Unit 17 Basics of Communication

UNIT 17 BASICS OF COMMUNICATION Structure Introduction Objectives Communication Systems and Processes Kinds of Signals Modulation Amplitude Modulation Frequency Modulation Pulse Modulation Demodulation Data Transmission and Retrieval Digital Communication Systems FAX Machine Modem E-mail Summary Terminal Questions Solutions and Answers 17.1 INTRODUCTION Communication comes naturally to us. The simplest way of communicating is talking and listening. You could introduce this concept through a simple example. When we speak, the sound reaches the listener's ears through the air. Our eyes can process a picture or view, which is in 2 or 3 dimensions. Audio and visual communications are two major modes of information transport in communication. You can tell students about how the importance of communication grew manifold during the last few decades. As the means of transport allowed people to travel far and wide, the need for quicker communication grew. The rapid growth in space technology led to better and faster means of con~munication.With e-mail and INTERNET, it is now possible to communicate with any one in real time at almost no cost. With a brief description of the current status of communication you could start teaching about the communication process: What happens when we communicate using an electronic communication system? How are our sounds and pictures altered for transmission and how the original information is recovered at the receiving end? Finally, you could discuss the latest popular communication systems. Objectives After studying this unit you will be able to: teach better - the principles and applications of communication systems, - modulation and demodulation, and - modern communication technologies; devise strategies to evaluate the teaching - learning of these concepts. 17.2 COMMUNICATION SYSTEMS AND PROCESSES Communication involves sending (transmitting) and receiving information fiomV,dne person (place) to another. The personlplace from which the messages are coming is called information source, the set of messages is termed as information and the Com~nunicationPhysics system used for communication from one place to another is called the communication system. In ancient times human beings successfully used sound (spoken word, clapping of hands, beating of drums, ctc.) and light (eyes, fire and smoke, pictures, lamps etc.) to convey information from one place to other. With the invention of written scripts, the forms of conmunication diversified. Letters, books, print materials came into being. Postal services were set up; the newspapers and journals became an influential print medium. The invention of the telephone, radio and TV revolutionized communication. With the invention of modem electronic devices, newer and quicker methods of communication have been established. We now have E-mail: Through use of our computers, we are able to read a message, reply or forward a message by a click of a mouse button. Satellite/video conferencing: With multi-point video conferencing, video meetings can be conducted easily and spontaneously between a number of remote sites. A two-way video conference allows participants to see and hear - and to be seen and heard by - everyone at a distant location via telephone line connections. Fax: The machine allows for letters, memos, pictures etc. to reach us by electronic means within a few minutes time. Voice mail: It is a modem day answering machine. It is an electronic medium where we can leave a message, reply or forward a message by telephone. The basic communication syste~rrbeing used in all these forms of communication can be modelled in a simple way. It has three major constituents: transmitter conmunication charmel receiver The transmitter transmits the message from a source, a place from where the information originates. Examples of a source are human speech, pictures, musical instruments, written script, etc. To send any of the signals over a long distance, effectively and reliably, it is necessary to convert it into another form. For example, a microphone converts sound signals into time varying electrical signals, called a message signal. It is also called a transducer. Source Transducer Fig.17.l: A schematic arrangement for the communication system Transmitter: It is a setup that transmits the message to the receiver through the con~municationchannel. In most of the cases, the destination is at a very large distance from the source. As the distance increases, the strength of the signal decreases. Therefore, it is not possible to send the signal as it is. We need to modify the signal. The transmitter basically modifies or transforms the message signal into a suitable form for transmission over the channel. The basic components of a transmitter are : message signal generator, modulator or encoder, and antenna. Basics of Communication In the transmitter, the power of the message signal is also boosted up by using a suitable amplifier and then the signal is radiated in space with the help of the antenna. Communication Channel or Transmission Medium: It is the medium through r which the signal moves. The medium of propagation between the transmitter and 1 receiver plays an important role in communication. The signal should be modified in a way that it is transmitted without distortion in the medium. Twisted pair, coaxial cable, optical fibre are examples of guided media, while free space is an example of unguided medium. Receiver: The main function of the receiver is to construct the original message after C it has propagated through the communication channel. It receives and detects the signal. A receiver can be as simple as a human ear or eye or a receiver antenna or a photo detector. Special techniques are needed to retrieve the message in electronic communication systems. I Destination: After extracting the information in signal form, there is a need to process I the information further to bring it back to its original form. The message signal is in the form of electrical current or voltage signal. Another transducer can do this. For example, the loudspeaker converts electrical signals into sound signals, due to which it is possible for us to hear the original signal. The quality of the transmission depends on the type of medium as well as the nature of the signal used for the transmission. 17.3 KINDS OF SIGNALS Usually, the messages to be communicated (e.g. speech, picture, etc.) are not electrical in nature. We need to convert them into time dependent electrical signals. There are two types of signals: a) analog and b) digital. An analog signal is a signal in which current or voltage value changes continuously with time. A simple example of an analog signal is human speech. Speech consists of several frequencies with differing amplitudes. The oscillation in any one such frequency can be represented in the form of a sinusoidal wave: F(t)= Asinot Such signals can have any value at different instants, within the range of a maximum value and a minimum value. Every analog signal is characterized by its amplitude (A) and frequency (a).Its amplitude is a continuous function of time. Human speech is a mixture of various frequencies, ranging from 20 Hz to 20 kHz. The range' over which the frequencies in a sigi~alvary is called its band width and the band of Frequencies which is deliverer! by the source as an information is called base band. (a) (b) FAg.173: a) Analog signal and b) digital Signal Communication Physics Analog quantities can take any value within a given range. Most physical measures 1 (i.e., temperature, length, voltage) are analog. Digital quantities may take only specific ; values within a range. A digital signal is discrete (Fig. 17.2b). It usually has two levels of current or voltage, represented by 0 and 1, defined at specific instants of time. The binary signals that computers and telecommunications systems use are digital signals. An analog signal can be converted into a digital form by sampling it in time, then quantizing it and coding it. You can explain this process as follows: Sampling a signal in time: In this process, the continuous signal is cut into a sequence of samples (i.e., discrete points in time). In other words, it limits the number of places after the decimal point of the analog signal on the time axis. You can useFig. 17.3 to explain what sampling means to your students. A basic J (base-band) signal with frequency 0, is sampled using pulse carrier or sampling carrier with frequency 0,. The sampled output is shown in Fig. 17.3a. The sampling frequency o,should be sufficiently high with respect to om.It is essential to have 0, 1 20, (Fig. 17.3b). 1 The output in case of amplitude modulation is given by Analog FET Sampled ln input Switch output input ... t -TvvvL t where k = B, T 2 4JA%F Thus, the modulated (sampled) output Sampling Pulse has three frequency components viz. a,, (as - 0,) and (o, + w,). 'The last component is the up-converted frequency where the modulated carrier frequency is higher than w, by om. I , Base band 1 I In Fig. 17.3b, ffeqtiency w, 2 2om1 hence the modulated frequency band by from o,- om,to o, + omlis well t 1 + Frequency separated .from om,and we can use a Om, 0,-a,, Ws Os+Q rnl filter to remove the base band frequency oml. Now, in Fig. 17.3c, the modulated base band frequency is B~~,,band ,Distorted Signal band such that o, < 2om2. In this case, the I I modulated frequency band and the base band frequency overlap; and when we b Frequency try to use a filter to take away , we (0s as+0m2 get a distorted signal due to overlapping bands shown as double shaded frequency band. Fig.17.3: a) Sampling of a signal.

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