资源描述：
《专业英语课文》由会员上传分享，免费在线阅读，更多相关内容在工程资料-天天文库。
1、L2：UNIT1ThePrincipleofPCM(脉冲编码调制)PCMisdependentonthreeseparateoperations,sampling,quantizing,andcoding.Manydifferentwaysforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones・Inthesedescription,weshallseehowaspeechchanneloftelephonequalitymaybecon
2、veyedasaseriesofamplitudevalues,eachvaluebeingrepresented,that匾coded,asasequenceof8binarydigits.Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange300Hzto3.4kHzPracticalequipments,however,normallyus
3、easamplingrateof8kHz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64kHz.Fig.1・1illustratesthesampling,quantizing,andcodingprocesses.Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4kHzhasbeenre
4、presentedbyasignaloffrequencyof64kHzHoweverifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32kHz・ThusthequalityoftransmissionisdependentonthepulserepetitionTate,andfordigitalcommunicationsystemsthesetwovariable
5、smaybeinterchangedmostefficiently.Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments・Noisecanbeintroducedintotransmissionpathinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequ
6、ipmentitself.Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.Basically,ifthesignalisverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.Forexample,thesign
7、alreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise・Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepowe匚Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularme
8、ssagewillalwaysbeob
