資源描述：
《新建文件夾通信英語(yǔ)The_Principle_of_PCM譯文.doc》由會(huì)員上傳分享，免費(fèi)在線閱讀，更多相關(guān)內(nèi)容在應(yīng)用文檔-天天文庫(kù)。
1、圖解:譯文“藍(lán)色”UNIT1ThePrincipleofPCMPCM原理Pcmisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.Inthesedescriptionsweshallseehowaspeechchanneloftel
2、ephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,codedasasequenceof8binarydigits.Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(khz)isrequiredtoconveyavoicechanneloccupyingtherange300HZto3.4Khz.
3、Practicalequipments,however,normallyuse3asamplingrateof8khz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64khz.Fig.1-1illustratesthesampling,quantizing,andcodingprocesses.PCM的構(gòu)成依賴于三個(gè)環(huán)節(jié)，即采樣、量化和編碼。近年來(lái)，人們對(duì)這三個(gè)環(huán)節(jié)
4、的實(shí)現(xiàn)提出了許多不同的方案，我們將對(duì)其中的一些主要的方案進(jìn)行討論。在這些討論中，我們會(huì)看到話路中的語(yǔ)聲信號(hào)是如何轉(zhuǎn)換成幅值序列的，而每個(gè)幅值又被編碼，即以8位二進(jìn)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(hào)，理論上最小采樣頻率須為6.8khz。但是，實(shí)際設(shè)備通常用8khz的采樣速率，而如果每個(gè)樣值用8位碼的話，則話路是由一個(gè)重復(fù)速率為64khz的脈沖流來(lái)表示的。圖1-1表示了采樣、量化、編碼的過(guò)程。Reexaminationofoursimpleexampleshowsusthatthes
5、peechsignalofmaximumfrequency3.4khzhasbeenrepresentedbyasignaloffrequency64khz.However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32khz.Thusthequalityoftransmissionisdependentonthepulserepet
6、itionrate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.讓我們?cè)傺芯恳幌律厦嫣岬降暮?jiǎn)單例子?？梢钥闯觯罡哳l率為3.4khz的話音信號(hào)適用64khz的（脈沖流）信號(hào)來(lái)表示的。但是，如果每個(gè)樣值中用4位（碼）表示，則傳輸質(zhì)量會(huì)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(duì)于數(shù)字通信系統(tǒng)，這兩個(gè)量之間極明顯的互相影響著。Digitaltransmissionprovide
7、sapowerfulmethodforovercomingnoisyenvironments.Noisecanbeintroducedintotransmissionpatchinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Itistherelationshipofthetruesig
8、naltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.Basically,ifthesignali
