM wave.
The amplifiers used in FM transmitters handle constant power and therefore, they are more efficient. Further, all the power transmitted in FM is useful, whereas in AM, most of the power is in the transmitted carrier.
(ii) In FM wave, amplitude variations may be caused by noise. By using amplitude limiters in the FM receivers, we can almost eliminate noise.
(iii) The noise can be reduced still further by increasing the ‘deviation’, in FM wave. This feature is not available in the AM wave, as depth of amplitude modulation cannot be increased beyond 100% without causing distortion.
(iv) Standard frequency allocations by the International Radio Consultative Committee (IRCC) provides a guard band between commercial FM stations. This reduces adjacent channel interference.
(v) FM broadcasts operate in VHF and UHF frequency ranges – where the noise is much less than in MF and HF ranges for AM broadcasts.
(vi) With the use of space wave in FM broadcasts, radius of operation is limited to slightly more than the line of sight. This reduces the chances of cochannel interference.
Some of the disadvantages of FM are :
(i) The channel width required by FM is upto 10 times as large as that needed by AM.
(ii) The equipment used in transmission and reception of FM is more complex.
(iii) The area of reception for FM is much smaller than for AM, as reception in FM is limited to the line of sight. This limits the FM mobile communications over a wide area.
Ranges of frequencies allotted for commercial FM radio and TV broadcast.
Nature of broadcast —— Frequency band
FM radio —————– 88–108 MHz
VHF TV —————– 47–230 MHz
UHF TV —————– 470 – 960 MHz
Demodulation:
Demodulation is the reverse process of modulation which is performed in a receiver to recover the original modulating signals.
We know that a signal to be transmitted is impressed on to a carrier wave using any of the modulation methods described above. It is then suitably treated, amplified and applied to a transmitting antenna – which radiates or propagates the same.
At the receiving end, the signal is generally quite weak having powers in picowatt. Therefore, the receiver must amplify the received signal first. As the signal is usually accompanied by lots of other (unwanted) signals, the desired signal is selected and others are rejected by the receiver. Finally, demodulation is performed in the receiver to recover the original modulating signals.
Thus a receiver has the following functions :
(i) Selecting the desired signal and rejecting the unwanted signals
(ii) Amplifying and demodulating the desired signal
(iii) Displaying the demodulated signal in a desired manner
Q : Why is a AM signal likely to be more noisy than a FM signal on transmission through a channel ?
Solution : In case of AM , the instantaneous voltage of carrier waves is varied by the modulating wave voltage . So , during the transmission , noise signals can also be added and receiver assumes noise a part of the modulating signal .
In case of FM , the frequency of carrier waves is changed as the change in the instantaneous voltage of modulating waves . This can be done by mixing and not while the signal is transmitting in channel . So , noise does not affect FM signal .
How does Modem work ?
Ans: We know that computers operate in the digital world, but telephone lines require a different method of transmission. The modem modulates the digital signal into a sine wave. This can be broadcast over a telephone line. The signal is transmitted over the line, until it reaches the IP hub. At this point, the sine wave is demodulated into a digital signal once more, and the connection with the internet is complete. Modems can perform this work at a variety of speeds, depending on the technology and availability of access lines.
Q: The frequency response curve (figure) for the filter circuit used for production of AM wave should be
(a) (i) followed by (ii)
(b) (ii) followed by (i)
(c) (iii)
(d) (iv)
Ans: (a) & (b)
Q: An amplitude modulated wave is as shown in figure. Calculate
(i) the percentage modulation,
(ii) peak carrier voltage and
(iii) peak value of information voltage
Sol: (i) From figure ,
Maximum Voltage , $V_{max} = \frac{100}{2}= 50 V$
Minimum Voltage, $V_{min} = \frac{20}{2}= 10 V$
Percentage modulation , $ \mu = \frac{V_{max}-V_{min}}{V_{max} + V_{min}}\times 100$
$\mu = \frac{50-10}{50+10} \times 100 $
= 66.67 %
(ii) Peak carrier voltage ,
$V_c = \frac{V_{max} + V_{min}}{2} $
$V_c = \frac{50 + 10}{2} $
= 30 V
(iii) peak value of information voltage,
$V_m = \mu V_c = \frac{66.67}{100} \times 30$
= 20 V