通信专业英语ppt

ENGLISHONTELECOMMUNICATIONS课程名称：通信专业外语UNIT1ThePrincipleofPCM1234UNIT5IntroductiontoOpticalFiberCommunicationUNIT4InternetUNIT2AsynchronousSerialDataTransmission567UNIT10GSMUNIT8CellularMobileTelephoneSystemUNIT7WDMUNIT11CircuitSwitchingandPacketSwitching11UNIT16CurrentSituationandtheFutureIntheTelecommunicationWorld161ThePrincipleofPCMUnit1ThePrincipleofPCMNEWWORDSANDPHRASES»principlen.原理»bedependenton依赖，取决于»samplev.采样»quantizev.量化，分层»codev.编码n.码»schemen.方案，设计，安排»describevt．叙述，描述»descriptionn．叙述，描述»amplituden．幅，幅度»binarya．二进制的»minimumn．最小值，最小量1ThePrincipleofPCMUnit1ThePrincipleofPCMNEWWORDSANDPHRASES»theoreticala．理论上的»repetitionn．重复，反复»reexaminationn．再审查，重考»maximumn．最大值»reducevt．；vi．减少，缩小»interchangey．互换，转换，相互影响»methodn．方式．方法，手段»overcomevt．克服，打败，征服»environmentn．环境，周围情况»lightningn．电光，闪电，雷电»strikev．击，敲，打»sparkv．发火花，打火，闪光1ThePrincipleofPCMUnit1ThePrincipleofPCMNEWWORDSANDPHRASES»ignitionn．点火，点火装置»signal-to-noiseratio信噪比»satelliten．卫星»terrestriala．地球的，地面的，大地的»bycomparison比较起来，相对之下»parameter[pə'ræmitə]n．参数，系数technicalparameter技术参数parameterestimation参数估计keyparameter关键参数1ThePrincipleofPCMUnit1ThePrincipleofPCMNEWWORDSANDPHRASES»attenuationn．衰减，衰耗attenuationcoefficient衰减系数attenuationdistortion衰减失真»inherenta．固有的，内在的»assumevt．假设，假定»decodern．解(译)码器»codecn．编译码器1ThePrincipleofPCMUnit1ThePrincipleofPCMNEWWORDSANDPHRASES»interleavevt．交插，交错，插接»appropriatea．适当的，合适的»uniquea．惟一的，独特的»uniquesolution唯一解»Insomeways,eachofusislikeerveyperson;andinsomeways,eachofusisunique.»reoccurvi．再发生，再次发生1①PCMisdependentonthreeseparateoperations,sampling,quantizing,andcoding.②Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.③Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,coded,asasequenceof8binarydigits.ThePrincipleofPCM1④Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange300Hzto3.4kHz.⑤Practicalequipments,however,normallyuseasamplingrateof8kHz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64kHz.⑥Fig.1-1illustratesthesampling,quantizing,andcodingprocesses.ThePrincipleofPCM1①Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4kHzhasbeenrepresentedbyasignaloffrequency64kHz.②However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32kHz.③Thusthequalityoftransmissionisdependentonthepulserepetitionrate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.ThePrincipleofPCM1①Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments.②Noisecanbeintroducedintotransmissionpathinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.③Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.ThePrincipleofPCM1④Basically,ifthesignalisverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.⑤Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise.⑥Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepower.ThePrincipleofPCM1①Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularmessagewillalwaysbeobtainedbysimplydetectingthepresenceorabsenceofthepulse.②Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformationusingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.③Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigitaltransmission.ThePrincipleofPCM1①Sofarinthisdiscussionwehaveassumedthateachvoicechannelhasaseparatecoder,theunitthatconvertssampledamplitudevaluestoasetofpulses;anddecoder,theunitthatperformsthereverseoperation.②Thisneednotbeso,andsystemsareinoperationwhereasinglecodec(i.e.,coderanditsassociateddecoder)issharedbetween24,30,oreven120separatechannels.③Ahigh-speedelectronicswitchisusedtopresenttheanaloginformationsignalofeachchannel,takeninturn,tothecodec.ThePrincipleofPCM1④Thecodecisthenarrangedtosequentiallysampletheamplitudevalue,andcodethisvalueintothe8-digitsequence.⑤Thustheoutputtothecodecmaybeseenasasequenceof8pulsesrelatingtochannel1,thenchannel2,andsoon.⑥Thisunitiscalledatimedivisionmultiplexer(TDM),andisillustratedinFig.1-2.⑦Themultiplexingprinciplethatisusedisknownaswordinterleaving.Sincethewords,or8-digitsequences,areinterleavedintime.ThePrincipleofPCM1①Atthereceiveterminalademultiplexerisarrangedtoseparatethe8-digitsequencesintotheappropriatechannels.②Thereadermayask,howdoesthedemultiplexerknowwhichgroupof8-digitsrelatestochannel1,2,andsoon?③Clearlyth