Архитектура Аудит Военная наука Иностранные языки Медицина Металлургия Метрология
Образование Политология Производство Психология Стандартизация Технологии
Television and telecommunication
Television is a widely used telecommunication medium for sending (broad-
casting) and receiving moving images, either monochromatic (black and white) or color, usually accompanied by sound.
In its early stages of development, television included only those devices employing a combination of optical, mechanical and electronic technologies to capture, transmit and display a visual image. As it is known, all modern television systems are based on electronic technologies, however the knowledge gained from the work on mechanical-dependent systems was important in the development of electronic television.
In 1884 Paul G. Nipkow, a 20-year old university student in Germany invented the first electromechanical television system which employed a scanning disk, a spinning disk with a series of holes spiraling toward the center, for “rasterization”, the process of converting a visual image into a stream of electrical pulses. The beginning of the 20-th century brought advances in amplifier tube technology and the use of a rotating mirror-drum scanner to capture the image. Most of the 20-th century television sets depended also upon the cathode-ray tube invented by Karl Braun in 1921.
Telecommunication, transmission of signals over a distance for the purpose of communication, is an important part of modern society. In telecommunication, a communication system is a collection of individual communications networks, transmission systems, relay stations, substations and data terminal equipment usually capable of interconnection and interoperation to form an integral whole. The components of a communication system serve a common purpose, they are technically compatible, use common procedures, respond to controls and operate in unison.
The word “radar” means Radio Determination and Ranging. Radar equipment is capable of determining by radio echoes the presence of objects, their direction, range and recognizing their character. Radar detects objects at a distance by reflecting radio waves off them. The delay caused by the echo measures he distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface.
There are several types of radar sets, all of them consisting of six essential components, namely: a transmitter, a receiver, an antenna system, an indicator, a timer and, of course, a power supply.
A radar set detects by sending out short powerful pulses of ultra-high frequency radio wave energy from a high power transmitter. The directional antenna takes this energy from the transmitter and radiates it in a beam (similar to that of a searchlight). As the transmitted energy strikes an object, a portion of it is reflected back. The receiver picks up the returning echo through its antenna and translates it into visual readable signals on a fluorescent screen. The appearance of these signals show the presence of an object within the field of view of radar.
Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on ships and long-distance aircraft. General-purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so he receiver can determine the type of surface of the reflector. Search radars scan a wide area with pulses of short radio waves and sometimes use the Doppler effect to separate moving vehicles from clutter. Weather radars can even measure wind speed.
4. Coordinate the words given in the left column with their interpretation in
Unit 4. Supplementary texts
Communication is the basic process of exchanging information. The basic components of an electronic communication system are:
2) Communication channel.
A Transmitter is a collection of electronic circuits designed to convert the information into a signal suitable for transmission over a given communication medium.
A radio communication system sends signals by radio. Types of radio communication systems deployed depend on technology, standards, regulations, radio spectrum allocation, user requirements, service positioning, etc. The radio equipment involved in communication systems includes a transmitter and a receiver, each having an antenna and appropriate terminal equipment such as a microphone at the transmitter and a loudspeaker at the receiver in the case of a voice-communication system.
The power consumed in a transmitting station varies depending on the distance of communication and the transmission conditions. The power received at the receiving station is usually only a tiny fraction of the transmitter's output, since communication depends on receiving the information, not the energy that was transmitted.
Classical radio communications systems use frequency-division multiplexing (FDM) as a strategy to split up and share the available radio-frequency bandwidth for use by different parties communications concurrently. Modern radio communication systems include those that divide a radio-frequency band by time-division multiplexing (TDM) and code-division multiplexing (CDM) as alternatives to the classical FDM strategy. These systems offer different tradeoffs in supporting multiple users, beyond the FDM strategy that was ideal for broadcast radio but less so for applications such as mobile telephony.
A radio communication system may send information only one way. For example, in broadcasting a single transmitter sends signals to many receivers. Two stations may take turns sending and receiving, using a single radio frequency; this is called "simplex." By using two radio frequencies, two stations may continuously and concurrently send and receive signals - this is called "duplex" operation.
Do the following tasks:
1) Translate into Russian.
2) Write a summary.
What is Modulation?
Modulation is the process of superimposing the information contents of a modulating signal on a carrier signal (which is of high frequency) by varying the characteristics of carrier signal according to the modulating signal.
Modulation is a process in which the base band signal modifies another high-frequency signal called the carrier.
Types of Modulation
We can modulate the information-bearing signal into two types namely.These
are called Modulation Techniques.
1. Analog Modulation
2. Digital Modulation
Analog modulation is the process of converting an analog input signal into a
signal that is suitable for RF transmission.
Digital modulation is the process of converting a digital bistream into an analog signal suitable for RF transmission.
Modulation Index indicates the depth of modulation. As the amplitude of the modulating signal increases, modulation index increases. For amplitude modulation, the modulation index is given as
m =Amplitude of modulating signal Amplitude of the carrier.
The Analog carrier signal is modulated by analog information signal so that information bearing analog signal can travel larger distance without the fear of loss due to absorption.
