Unit17CommunicationSystemTextKeywordsTechnicaltermsExpressionsNotestothetextText

Inthemostfundamentalsense,“communication”meanstheconveyingortransmissionofinformationfromonepointtoanotherthroughasuccessionofprocesses.Thisdefinitionisnotveryprecise,butthesubjectofcommunicationisverybroad.Today,communicationentersourdailylivesinsomanydifferentwaysthatitisveryeasytooverlookthemultitudeofitsfacets.

Thetelephonesatourhands,theradiosandtelevisionsinourlivingrooms,thecomputerterminalswithaccesstotheInternetinourofficesandhomes,andournewspapersareallcapableofprovidingrapidcommunicationsfromeverycorneroftheglobe.Communicationprovidesthesensesofshipsonthehighseas,aircraftinflight,androcketsandsatellitesinspace.Communicationthroughawirelesstelephonekeepsacardriverintouchwiththeofficeorhomemilesaway.Communicationkeepsaweatherforecasterinformedofconditionsmeasuredbyamultitudeofsensors.Indeed,thelistofapplicationsinvolvingtheuseofcommunicationinonewayoranotherisalmostendless.

AdiagramofthebasicunitscomprisingacommunicationsystemisshowninFig.17-1.Noteverycommunicationsystemmakesuseofallindicatedoperations,buteachalwaysinvolvesatransmissionmediumofsomekind.Theencoderchoosesthebestformforthesignalinordertooptimizeitsdetectionattheoutput.Thedecoderperformstheinverseoperationtomakethebestdecision,basedontheavailablesignals,thatagivenmessagewasindeedsent.Thedesignoftheencoderanddecodermustrelyonadetailedmathematicaldescriptionofinformationtransmission.Whilethesubjectofcodingoftencarrieswithitanairofsecrecy,amoreimportantmotiveinmanymoderncodingsystemsistheimprovedefficiencyinconveyinginformation.

Fig.17-1aCommunicationSystem

Themodulatorproducesavaryingsignalatitsoutputwhichisproportionalinsomewaytothesignalappearingacrossitsinputterminals.Forexample,asinusoidalmodulatormayvarytheamplitude,frequency,orphaseofasinusoidalindirectproportiontothevoltageinput.Therolesoftheencoderandthemodulatoraresimilarinthatbothpreparethesignalformoreefficienttransmission.However,theprocessofcodingisdesignedtooptimizetheerror-freedetectionthatagivenmessageisbeingsent,whereastheprocessofmodulationisdesignedtoimpresstheinformationsignalontothewaveformtobetransmitted.Thedemodulatorperformstheinverseoperationofthemodulatorisoftenreferredtoasa“modem”indatatransmission.

Thetransmissionmediumisthecruciallinkinthesystem.Withoutit,therewouldbenocommunicationproblem.Thetransmissionmediummayincludetheionosphere,thetroposphere,freespace,orsimplyatransmissionline.Inanycase,attenuationanddistortion,aswellasnoisesignalsgeneratedinthemediaandthetransmittingandreceivingequipment,areintroduced.Forourpurpose,noisesignalsareanyelectricalsignals(voltagesorcurrents)whichinterferewiththeerror-freereceptionofthemessage-bearingsignal.Keywords

convey vt.運(yùn)輸，輸送

transmission n.發(fā)射，傳送，傳輸

precise adj.精確的，準(zhǔn)確的

facet n.方面

globe n.地球，世界

encoder n.譯碼器，編碼器

optimize vt.使最優(yōu)化

decoder n.解碼器

inverse adj.倒轉(zhuǎn)的，反轉(zhuǎn)的

description n.描述

motive n.動(dòng)機(jī)，目的；adj.發(fā)動(dòng)的，運(yùn)動(dòng)的

crucial adj.至關(guān)緊要的

ionosphere n.電離層

troposphere n.

[氣]對(duì)流層Technicalterms

communicationsystem 通信系統(tǒng)

computerterminals 計(jì)算機(jī)終端設(shè)備

wirelesstelephone 無(wú)線電話

highseas 公海

transmissionmedium 傳輸媒質(zhì)

codingsystems 譯碼系統(tǒng)

asinusoidalmodulator 正弦調(diào)制器

theerror-freedetection 無(wú)誤差檢測(cè)Expressions

becapableof... 能夠…

keeps…intouchwith… 與…保持聯(lián)系

makeuseof... 使用…

inorderto... 為了…

relyon 依靠

beproportionalto… 與…成比例Notestothetext

1.Today,communicationentersourdailylivesinsomanydifferentwaysthatitisveryeasytooverlookthemultitudeofitsfacets.

今天，我們很容易觀察到通信在我們的生活中幾乎無(wú)處不在。句中，“so…that…”意為：如此…以致于…，引導(dǎo)結(jié)果狀語(yǔ)從句，“that”后面的動(dòng)詞不定式短語(yǔ)作從句的形式

主語(yǔ)。

2.Thedecoderperformstheinverseoperationtomakethebestdecision,basedontheavailablesignals,thatagivenmessagewasindeedsent.

解碼器完成相反的操作，它根據(jù)提供的信號(hào)作出最佳判斷：某個(gè)消息確實(shí)已發(fā)出。句中，動(dòng)詞不定式“tomakethebestdecision”作目的狀語(yǔ)；“basedontheavailablesignals”為插入語(yǔ)；“thatagivenmessagewasindeedsent”為定語(yǔ)從句，修飾先行詞“decision”。

3.However,theprocessofcodingisdesignedtooptimizetheerror-freedetectionthatagivenmessageisbeingsent,whereastheprocessofmodulationisdesignedtoimpresstheinformationsignalontothewaveformtobetransmitted.

可是