《電子信息類專業(yè)英語 》課件-第6章

Unit6ElectronicCommunicationsPassageA

IntroductiontoElectronicCommunicationsPassageB

ThePrincipleofPCMPassageC

InformationSources

PassageA

Introduction

to

Electronic

Communications

1.HistoricalPerspective

Thefundamentalpurposeofanelectroniccommunicationsystemistotransferinformationfromoneplacetoanother.[1]

Thus,electroniccommunicationscanbesummarizedasthetransmission,reception,andprocessingofinformationbetweentwoormorelocationsusingelectroniccircuits.

Theoriginalsourceinformationcanbeinanalog(continuous)form,suchasthehumanvoiceormusic,orindigital(discrete)form,suchasbinary-codednumbersoralphanumericcodes.[2]Allformsofinformation,however,mustbeconvertedtoelectromagneticenergybeforebeingpropagatedthroughanelectroniccommunicationssystem.

SamuelMorsedevelopedthefirstelectroniccommunicationssystemin1837.Morseusedelectromagneticinductiontotransferinformationintheformofdots,dashes,andspacesbetweenasimpletransmitterandreceiverusingatransmissionlineconsistingofalengthofmetallicwire.Hecalledhisinventionthetelegraph.In1876AlexanderGrahamBellandThomasA.Watsonwerethefirstsuccessfullytransferhumanconversationoveracrudemetallicwirecommunicationssystem,theycalledthetelephone.

GuglielmoMarconisuccessfullytransmittedthefirstwirelessradiosignalsthroughEarth’satmospherein1894,andin1908LeeDeforestinventedthetriodevacuumtubewhichprovidedthefirstpracticalmeansofamplifyingelectricalsignals.Commercialradiobeganin1920whenradiostationsbeganbroadcastingamplitude-modulated(AM)signals,andin1933,MajorEdwinHowardArmstronginventedfrequencymodulation(FM).CommercialbroadcastingofFMbeganin1936.

Althoughthefundamentalconceptsandprinciplesofelectroniccommunicationshavechangedlittlesincetheirinception,themethodsandcircuitsusedtoimplementthemhaveundergoneconsiderablechange.Inrecentyears,transistorsandlinearintegratedcircuitshavesimplifiedthedesignofelectroniccommunicationscircuits,thusallowingforminiaturization,improvedperformanceandreliability,andreducedoverallcosts.Inrecentyears,therehasbeenanoverwhelmingneedformoreandmorepeopletocommunicatewitheachother.Thistremendousneedhasstimulatedamonumentalgrowthintheelectroniccommunicationsindustry.Modernelectroniccommunicationssystemsincludemetalliccablesystems,microwaveandsatelliteradiosystems,andopticalfibersystems.

AtimechartshowingthehistoricaldevelopmentofcommunicationsisgiveninTable6.1.Thereaderisencouragedtospendsometimestudyingthistabletoobtainappreciationforthechronologyofcommunications.Notethatalthoughthetelephonewasdevelopedlateinthenineteenthcentury,thefirsttransatlantictelephonecablewasnotcompleteduntil1954.Previoustothisdate,transatlanticcallswerehandledviashortwaveradio.Similarly,althoughtheBritishbegantelevisionbroadcastingin1936,transatlantictelevisionrelaywasnotpossibleuntil1962whentheTelstarIsatellitewasplacedintoorbit.Digitaltransmissionsystemembodiedbytelegraphsystems-weredevelopedinthe1850sbeforeanalogsystems-thetelephone-inthetwentiethcentury.[3]

Figure6.1CommunicationSystem

Thesignal-processingblockatthetransmitterconditionsthesourceformoreefficienttransmission.[4]Forexample,inananalogsystem,thesignalprocessormaybeananaloglow-passfilterthatisusedtorestrictthebandwidthofm(t).Inahybridsystem,thesignalprocessormaybeananalog-to-digitalconverter(ADC).Thisproducesa“digitalword”thatrepresentssamplesoftheanaloginputsignal.Inthiscase,theADCinthesignalprocessorisprovidingsourcecodingoftheinputsignal.Inaddition,thesignalprocessormayalsoaddparitybitstothedigitalwordtoprovidechannelcodingsothaterrordetectionandcorrectioncanbeusedbythesignalprocessorinthereceivertoreduceoreliminatebiterrorsthatarecausedbynoiseinthechannel.[5]Thesignalattheoutputofthetransmittersignalprocessorisabasebandsignalbecauseithasconcentratednear

f=0.

