资源描述：
《电子信息工程专业英语B篇译文.docx》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。
1、果粒橙图解:译文“蓝色”Unit6ThePrincipleofPCMPCM原理Pcmisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.Inthesedescriptionsweshallseehowa
2、speechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,codedasasequenceof8binarydigits.Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(khz)isrequiredtoconveyavoicechann
3、eloccupyingtherange300HZto3.4Khz.Practicalequipments,however,normallyuse3asamplingrateof8khz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64khz.Fig.1-1illustratesthesampling,quantizing,andcodi
4、ngprocesses.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的脉冲流来表示的。图1
5、-1表示了采样、量化、编码的过程。Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4khzhasbeenrepresentedbyasignaloffrequency64khz.However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepuls
6、eswouldbereducedto32khz.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，如果每个样值中用4位（码）表示，则传输质
7、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,thesparkingofacarig
8、nitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.Basically,ifthesign
