外文翻译--数字通信系统

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1、英文资料及中文翻译DigitalCommunicationSystemFormodernpeople,ifyoudonotknow“DigitalCommunication”or“DigitalSignal”,itisseemtobeoutdated,evenwouldbelaughedas“aignorantmen”.wellthen,whatisthe“DigitalCommunication”andwhatisthe“DigitalSignal”?Thistextistodoangeneralizeinthewayofbriefandp

2、lain.1、WhyDigital?Whyarecommunicationsystem，militaryandcommercialalike，“goingdigital”？Theprimaryadvantageistheeasewithwhichdigitalsignals，comparedwithanalogsignals，areregenerated.Figure1illustratesanidealbinarydigitalpulsepropagatingalongatransmissionline.Theshapeofthewavef

3、ormisaffectedbytwobasicmechanisms:(1)asalltransmissionlinesandcircuitshavesomenonidealfrequencetransferfunction，thereisadistortingeffectontheidealandpulse;and(2)unwantedelectricalnoiseorotherinterferencefurtherdistortsthepulsewaveform.Bothofthesemechanismscausethepulseshape

4、todegradeasafunctionoflinelenge，asshowninFigure1.Duringthetimethatthetransmittedpulsecanstillbereliablyidentifiedbeforeitisdegradedtoanambiguousstate，thepulseisamplifiedbyadigitalamplifierthatrecoversitsoriginalidealshape.Thepulseisthus“rebor”orregenerated.Circuitsthatperfo

5、rmthisfunctionatregularintervalsalongatransmissionsystemarecalledregenerativerepeaters.Distance1Distance2Dance3stance4Distance5OriginalSomesignalDegradedSignalisbadlyAmplificationdstortiuondstortiuondstortiuondegradedregeneratepulsePropagationdistanceFigure1Pulsedegradation

6、andregenerationDigitalcircuitarelesssubjecttodistortionandinterferncethananalogcircuit.Becansebinarycircuitsopoerateinoneoftwostatesfullyonfullyof—tobemeaningful，adisturbancemustbelargeenoughtochangethecircuitooperatingpointfromonestatetotheother.Suchtwo-stateoperationfacil

7、itatessignalregenerationandthuspreventsnoiseandotherdisturbancesfromaccumulatingintransmission.Analogsignals，however，arenottwo-statesignals;theycantakeaninfinitevarietyofshapes.Withanalogcircuits，evenasmalldisturbancecanrenderthereproducedwaveformunacceptablydistorted.Oncet

8、heanalogsignalisdistorted，thedistortioncannotberemovedbyamplification.Becauseaccum