Thetransmittercarriercircuitconvertstheprocessedbasebandsignalintoafrequencybandthatisappropriateforthetransmissionmediumofthechannel.Forexample,ifthechannelconsistsofafiberopticcable,thecarriercircuitsconvertthebasebandinputtolightfrequencies,andthetransmittedsignal

s(t)islight.Ifthechannelpropagatesbasebandsignals,nocarriercircuitsareneeded,ands(t)canbeoutputoftheprocessing

circuitatthetransmitter.Carriercircuitsareneededwhenthetransmissionchannelislocatedinabandoffrequenciesaroundfc>>0.

Inthiscase,s(t)issaidtobeabandpassbecauseitisdesignedtohavefrequencieslocatedinabandaboutfc.Forexample,anamplitudemodulated(AM)broadcastingstationwithanassignedfrequencyof850kHzhasacarrierfrequencyof

fc=850kHz.Themappingofthebasebandinputinformationwaveformm(t)intothebandpasssignals(t)iscalledmodulation.

3.TheElectromagneticSpectrum

Incommunicationsystemsthatusetheatmosphereforthetransmissionchannel,

interferenceandpropagationconditionsarestronglydependentonthetransmissionfrequency.Theoretically,anytypeofmodulation(e.g.,amplitudemodulation,frequencymodulation,singlesideband,phase-shiftkeying,frequency-shiftkeying,etc.)couldbeusedatanytransmissionfrequency.However,toprovidesomesemblanceoforderandforpoliticalreasons,governmentregulationsspecifythemodulationtype,bandwidth,andtypeofinformationthatcanbetransmittedoverdesignedfrequencybands.

Onaninternationalbasis,frequencyassignmentsandtechnicalstandardsaresetbytheInternationalTelecommunicationsUnion(ITU).TheITUisaspecializedagencyoftheUnitedNations,andtheITUadministrativeheadquartersarelocatedinGeneva,Switzerland,withastaffof700persons.Thisstaffisresponsibleforadministeringthe

agreementsthathavebeenratifiedbythe184membernationsoftheITU.In1992,theITUwasrestructuredintothreesectors.TheRadioCommunicationSector(ITU-R)providesfrequencyassignmentsandisconcernedwithefficientuseoftheradiofrequencyspectrum.TheTelecommunicationsStandardizationSection(ITU-T)examinestechnical,operating,

andtariffquestions.Itrecommendsworldwidestandardsforthepublic

telecommunicationsnetwork(PTN)andrelatedradiosystems.TheTelecommunication

DevelopmentSector(ITU-D)providestechnicalassistance,especiallyfordevelopingcountries.Thisencouragesafullarrayoftelecommunicationservicestobeeconomically

providedandintegratedintotheworldwidetelecommunicationsystem.Before1992,theITUwasorganizedintotwomainsectors:theInternationalTelegraphandTelephoneConsultativeCommittee(CCITT)andtheInternationalRadioConsultativeCommittee(CCIR).

EachmembernationoftheITUretainssovereigntyoverthespectralusagesandstandardsadoptedinitsterritory.However,eachnationisexpectedtoabidebytheoverallfrequencyplanandstandardsthatareadoptedbytheITU.Usually,eachnationestablishesanagencythatisresponsibleforadministrationoftheradiofrequencyassignmentswithinitsborders.IntheUnitedStates,theFCCregulatesandlicensesradiosystemsforthegeneralpublic,andstateandlocalgovernment.

Inaddition,theNationalTelecommunicationandInformationAdministration(NTIA)isresponsibleforU.S.governmentandU.S.militaryfrequencyassignments.TheinternationalfrequencyassignmentsaredividedintosubbandsbytheFCCtoaccommodate70categoriesofservicesand9milliontransmitters.Table6.2givesagenerallistingoffrequencybands.Theircommondesignations,typicalpropagationconditions,andtypicalservicesassignedtothesebands.

4.BandwidthandInformationCapacity

Thetwomostsignificantlimitationsontheperformanceofacommunicationssystemarenoiseandbandwidth.Noisewillbediscussedlater.Thebandwidthofaninformationsignalissimplythedifferencebetweenthehighestandlowestfrequenciescontainedintheinformation,andthebandwidthofacommunicationschannelisthedifferencebetweenthehighestandlowestfrequenciesthatthechannelwillallowtopassthroughit(i.e.,itspassband).Thebandwidthofacommunicationschannelmustbelarge(wide)enoughtopassallsignificantinformationfrequencies.

