Т е к с т 1. Telecommunication.

(3.800)

The basic elements of a telecommunication system are:

– a transmitter, that takes information and converts it to a signal for transmission;

– a transmission medium, over which the signal is transmitted;

– a receiver, that receives and converts the signal back into usable information.

For example, consider a radio broadcast. In this case the broadcast tower is the transmitter, the radio is the receiver and the transmission medium is free space. Often telecommunication systems are two-way and devices act as both, a transmitter and receiver or transceiver. For example, a mobile phone is a transceiver. Telecommunication over a phone line is called point-to-point communication, because it is between one transmitter and one receiver; telecommunication through radio broadcasts is called broadcast communication, because it is between one powerful transmitter and numerous receivers. Signals can either be analogue or digital. In an analogue signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (e.g. 1's and 0's).

Telecommunications devices convert different types of information, such as sound and video, into electrical or optical signals. Electrical signals typically travel along a medium, such as copper wire or are carried over the air as radio waves. Optical signals typically travel along a medium, such as strands of glass fibers. When a signal reaches its destination, the device on the receiving end converts the signal back into an understandable message, such as sound over a telephone, moving images on a television, or words and pictures on a computer screen.

A collection of transmitters, receivers or transceivers, that communicate with each other is known as a network. Digital networks may consist of one or more routers that route data to the correct user. An analogue network may consist of one or more switches that establish a connection between two or more users. For both types of network, a repeater may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from noise. A channel is a division in a transmission medium, so that it can be used to send multiple independent streams of data. For example, a radio station may broadcast at 96 MHz, while another radio station may broadcast at 94.5 MHz. In this case the medium has been divided by frequency and each channel received a separate frequency to broadcast on. Alternatively one could allocate each channel a recurring segment of time over which to broadcast.

The shaping of a signal to convey information is known as modulation. Modulation is a key concept in telecommunications and is frequently used to impose the information of one signal on another. Modulation is used to represent a digital message as an analogue waveform. This is known as keying and several keying techniques exist; these include phase-shift keying, frequency-shift keying, amplitude-shift keying and minimum-shift keying. Bluetooth, for example, uses phase-shift keying for exchanges between devices.

However, more relevant to earlier discussion, modulation is also used to boost the frequency of analogue signals. This is because a raw signal is often not suitable for transmission over long distances of free space due to its low frequencies. Hence its information must be superimposed on a higher frequency signal (known as a carrier wave) before transmission.

There are several different modulation schemes available to achieve this; some of the most basic being amplitude modulation and frequency modulation. An example of this process is a DJ's voice being superimposed on a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio).

Vocabulary

transceiver – приемо-передатчик

point-to-point – двухточечный, двухпунктовый

continuously – непрерывно, постоянно

with respect to – зд.: относительно

strand – пучок, кабель

router – маршрутизатор

route – направлять

amplify – усиливать

combat – борьба

attenuation – затухание

render – передавать

indistinguishable – неотличимый

alternatively – в качестве альтернативы

allocate – распределять

recurring – периодический, повторяющийся, рекуррентный

convey – передавать

impose – налагать

keying – работа на клавиатуре, манипуляция

phase-shift – изменение фазы

raw – необработанный

superimposed – наложенный

carrier wave – несущее колебание

Т е к с т  2. Reciprocating Internal Combustion Engines

( 2 .000)

Introduction

Perhaps the best-known engine in the world is the reciprocating internal combustion (IC) engine. Virtually every person, who has driven an automobile or pushed a power lawn - mower has used one. By far the most widely used IC engine is the spark-ignition gasoline engine, which takes us to school and work and on pleasure jaunts. Although others had made significant contributions, Nikolaus Otto is generally credited with the invention of the engine and with the statement of its theoretical cycle.

Another important engine is the reciprocating engine, that made the name of Rudolf Diesel famous. The Diesel engine, the workhorse of the heavy truck industry, is widely used in industrial power and marine applications. It replaced the reciprocating steam engine in railroad locomotives about fifty years ago and remains dominant in that role today.

The piston, cylinder, crank, and connecting rod provide the geometric basis of the reciprocating engine. While two-stroke-cycle engines are in use and of continuing interest, the discussion here will emphasize the more widely applied four-stroke-cycle engine. In this engine the piston undergoes two mechanical cycles for each thermodynamic cycle. The intake and compression processes occur in the first two strokes, and the power and exhaust processes in the last two. These processes are made possible by the crank-slider mechanism, discussed next.