The Analog modulation is of two types:
1) Amplitude Modulation
2) Angle Modulation
The Angle modulation is further classified as Frequency modulation and Phase Modulation.
In this type of modulation, the strength of the carrier signal is varied with the modulating signal.
Frequency Modulation: In this type of modulation, the frequency of the carrier signal is varied with the modulating signal.
Phase Modulation: In this type of modulation, the phase of the carrier signal is varied with the modulating signal. It is the variant of the frequency modulation.
The analog carrier signal is modulated by digital information signal. It is also considered as digital to analog conversion.
Do the following tasks:
1)Translate the texts.
2)Speak about types of modulation.
Channels of communications
Old telephone wires are a challenging communications channel for modern digital communications.
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example, a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.
Communicating data from one location to another requires some form of pathway or medium. These pathways, called communication channels, use two types of media: cable (twisted-pair wire, cable, and fiber-optic cable) and broadcast (microwave, satellite, radio, and infrared). Cable or wire line media use physical wires of cables to transmit data and information. Twisted-pair wire and coaxial cables are made of copper, and fiber-optic cable is made of glass.
In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is also a kind of channel, which can be sent to (written) and received from (read).
A channel can take many forms. Examples of communications channels include:
1) A connection between initiating and terminating nodes of a circuit.
2) A single path provided by a transmission medium via either physical
separation, such as by multipair cable or electrical separation, such as by frequency-division or time-division multiplexing.
3) A path for conveying electrical or electromagnetic signals, are usually
distinguished from other parallel paths. It includes:
-A storage that can communicate a message over time as well as space.
-The portion of a storage medium, such as a track or band, that is accessible to a given reading or writing station or head.
-A buffer from which messages can be 'put' and 'got'.
In a communication system, the physical or logical link connects a data source to a data sink.
4) A specific radio frequency, pair or band of frequencies, are usually named
with a letter, number, or code word, and often allocated by international agreement.
A channel can be modelled physically by trying to calculate the physical processes, which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.
Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p (i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel.
Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations.
Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined.
Communication channels are also studied in a discrete-alphabet setting. This corresponds to abstracting a real world communication system in which the analog->digital and digital->analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input.
A channel model may either be digital (quantified, e.g. binary) or analog.
Do the following tasks:
1) Read and translate the text.
2) Write a summary.
3) Speak on the forms, which channels of communication can take.
Transmitter and modulation
Each system contains a transmitter. This consists of a source of electrical energy, producing alternating current of a desired frequency of oscillation. The transmitter contains a system to modulate (change) some property of the energy produced to impress a signal on it. This modulation might be as simple as turning the energy on and off, or altering more subtle properties such as amplitude, frequency, phase, or combinations of these properties. The transmitter sends the modulated electrical energy to a tuned resonant antenna; this structure converts the rapidly changing alternating current into an electromagnetic wave that can move through free space (sometimes with a particular polarization).
An audio signal (top) may be carried by an AM or FM radio wave.
Amplitude modulation of a carrier wave works by varying the strength of the transmitted signal in proportion to the information being sent. For example, changes in the signal strength can be used to reflect the sounds to be reproduced by a speaker, or to specify the light intensity of television pixels. It was the method used for the first audio radio transmissions, and remains in use today. "AM" is often used to refer to the medium wave broadcast band.
Frequency modulation varies the frequency of the carrier. The instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.
FM is commonly used at VHF radio frequencies for high fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM.
Angle modulation alters the instantaneous phase of the carrier wave to transmit a signal. It is another term for phase modulation.
An antenna (or aerial) is an electrical device, which converts electrical currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving.
Once generated, electromagnetic waves either travel through space directly, or have their path altered by reflection, refraction or diffraction. The intensity of the waves diminishes due to geometric dispersion (the inverse-square law); some energy may also be absorbed by the intervening medium in some cases. Noise will generally alter the desired signal; this electromagnetic interference comes from natural sources, as well as from artificial sources such as other transmitters and accidental radiators. Noise is also produced at every step due to the inherent properties of the devices used.
If the magnitude of the noise is large enough, the desired signal will no longer be discernible; this is the fundamental limit to the range of radio communications.
Electrical resonance of tuned circuits in radios allow individual stations to be selected. A resonant circuit will respond strongly to a particular frequency and much less so to differing frequencies. This allows the radio receiver to discriminate between multiple signals differing in frequency.
Receiver and demodulation
A crystal receiver consists of an antenna, rheostat, coil, crystal rectifier, capacitor, headphones and ground connection. The electromagnetic wave is intercepted by a tuned receiving antenna; this structure captures some of the energy of the wave and returns it to the form of oscillating electrical currents. At the receiver, these currents are demodulated, which is conversion to a usable signal form by a detector sub-system. The receiver is "tuned" to respond preferentially to the desired signals, and reject undesired signals.
Early radio systems relied entirely on the energy collected by an antenna to produce signals for the operator. Radio became more useful after the invention of electronic devices such as the vacuum tube and later the transistor, which made it possible to amplify weak signals. Today radio systems are used for applications from walkie-talkie children's toys to the control of space vehicles, as well as for broadcasting, and many other applications.
A radio receiver receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna. It amplifies the signal to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.
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