Inotherwords,thebandwidthofthecommunicationschannelmustbeequaltoorgreaterthanthebandwidthofinformation.For

example,voicefrequenciescontainsignalsbetween300Hzand3000Hz.Therefore,avoice-frequencychannelmusthaveabandwidthequaltoorgreaterthan2700Hz.Ifacabletelevisiontransmissionsystemhasapassbandfrom500kHzto5000kHz,ithasabandwidthof4500kHz.Asageneralrule,acommunicationschannelcannotpropagateasignalthatcontainsafrequencythatischangingatarategreaterthanthebandwidthofthechannel.

Informationtheoryisahighlytheoreticalstudyoftheefficientuseofbandwidthtopropagateinformationthroughelectroniccommunicationssystems.Informationtheorycanbeusedtodeterminetheinformationcapacityofacommunicationssystem.Information

capacityisameasureofhowmuchinformationcanbetransferredthroughacommunicationssysteminagivenperiodoftime.Theamountofinformationthatcanbepropagatedthroughatransmissionsystemisafunctionofsystembandwidthandtransmissiontime.In1920,R.HartleyofBellTelephoneLaboratoriesdevelopedtherelationshipamongbandwidth,transmissiontime,andinformationcapacity.

Hartley’slawsimplystatesthatthewiderthebandwidthandthelongerthetimeoftransmission,themoreinformationthatcanbeconveyedthroughthesystem.Mathematically,Hartley’slawisstatedas

I∞B×t,whereI=informationcapacity,B=systembandwidth(Hz),t=transmissiontime(second).Thisequationshowsthatinformationcapacityisalinearfunctionanddirectlyproportionaltobothsystembandwidthandtransmissiontime.Ifthebandwidthofacommunicationschanneldoubles,theamountofinformationcancarryalsodoubles.Ifthetransmissiontimeincreaseordecrease,thereisaproportionalchangeintheamountofinformationthatcanbetransferredthroughthesystem.

Ingeneral,themorecomplextheinformationsignal,themorebandwidthrequiredtotransportitinagivenperiodoftime.Approximately3kHzofbandwidthisrequiredtotransmitvoice-qualitytelephonesignals.Incontrast200kHzofbandwidthisallocatedforcommercialFMtransmissionofhigh-fidelitymusic,andalmost6MHzofbandwidthisrequiredforbroadcast-qualitytelevisionsignals.

In1948,C.E.Shannon(alsoofBellTelephoneLaboratories)publishedapaperintheBellSystemTechnicalJournalrelatingtheinformationcapacityofacommunicationschannelinbits-per-second(bps)tobandwidthandsignal-to-noiseratio.Mathematicallystated,theShannonlimitforinformationcapacityis

whereI=informationcapacity(bps),B=bandwidth(Hz),

S/N=signal-to-noisepowerratio(unit-less).Forastandardvoice-bandcommunicationschannelwithasignal-to-noisepowerratioof1000(30dB)andabandwidthof2.7kHz,theShannonlimitforinformationcapacityis

I=26.9kbps.

Shannon’sformulaisoftenmisunderstood.Theresultsoftheprecedingexampleindicatethat26.9kbpscanbetransferredthrougha2.7kHzchannel.Thismaybetrue,butitcannotbedonewithabinarysystem.Toachieveaninformationtransmissionrateof26.9kbpsthrougha2.7kHzchannel,eachsymboltransmittedmustcontainmorethanonebitofinformation.Therefore,toachievetheShannonlimitforinformationcapacity,digitaltransmissionsystemthathavemorethantwooutputconditions(symbols)mustbeused.

Notes

［1］Thefundamentalpurposeofanelectroniccommunicationsystemistotransfer

informationfromoneplacetoanother.

電子通信系統(tǒng)的基本作用是把信息從此地傳送到彼地。

·information,intelligence,message三詞在通信技術(shù)中可能混用,都可譯為信息、消息、情報等,但information似乎用得更普遍。例如,信息技術(shù)就只能用informationtechnology(IT)。

［2］Theoriginalsourceinformationcanbeinanalog(continuous)form,suchasthehumanvoiceormusic,orindigital(discrete)form,suchasbinary-codednumbersoralphanumericcodes.

