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Analog and Digital Communication |
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The concept of information is central to communication. There is no precise definition of the word "information". So, instead of information, we deal with "message". |
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Communication Systems with Input and Output Transducers |
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The input transducer converts the message to an electrical signal say a voltage or current. Another transducer at the destination converts the output signal to the desired message form. |
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Elements of a Communication System |
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The above figure depicts the elements of a communication system. There are three essential parts of any communication system, the transmitter, transmission channel, and receiver. |
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Advantages of Digital Communication Systems over Analog Communication Systems |
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A common format for encoding different kinds of message signals (e.g., speech signal, video signal, computer data, etc.) for the purpose of transmission. |
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Undesirable Effects in the Course of Signal Transmission |
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Attenuation is undesirable because it reduces signal strength at the receiver. More serious, however, is distortion interference, and noise, which appear as alternations of the signal shape. Although such contaminations may occur at any point, the standard convention is to blame them entirely on the channel. The transmitter and receiver are always treated as ideal. |
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Modulation |
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In earlier times, the most widely used form of communication was a system based on the transmission of a continuous-wave (CW) signal. With this system, the signal was interrupted periodically (Morse code) to produce a coded message. |
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Need for Modulation |
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The velocity of electromagnetic waves is 3 x 108ms-1. On the other hand, the velocity of sound waves cannot be used to transmit intelligence to far off places. Only electromagnetic waves can be made to do this. |
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Different Types of Modulation |
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In this type if modulation, wc and q are kept constant. The amplitude Ec of the carrier wave is varied in accordance with the modulating wave. |
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Amplitude Modulation |
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When the amplitude of high frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation. |
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Modulation Factor |
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The ratio of change of amplitude of carrier wave to the amplitude of normal carrier wave is called the modulation factor m. |
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Disadvantages of Amplitude Modulation |
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In amplitude modulation, the sidebands contain the signal. The power in the sidebands is the only useful power. The power carrier by the side bands is only 33.3% even when there is 100% modulation. If modulation is 50%, then power carried by the sidebands is 11.1%. Clearly, the useful power is small. So, the amplitude modulation has low efficiency. |
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Frequency Modulation |
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In frequency modulation, the frequency of the carrier wave is modified in accordance with the amplitude of the modulating wave. |
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Need for Frequency Modulation |
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Various electrical machines and noises cause amplitude disturbance in the transmission of amplitude-modulated wave. This makes the reception noisy. |
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Pulse Modulation |
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In amplitude modulation and frequency modulation, the modulation was done by sinusoidal signals. In pulse modulation, short pulses may do the modulation of a radio-frequency carrier. The simplest of these is the conventional telegraph, whereby information is transmitted by a code of dots and dashes, or by turning the carrier on and off. |
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Demodulation |
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Demodulation is the process of recovering the signal intelligence from a modulated carrier wave. This process, also called detection, is the reverse process of modulation. |
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Data Transmission and Reception |
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Telefacsimile's or Fax machines are a must for many businesses around the world. The fax machine however, did not become widely used until the late 1980's. |
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Principle of Fax Machine |
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The essential parts of a fax system are the transmitting devices that translate the graphic material into electrical impulses according to a set pattern, and a synchronized receiving device that retranslates these impulses and prints that. |
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Modems |
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The 'modem' is a contraction of the term modulator-demodulator. It is a conversion that facilitates the transmission and reception of the data over the public switched telephone network (PSTN). |
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Symmetric and Asymetric Modem Configuration |
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The simplest approach to the design of modems is to treat the entire PSTN as a linear analog network. [Note that PSTN is almost entirely due to the use of pulse-code modulation (PCM) for the transmission of voice signals. |
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Modems with reference to computers |
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A modem (a modulator/demodulator) lets you connect your computer to a standard telephone line so that you can transmit and receive electronically transmitted data. |
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Types of Modems |
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Depending upon how your computer is configured and your preferences, you can have an external, internal or PC modem card. All three types work the same way, but each has its advantages and disadvantages. |
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How Modems Work? |
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When a modem first makes a connection, you will hear screeching sounds coming from the modem. These are digital signals coming from the computer to which you are connecting being modulated into audible sounds. The modem sends a higher-pitched tone to represent the digit 1 and a lower-pitched tone to represent the digit 0. |
