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1、外文翻译CommunicationSystemAgeneralizedcommunicationsystemhasthefollowingcomponents:(a)InformationSource.Thisproducesamessagewhichmaybewrittenorspokenwords,orsomeformofdata.(b)Transmitter.Thetransmitterconvertsthemessageintoasignal,theformofwhichissuitablefortrans
2、missionoverthecommunicationchannel.(c)CommunicationChannel.Thecommunicationchannelisthemediumusedtransmitthesignal,fromthetransmittertothereceiver.Thechannelmaybearadiolinkoradirectwireconnection.(d)Receiver.Thereceivercanbethoughtofastheinverseofthetransmitte
3、r.Itchangesthereceivedsignalbackintoamessageandpassesthemessageontoitsdestinationwhichmaybealoudspeaker,teleprinterorcomputerdatabank.Anunfortunatecharacteristicofallcommunicationchannelsisthatnoiseisaddedtothesignal.Thisunwantednoisemaycausedistorionsofsoundi
4、natelephone,orerrorsinatelegraphmessageordata.FrequencyDiversionMultiplexingFrequencyDiversionMultiplexing(FDM)isaoneofanalogtechnologies.Aspeechsignalis0~3kHz,singlesidebandamplitude(SSB)modulationcanbeusedtotransferspeechsignaltonewfrequencybands,foursimilar
5、signals,forexample,movedbySSBmodulationtosharethebandfrom5to20kHz.Thegapsbetweenchannelsareknownasguardspacesandtheseallowforerrorsinfrequency,inadequatefiltering,etcintheengineeredsystem.Oncethisnewbasebandsignal,a“group”of4channels,hasbeenformeditismovedarou
6、ndthetrunknetworkasasingleunit.Ahierarchycanbesetupwithseveralchannelsforminga“group”,severalgroupsa“supergroup”andseveral“supergroup”eithera“mastergroup”or“hypergroup”.Groupsorsupergroupsaremovedaroundassingleunitsbythecommunicationsequipmentanditisnotnecessa
7、ryfortheradiostoknowhowmanychannelsareinvolved.Aradiocanhandleasupergroupprovidedsufficientbandwidthisavailable.Thesizeofthegroupsisacompromiseastreatingeachchannelindividuallyinvolvesfarmoreequipmentbecauseseparatefilters,modulatorsandoscillatorsarerequiredfo
8、reverychannelratherthanforeachgroup.Howeverthefailureofonemodulewillloseallofthechannelsassociatedwithagroup.TimeDiversionMultiplexingItispossible,withpulsemodulationsystems,touset