原始的信息源既可以是模擬(連續(xù))的,例如語音或音樂;也可以是數(shù)字(離散)的,例如二進制編碼數(shù)或字符碼。

·analogsignal:時間和幅值皆連續(xù)分布的信號。

·discretesignal:時間離散分布的信號。

·digitalsignal:時間和幅值皆離散分布的信號。

［3］Digitaltransmissionsystem-embodiedbytelegraphsystems-weredevelopedinthe1850sbeforeanalogsystems-thetelephone-inthetwentiethcentury.

以電報為代表的數(shù)字傳輸系統(tǒng)研發(fā)于19世紀(jì)50年代,早于以電話為代表的模擬系統(tǒng),它是20世紀(jì)才開發(fā)的。

［4］Thesignal-processingblockatthetransmitterconditionsthesourceformoreefficienttransmission.

發(fā)射機中的信號處理單元對信源進行調(diào)理,以便更有效地傳送。

·condition是動詞,作謂語,而非名詞。

·for引導(dǎo)目的狀語。

［5］Inaddition,thesignalprocessormayalsoaddparitybitstothedigitalwordtoprovidechannelcodingsothaterrordetectionandcorrectioncanbeusedbythesignal

processorinthereceivertoreduceoreliminatebiterrorsthatarecausedbynoiseinthe

channel.

此外,信號處理器還可以給數(shù)碼字加入奇偶校驗位,即提供信道編碼,使得接收機的信號處理器可以進行誤碼檢測和糾錯,以減少或消除由信道噪聲所引起的誤碼。

·paritybit奇偶校驗位。

·errordetectionandcorrection誤碼檢測和糾錯。

Exercises

1.TranslatethefollowingphrasesintoChinese.

(1)binary-codednumber

(2)Hi-Fi

(3)electromagneticinduction

(4)ADC

(5)VLSI

(6)blockdiagram

(7)PSK

(8)discretesignal

(9)PTN

(10)afullarray

(11)amplitudemodulation(AM)

(12)negative-feedbackamplifier

(13)PCM(Pulse-codeModulation)

(14)time-divisionmultiplexing(TDM)

(15)adaptiveequalization

(16)randomaccessmemory(RAM)

(17)ISDN(IntegratedServicesDigitalNetwork)

(18)Lospropagation

2.TranslatethefollowingphrasesintoEnglish.

(1)真空三極管(2)糾錯編碼

(3)視線傳播(4)電離層反射

(5)多路通信系統(tǒng)(6)奇偶校驗位

(7)頻率配置(8)頻率的整體規(guī)劃

(9)擴頻系統(tǒng)(10)高清晰度電視

(11)帶通信號(12)單邊帶

(13)國際電信聯(lián)盟(14)業(yè)余無線電

(15)信噪比

3.Answerthefollowingquestions.

(1)Whenandbywhomwasthefirstelectroniccommunicationssystemdeveloped?

(2)Whatisthefirstamplifyingdeviceforelectricalsignals?

(3)Whatsystemsareincludedinmodernelectroniccommunicationssystems?

(4)What’sthedifferencebetweenanalogsignal,discretesignalanddigitalsignal?

(5)CouldyoustateHartley’slawandShannon’sformula?

PassageBThe

Principle

of

PCM

PCMisdependentonthreeseparateoperations:sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.[1]Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,coded,asasequenceof8binarydigits.[2]

Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange300Hzto3.4kHz.[3]Practicalequipments,however,normallyuseasamplingrateof8kHz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64kHz.Figure6.2illustratesthesampling,quantizing,andcodingprocesses.

Figure6.2TheSamplingandCodingProcesses,andtheResultantPCMSignalFigure6.2TheSamplingandCodingProcesses,andtheResultantPCMSignalFigure6.2TheSamplingandCodingProcesses,andtheResultantPCMSignal

Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4kHzhasbeenrepresentedbyasignaloffrequency64kHz.However,if

only4digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32kHz.Thusthequalityoftransmissionisdependentonthepulserepetitionrate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.

Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments.Noisecanbeintroducedintotransmissionpathinmanydifferentways;perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofmostinteresttothecommunicationengineer.Basically,ifthesignalisverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise.[4]

Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepower.

Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularmessagewillalwaysbeobtainedbysimplydetectingthepresenceorabsenceofthepulse.Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformation

usingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.[5]Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigita1transmission.

Sofarinthisdiscussionwehaveassumedthateachvoicechannelhasaseparate

coder,theunitthatconvertssampledamplitudevaluestoasetofpulses;anddecoder,theunitthatperformsthereverseoperation.Thisneednotbeso,andsystemsareinoperationwhereasinglecodec(i.e.,coderanditsassociateddecoder)issharedbetween24,30,oreven120separatechannels.Ahigh-speedelectronicswitchisusedtopresenttheanaloginformationsignalofeachchannel,takeninturn,tothecodec.

