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1、图解:译文“蓝色”UNIT1ThePrincipleofPCMPCM原理Pcmisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,codedasasequenceof8binarydigits.Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(khz)isrequiredtoconveyavoicec。
2、hanneloccupyingtherange300HZto3.4Khz.Practicalequipments,however,normallyuse3asamplingrateof8khz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64khz.Fig.1-1illustratesthesampling,quantizing,andcodingprocesses.PCM的构成依赖于三个环节,即采样、量化和编码。近年来,人们对这三个环节的实现提出了许多不同的方案,我们将对其中的一些主要的方案进行讨论。在这些讨论中,我们会看到话路中的语声信号是如何转换成幅值序列的,而每个幅值又被编码,即以8位二进制数的序列表示。而且,我们将证明,为了转换频率范围为300HZ—3.4KHZ的话路信号,理论上最小采样频率须为6.8khz。但是,实际设备通常用8khz的采样速率,而如果每个样值用8位码的话,则话路是由一个重复速率为64khz的。
3、脉冲流来表示的。图1-1表示了采样、量化、编码的过程。Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4khzhasbeenrepresentedbyasignaloffrequency64khz.However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32khz.Thusthequalityoftransmissionisdependentonthepulserepetitionrate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.让我们再研究一下上面提到的简单例子。可以看出,最高频率为3.4khz的话音信号适用64khz的(脉冲流)信号来表示的。但是,如果每个。
4、样值中用4位(码)表示,则传输质量会下降,而脉冲的重复速率也将减小到32khz。因而传输质量是取决于脉冲重复速率的。对于数字通信系统,这两个量之间极明显的互相影响着。Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments.Noisecanbeintroducedintotransmissionpatchinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.Basically,ifthesignal。
5、isverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise.Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepower.数字传输对于克服噪声环境的影响提供了一个强有力的手段。噪声可以以多种不同方式进入传输信道,比如说因为附近的闪电、汽车点火系统的打火或因通信设备本身低电平的热噪声所致。正是这种被称为信噪比的东西,即真实信号与噪声的关系引起了通信工程师的极大的兴趣。从本质上讲,如果信号比噪声电平大得多,则信息的传输是完美的。但是,实际情况并不总是这样,例如,从。
6、位于遥远太空中的卫星接收到的信号极其微弱,其电平仅比噪声稍高一点。地面系统则是另一类例子,尽管信号很强,噪声也很强。Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularmessagewillalwaysbeobtainedbysimplydetectingthepresenceorabsenceofthepulse.Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformationusingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigitaltra。
本文标题:通信专业英语
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