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1、圖解:譯文“藍色”UNIT1ThePrincipleofPCMPCM原理Pcmisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.Inthesedescriptionsweshallseehowaspeechchanneloftele
2、phonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,codedasasequenceof8binarydigits.Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(khz)isrequiredtoconveyavoicechanneloccupyingtherange300HZto3.4Khz.Pr
3、acticalequipments,however,normallyuse3asamplingrateof8khz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64khz.Fig.1-1illustratesthesampling,quantizing,andcodingprocesses.PCM的構(gòu)成依賴于三個環(huán)節(jié)，即采樣、量化和編碼。近年來，人們對這三個環(huán)節(jié)的實現(xiàn)
4、提出了許多不同的方案，我們將對其中的一些主要的方案進行討論。在這些討論中，我們會看到話路中的語聲信號是如何轉(zhuǎn)換成幅值序列的，而每個幅值又被編碼，即以8位二進制數(shù)的序列表示。而且，我們將證明，為了轉(zhuǎn)換頻率范圍為300HZ—3.4KHZ的話路信號，理論上最小采樣頻率須為6.8khz。但是，實際設(shè)備通常用8khz的采樣速率，而如果每個樣值用8位碼的話，則話路是由一個重復(fù)速率為64khz的脈沖流來表示的。圖1-1表示了采樣、量化、編碼的過程。Reexaminationofoursimpleexampleshowsusthatthespeec
5、hsignalofmaximumfrequency3.4khzhasbeenrepresentedbyasignaloffrequency64khz.However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32khz.Thusthequalityoftransmissionisdependentonthepulserepetition
6、rate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.讓我們再研究一下上面提到的簡單例子?？梢钥闯?，最高頻率為3.4khz的話音信號適用64khz的（脈沖流）信號來表示的。但是，如果每個樣值中用4位（碼）表示，則傳輸質(zhì)量會下降，而脈沖的重復(fù)速率也將減小到32khz。因而傳輸質(zhì)量是取決于脈沖重復(fù)速率的。對于數(shù)字通信系統(tǒng)，這兩個量之間極明顯的互相影響著。Digitaltransmissionprovidesapowe
7、rfulmethodforovercomingnoisyenvironments.Noisecanbeintroducedintotransmissionpatchinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesignaltoth
8、enoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.Basically,ifthesignali