Thecodecisthenarrangedtosequentiallysampletheamplitudevalue,andcodethisvalueintothe8-digitsequence.Thustheoutputtothecodecmaybeseenasasequenceof8pulsesrelatingtochannel1,thenchannel2,andsoon.Thisunitiscalledatimedivisionmultiplexer(TDM),and15illustratedinFigure6.3.Themultiplexingprinciplethatisusedisknownaswordinterleaving.Sincethewords,or8-digitsequences,areinterleavedintime.

Atthereceiveterminalademultiplexerisarrangedtoseparatethe8-digitsequencesintotheappropriatechannels.Thereadermayask,howdoesthedemultiplexerknowwhichgroupof8-digitsrelatestochannel1,2,andsoon?Clearlythisisimportant!Theproblemiseasilyovercomebyspecifyingaframeformat,whereatthestartofeachframeauniquesequenceofpulsescalledtheframecode,orsynchronizationword,isplacedsoastoidentifythestartoftheframe.Acircuitofthedemultiplexerisarrangedtodetectthesynchronizationword,andtherebyitknowsthatthenextgroupof8-digitscorrespondstochannel1.Thesynchronizationwordreoccursonceagainafterthelastchannelhasbeenreceived.[6]

Figure6.3TheFunctionoftheTimeDivisionMultiplexer(TDM)

Notes

［1］Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.

近年來,人們對這三個環(huán)節(jié)的實現(xiàn)提出了許多不同的方案,我們將對其中一些主要的方案進行討論。

·performing是perform的動名詞。動名詞雖為名詞,但仍保留著動詞的某些特征,例如它仍可帶有動詞賓語。本句中的thesethreefunctions就是performing的賓語。本課中這類例子很多,例如第四段中的最后一句:

Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigitaltransmission.

句中的overcoming和choosing都是動名詞且?guī)в凶约旱膭釉~賓語。

［2］Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,coded,asasequenceof8binarydigits.

在這些討論中,我們會看到話路中的語音信號是如何轉(zhuǎn)換成一個幅值序列的,而每個幅值又被編碼,即以8位二進制數(shù)的序列表示。

·eachvaluebeingrepresent是一種獨立分詞結(jié)構(gòu),用來表示一種伴隨狀態(tài),作狀語，可譯成“而每一幅值被表示為……”。獨立分詞結(jié)構(gòu)在科技文章中相當(dāng)常見,例如:

Therearemanykindsofsteel,eachhavingitsusesinindustry.可譯為:“鋼有許多種,在工業(yè)中每種都有它自己的用途?！豹毩⒎衷~結(jié)構(gòu)還可以表示時間、原因或條件等,對主句進行補充說明。例如表示原因:

Theresistancebeingveryhigh,thecurrentinthecircuitwaslow.

可譯為:“由于電阻很高,故電路里的電流很小?！?/p>

·asasequenceof8binarydigits句中的as意為“作為,表示為,成為”。該短語可譯成“(表示為)8位二進制碼的序列”。

［3］Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyof

order6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange

300Hzto3.4kHz.

而且我們將證明,為了變換頻率范圍為300Hz~3.4kHz的話路信號,理論上的最小采樣頻率為6.8kHz。

·occupying是occupy的現(xiàn)在分詞。動詞的-ing形式可以作為名詞用,稱為動名詞,上面注釋1已對此作了介紹。動詞的-ing形式亦可作形容詞用(稱現(xiàn)在分詞)。它一般修飾該動作的發(fā)出者,而且它亦可帶有自己的賓語,例如本句中的avoicechanneloccupyingtherange300Hzto3.4kHz。該短語可譯為:“占有300kHz到3.4kHz頻率范圍的話路”。

［4］Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise.

例如,從位于遙遠太空中的衛(wèi)星接收到的信號極其微弱,其電平僅比噪聲稍高一點。

·locateinfarouterspace是一種分詞短語結(jié)構(gòu)。located是動詞的-ed形式,稱為過去分詞,起形容詞的作用,在句中作定語。這種分詞往往用來修飾原來動詞動作的承受者。例如:Thetreesplantedbymehavegrownup.“我種的樹已長大了?！痹倮?Electromotiveforceresultinelectricalpressure,comparedtowaterpressure.“電動勢產(chǎn)生電壓,電壓好比水壓?！彼哉n文中的這一短語可譯為:“位于遙遠太空中的(衛(wèi)星)”。

［5］Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformationusingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.

