L13-1

Lesson13DigitalTransmissionTechnologyText1BasebandandPassbandKeyPoints要點(diǎn)離散和連續(xù)、基帶和頻帶數(shù)據(jù)和數(shù)字、digit、digital、numeralelectric和electrical信息和消息PSK、ASK、FSK模數(shù)轉(zhuǎn)換數(shù)據(jù)壓縮數(shù)據(jù)data和數(shù)字digit、digital是不同的概念，數(shù)字是數(shù)據(jù)承載形式中的一種。數(shù)據(jù)傳輸datatransmission一般是指數(shù)字化了的傳輸digitalized/digitaltransmission，只有在和模擬傳輸analogtransmission相比較的時(shí)候才會(huì)加以區(qū)分。Information和message都可翻譯成“信息”，但information側(cè)重于“人理解了的”內(nèi)容，是不可數(shù)的（和news一樣）一條信息：apieceofinformation許多信息：manypiecesofinformation。Message和data一樣，側(cè)重于信息的載體，一般是可數(shù)名詞。通信側(cè)重information，通訊側(cè)重message。Data和digitalinformation、communication和transmission、information和message都是目的和手段、內(nèi)容和形式的關(guān)系，但界限其實(shí)是模糊的。數(shù)字：digit組成numeral（figure），而numeral（具體）代表number（抽象）Numeral：表示數(shù)字的，digital：屬性是數(shù)字的，名：數(shù)字電視Whileanalogcommunicationisthetransferofanaloginformationoriginallyrepresentedbyphysicalcontinuouslyvaryinganalogsignals,digital(data)communicationisthetransferofdiscretemessages,usingadigitalmodulationmethod,overdigitaloranalog,point-to-pointorpoint-to-multipointcommunicationchannel.Thediscretemessagesareeitherrepresentedbyasequenceofphysicalpulses,orbyalimitedsetofphysicalcontinuouslyvaryingwaveforms,Examplesofsuchchannelsarecopperwires,opticalfibers,wirelesscommunicationchannels,storagemediaandcomputerbuses.Thedata(digitalinformation)isrepresentedasanelectromagneticsignal,suchasanelectricalvoltage,radiowave,microwave,orinfraredsignal.Thephysicallytransmittedsignalmaybeoneofthefollowing:Abasebandsignal(digitalinformation-over-digitaltransmission):AsequenceofelectricalpulsesorlightpulsesproducedbymeansofalinecodingschemesuchasManchestercoding曼徹斯特編碼.Thisistypicallyusedinserialcables,wiredlocalareanetworkssuchasEthernet,andinopticalfibercommunication.Itresultsinapulseamplitudemodulated(PAM)signal,alsoknownasapulsetrain，usinginanaloginformation-over-digital

transmission.Apassbandsignal(digitalinformation-over-analogtransmission):Amodulatedsinewavesignal(digitizedanalogsignal)representingadigitalbit-stream.ThesignalisproducedbymeansofadigitalmodulationmethodsuchasPSK(PhaseShiftKeying),ASK(Amplitude

Shift

Keying)orFSK(FrequencyShiftKeying).Thepassbandmodulationandcorrespondingdemodulation(alsoknownasdetection檢波)arecarriedoutbymodemequipment.Thisisusedinwirelesscommunication,andovertelephonenetworklocal-loopandcable-TVnetworks.Note:Thisclassificationisbasedonwhatistransmitted.Accordingtothemostcommondefinitionofdigitalcommunication,bothbasebandandpassbandsignalsrepresentingbit-streamsareconsideredasdigitaltransmission,whileanalternativedefinitiononlyconsidersthedigitalbasebandsignalasdigitalcommunication,andpassbandtransmissionofdigitaldataasaformofdigital-to-analogconversion.Datatransmittedmaybedigitalmessagesoriginatingfromadatasource,forexampleacomputerorakeyboard.Itmayalsobeananalogsignalsuchasaphonecalloravideosignal,digitizedintoabit-streamforexampleusingpulse-codemodulatio