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What is the Difference Between Digital and Analog Signals? |
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A computer performs its tasks by turning on and off a series of electronic switches represented by the numerical digits of 0 and 1. A 0 is the code for off, and a 1 is the code for on. |
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Space Communication |
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In an Earth environment, electromagnetic waves propagate in ways that depend not only on their own properties but also on those of the environment itself. |
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Importance of Radio Waves in Modern Communication |
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Radio waves are electromagnetic waves of wavelength 10-3m and higher. Their frequency range is from a few kHz to nearly a few hundred MHz. |
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Important Terms Used in the Study of Sky Wave Propagation |
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As the wave is refracted, it is bent down gradually rather than sharply. However, below the ionised layer, the incident and refracted rays follow paths that are exactly the same as they would have been if reflection had taken place from a surface located at a greater height, called the virtual height of this layer. |
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Space Waves |
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Space waves travel in (more or less) straight lines. But they depend on line-of-sight conditions. So, they are limited in their propagation by the curvature of the Earth, except in very unusual circumstances. |
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Basic Principles of Wireless Radio Communication |
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Let us now discuss the basic principles of wireless radio communications. We shall mainly concentrate on the principle of amplitude modulation and demodulation. |
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Microwaves and Radar |
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The 'radar' beams microwaves towards a distant object and receives the reflected signals. Since both the outgoing and incoming waves travel with the velocity of light therefore by measuring the 'time delay' in receiving the signal back, we can measure the distance of the object from the radar. In this way, the precise location of the distant object is made possible. |
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Satellites |
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Celestial bodies revolving around a planet are called satellites. Moon is a natural satellite of the planet Earth. These days, the term satellite is used for manmade satellites, which revolve around the Earth. The closed path of a satellite around the Earth is called its orbit. |
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Satellites Communication |
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It is mainly done through geostationary satellites. These satellites are fitted with special devices. These devices can receive signals from an Earth station and transmit them again in different directions. These special devices are called transponders. |
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Remote Sensing |
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"Remote Sensing" can be defined as obtaining information about an object by observing it from a distance and without coming to actual contact with it. Infact when we see an object and understand what it is, our eye is sensing that object remotely. This is a broad definition. But we generally use this term for observing the surface of Earth from space using satellites. |
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Line Communication - 2 - Wire Lines, Cables |
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This type of wave propagation is said to be unguided. The uniform plane wave exists throughout all space. The electromagnetic energy associated with the wave spreads over a wide area. |
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Telephone Links |
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A microphone is a device that transforms sound pressure into electrical energy. In most types of microphone, the sound pressure acts upon a thin plate or diaphragm, setting it into vibration, and this mechanical motion is then utilised to produce electrical effects. |
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Telephone Circuits |
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Two-way operation is essential for satisfactory telephone service, and many of the problems of the industry arise from this fact. |
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Telephone Lines |
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The transmission lines used in telephony are of two kinds: open- wire and cable. The open-wire lines are gradually being superseded by cable construction, either overhead or underground, for two principal reasons. |
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Optical Communication - Optical Fibres |
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An optical fiber is a dielectric wave guide that transports light signals from one place to another just as a twisted-wire pair or a coaxial cable transports electrical signals. |
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Lasers-An Introduction |
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The laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The laser has been invented and developed in 1959 and 1960. |
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How a Laser Works |
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The below figure shows schematically how a population inversion can be achieved so that laser action-or lasing us it is called can occur. |
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Characteristics of Laser Light |
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A laser beam departs from strict parallelism only because of diffraction effects, determined by the wavelength and the diameter of the exit aperture. Light from other sources can be made into an approximately parallel beam by a lens or a mirror, but the beam divergence is much greater than for laser light. |
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Applications of Lasers |
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The smallest lasers used for telephone communication over optical fibres have as their active medium a semiconducting gallium arsenide crystal about the size of a pin-head. |
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Ruby Laser |
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Ruby is a crystalline form of silica(Al2O3) with a slight natural doping of chromium. It has the following advantages for being used as a laser. |
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Helium - Neon Gas Laser |
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The glass discharge tube is filled with a 80%-20% mixture of the inert gases helium and neon. |
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Principles of Light Modulation |
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Let us consider the case of electromagnetic radiation in an ionised medium. Suppose we increase/decrease the amplitude of an electromagnetic wave passing through an ionised gas. |