相比之下,許多其他形式的傳輸系統(tǒng)是利用被傳信號的波形或電平的高低來傳送信息的,而這些參數(shù)又極易受到傳輸路徑中的噪聲和衰耗的影響。

·bycomparison相比之下。

·usingtheshape,orleveloftransmittedsignal為現(xiàn)在分詞短語結(jié)構(gòu),作伴隨情況狀語,可譯為:“用傳輸信號的波形或電平(來傳送信息)”。類似的例子還有:Therocketsrosehissingoverthelaunchingsite.“火箭在發(fā)射場的上空嘶嘶地上升?！痹倮?Theysattogether,carefullystudyingthedesignofthecircuits.“他們坐在一起仔細(xì)地研究著那些電路的設(shè)計”。

［6］Thesynchronizationwordreoccursonceagainafterthelastchannelhasbeenreceived.

當(dāng)最后一個話路的碼字收到之后,同步碼字又再次出現(xiàn)。

Exercises

1.TranslatethefollowingphraseintoChinese.

(1)speechchannel(2)samplingfrequency

(3)streamofpulses

(4)codingprocess

(5)transmissionquality

(6)transmissionpath

(7)noisepower

(8)8-digitalsequence

2.TranslatethefollowingphraseintoEnglish.

(1)抽樣、量化與編碼(2)幅值

(3)抽樣速率(4)重復(fù)率

(5)模擬信號(6)含噪聲的環(huán)境

(7)信號電平(8)反向操作

(9)同步字

3.Answerthequestions.

(1)WhichthreeseparateoperationsofPCMisdependenton?

(2)Whatistheminimalsamplingfrequencyintheoryandpracticalequipments?

(3)Whychoosedigitaltransmissionforovercomingnoisyenvironments?

(4)Atthereceiveterminal,whichisneededtoseparatethe8-digitsequencesintotheappropriatechannels？

PassageCInformationSources

SpeechandMusic

Thetelecommunicationsenvironmentisdominatedbyfourimportantsourcesofinformation:speech,music,pictureandcomputerdata.Asourceofinformationmaybecharacterizedintermsofthesignalthatcarriestheinformation.Asignalisdefinedasasingle-valuedfunctionoftimethatplaystheroleoftheindependentvariable;ateveryinstantoftime,thefunctionhasauniquevalue.

Thesignalcanbeone-dimensional,asinthecaseofspeech,music,orcomputerdata;two-dimensional,asinthecaseofpictures;three

dimensional,asinthecaseofvideodata;andfour-dimensional,asinthecaseofvolume

dataovertime.Inthesequel,weelaborateondifferentsourcesofinformation.

Speechistheprimarymethodofhumancommunication.Specifically,thespeech

communicationprocessinvolvesthetransferinformationfromaspeakertoalistener,

whichtakesplaceinthreesuccessivestages:

◆

Production.

Anintendedmessageinthespeaker’smindisrepresentedbyaspeechsignalthatconsistsofsounds(i.e.,pressurewaves)generatedinsidethevocaltractandwhosearrangementisgovernedbytherulesoflanguage.

◆

Propagation.

Thesoundwavespropagatethroughtheairataspeedof300m/s,reachingthelistener’sears.

◆

Perception.

Theincomingsoundsaredecipheredbythelistenerintoareceivedmessage,therebycompletingthechainofeventsthatculminateinthetransferofinformationfromthespeakertothelistener.

Thespeech-productionprocessmaybeviewedasaformoffiltering,inwhichasoundsourceexcitesavocaltractfilter.Thevocaltractconsistsofatubeofnonuniformcross-sectionalarea,beginningattheglottis(i.e.,theopeningbetweenthevocalcords)andendatthelip.Asthesoundpropagatesalongthevocaltract,thespectrum(i.e.,frequencycontent)isshapedbythefrequencyselectivityofthevocaltract;thiseffectissomewhatsimilartotheresonance:phenomenonobservedinorganpipes.Theimportantpointtonotehereisthatthepowerspectrum(i.e.,thedistributionoflong-termaveragepowerversusfrequency)ofspeechapproacheszeroforzerofrequencyandreachesapeakintheneighborhoodofafewhundredhertz.

Toputmattersintoproperperspective,however,wehavetokeepinmindthatthehearingmechanismisverysensitive