《通信工程專業(yè)英語(yǔ)（第二版）》課件Unit 11

Unit11TextAModulationandDemodulationModulationandDemodulationInmanytelecommunicationssystems,itisnecessarytorepresentaninformation-bearingsignalwithawaveformthatcanpassaccuratelythroughatransmissionmedium.Thisassigningofasuitablewaveformisaccomplishedbymodulation,whichistheprocessbywhichsomecharacteristicofacarrierwaveisvariedinaccordancewithaninformationsignal,ormodulatingwave.Themodulatedsignalisthentransmittedoverachannel,afterwhichtheoriginalinformation-bearingsignalisrecoveredthroughaprocessofdemodulation.Modulationisappliedtoinformationsignalsforanumberofreasons,someofwhichareoutlinedbelow.TextTextWordsNotesDiscussionHome1)Manytransmissionchannelsarecharacterizedbylimitedpassbands—thatis,theywillpassonlycertainrangesoffrequencieswithoutseriouslyattenuatingthem(reducingtheiramplitude).Modulationmethodsmustthereforebeappliedtotheinformationsignalsinorderto“frequencytranslate”thesignalsintotherangeoffrequenciesthatarepermittedbythechannel.Examplesofchannelsthatexhibitpassbandcharacteristicsincludealternating-current-coupledcoaxialcables,whichpasssignalsonlyintherangeof60kilohertztoseveralhundredmegahertz,andfibre-opticcables,whichpasslightsignalsonlywithinagivenwavelengthrangewithoutsignificantattenuation.Intheseinstances,frequencytranslationisusedto“fit”theinformationsignaltothecommunicationschannel.TextTextWordsNotesDiscussionHome2)Inmanyinstancesacommunicationschannelissharedbymultipleusers.Inordertopreventmutualinterference,eachuser’sinformationsignalismodulatedontoanassignedcarrierofaspecificfrequency.Whenthefrequencyassignmentandsubsequentcombiningisdoneatacentralpoint,theresultingcombinationisafrequency-divisionmultiplexedsignal,asisdiscussedinMultiplexing.Frequentlythereisnocentralcombiningpoint,andthecommunicationschannelitselfactsasadistributedcombine.Anexampleofthelattersituationisthebroadcastradiobands(from540kilohertzto600megahertz),whichpermitsimultaneoustransmissionofmultipleAMradio,FMradio,andtelevisionsignalswithoutmutualinterferenceaslongaseachsignalisassignedtoadifferentfrequencyband.TextTextWordsNotesDiscussionHome3)Evenwhenthecommunicationschannelcansupportdirecttransmissionoftheinformation-bearingsignal,thereareoftenpracticalreasonswhythisisundesirable.Asimpleexampleisthetransmissionofathree-kilohertz(i.e.,voiceband)signalviaradiowave.Infreespacethewavelengthofathree-kilohertzsignalis100kilometres(60miles).Sinceaneffectiveradioantennaistypicallyaslargeashalfthewavelengthofthesignal,athree-kilohertzradiowavemightrequireanantennaupto50kilometresinlength.Inthiscasetranslationofthevoicefrequencytoahigherfrequencywouldallowtheuseofamuchsmallerantenna.TextTextWordsNotesDiscussionHome1.Analogmodulation1Asisnotedinanalog-to-digitalconversion,voicesignals,aswellasaudioandvideosignals,areinherentlyanaloginform.Inmostmodernsystemsthesesignalsaredigitizedpriortotransmission,butinsomesystemstheanalogsignalsarestilltransmitteddirectlywithoutconvertingthemtodigitalform.Therearetwocommonlyusedmethodsofmodulatinganalogsignals.Onetechnique,calledamplitudemodulation,variestheamplitudeofafixed-frequencycarrierwaveinproportiontotheinformationsignal.Theothertechnique,calledfrequencymodulation,variesthefrequencyofafixed-amplitudecarrierwaveinproportiontotheinformationsignal.TextTextWordsNotesDiscussionHome2.Digitalmodulation2Inordertotransmitcomputerdataandotherdigitizedinformationoveracommunicationschannel,ananalogcarrierwavecanbemodulatedtoreflectthebinarynatureofthedigitalbasebandsignal.Theparametersofthecarrierthatcanbemodifiedaretheamplitude,thefrequency,andthephase.TextTextWordsNotesDiscussionHome3.Amplitude-shiftkeying3Ifamplitudeistheonlyparameterofthecarrierwavetobealteredbytheinformationsignal,themodulatingmethodiscalledamplitude-shiftkeying(ASK).ASKcanbeconsideredadigitalversionofanalogamplitudemodulation.Initssimplestform,aburstofradiofrequencyistransmittedonlywhenabinary1appearsandisstoppedwhena0appears.Inanothervariation,the0and1arerepresentedinthemodulatedsignalbyashiftbetweentwopreselectedamplitudes.TextTextWordsNotesDiscussionHome4.Frequency-shiftkeying4Iffrequencyistheparameterchosentobeafunctionoftheinformationsignal,themodulationmethodiscalledfrequency-shiftkeying(FSK).InthesimplestformofFSKsignaling,digitaldataistransmittedusingoneoftwofrequencies,wherebyonefrequencyisusedtotransmita1andtheotherfrequencytotransmita0.SuchaschemewasusedintheBell103voicebandmodem,introducedin1962,totransmitinformationatratesupto300bitspersecondoverthepublicswitchedtelephonenetwork.IntheBell103modem,frequenciesof1,080+/-100hertzand1,750+/-100hertzwereusedtosendbinarydatainbothdirections.TextTextWordsNotesDiscussionHome5.Phase-shiftkeying5Whenphaseistheparameteralteredbytheinformationsignal,themethodiscalledphase-shiftkeying(PSK).InthesimplestformofPSKasingleradiofrequencycarrierissentwithafixedphasetorepresenta0andwitha180°phaseshift-thatis,withtheoppositepolarity-torepresenta1.PSKwasemployedintheBell212modem,whichwasintroducedabout1980totransmitinformationatratesupto1,200bitspersecondoverthepublicswitchedtelephonenetwork.TextTextWordsNotesDiscussionHome6.AdvancedmethodsInadditiontotheelementaryformsofdigitalmodulationdescribedabove,thereexistmoreadvancedmethodsthatresultfromasuperpositionofmultiplemodulatingsignals.Anexampleofthelatterformofmodulationisquadratureamplitudemodulation(QAM)6..QAMsignalsactuallytransmittwoamplitude-modulatedsignalsinphasequadrature(i.e.,90°apart),sothatfourormorebitsarerepresentedbyeachshiftofthecombinedsignal.CommunicationssystemsthatemployQAMincludedigitalcellularsystemsintheUnitedStatesandJapanaswellasmostvoicebandmodemstransmittingabove2,400bitspersecond.TextTextWordsNotesDiscussionHomeAformofmodulationthatcombinesconvolutionalcodeswithQAMisknownastrellis-codedmodulation(TCM7).Trellis-codedmodulationformsanessentialpartofmostofthemodernvoicebandmodemsoperatingatdataratesof9,600bitspersecondandabove,includingV.32andV.34modems.TextTextWordsNotesDiscussionHomedemodulationn.檢波；反調(diào)制；解調(diào)制amplituden.振幅；豐富，充足megahertzn.兆赫interferencen.干擾，沖突；干涉antennan.[電訊]天線conversionn.轉(zhuǎn)換；變換WordsWordsTextWordsNotesDiscussionHomedigitizevt.[計(jì)]數(shù)字化binaryadj.[數(shù)]二進(jìn)制的；二元的，二態(tài)的basebandn.基帶preselectvt.預(yù)選wherebyadv.憑借；通過(guò)…；借以；與…一致modemn.調(diào)制解調(diào)器（等于modulator-demodulator）WordsTextWordsNotesDiscussionHomesuperpositionn.[數(shù)]疊加，重合quadraturen.正交；求積；弦convolutionaladj.卷積的；回旋的；腦回的trellisn.格子；格子結(jié)構(gòu)；框架WordsTextWordsNotesDiscussionHome[1]analogmodulation：一般指調(diào)制信號(hào)和載波都是連續(xù)波的調(diào)制方式。它有調(diào)幅、調(diào)頻和調(diào)相三種基本形式。TextWordsNotesDiscussionNotesHome[2]digitalmodulation：一般指調(diào)制信號(hào)是離散的，而載波是連續(xù)波的調(diào)制方式。它有四種基本形式：振幅鍵控、移頻鍵控、移相鍵控和差分移相鍵控。TextWordsNotesDiscussionHome[3]AmplitudeShiftKeying（ASK）：即幅移鍵控，指的是振幅鍵控方式。這種調(diào)制方式是根據(jù)信號(hào)的不同，調(diào)節(jié)正弦波的幅度。幅度鍵控可以通過(guò)乘法器和開(kāi)關(guān)電路來(lái)實(shí)現(xiàn)。TextWordsNotesDiscussionHome[4]Frequency-shiftkeying（FSK）：頻移鍵控，指以數(shù)字信號(hào)控制載波頻率變化的調(diào)制方式。根據(jù)已調(diào)波的相位連續(xù)與否，頻移鍵控分為兩類：相位不連續(xù)的頻移鍵控和相位連續(xù)的頻移鍵控。頻移鍵控是信息傳輸中使用得較早的一種調(diào)制方式,它的主要優(yōu)點(diǎn)是：實(shí)現(xiàn)起來(lái)較容易，抗噪聲與抗衰減的性能較好。TextWordsNotesDiscussionHome[5]Phase-shiftkeying（PSK）：相位偏移調(diào)制，又稱移相鍵控，是一種利用相位差異的信號(hào)來(lái)傳送資料的調(diào)制方式。這種調(diào)制方式因此而得名。該傳送信號(hào)必須為正交信號(hào)，其基底更須為單位化信號(hào)。TextWordsNotesDiscussionHome[6]quadratureamplitudemodulation（QAM）：正交振幅鍵控是一種將兩種調(diào)幅信號(hào)（2ASK和2PSK）匯合到一個(gè)信道的方法，因此會(huì)雙倍擴(kuò)展有效帶寬，正交調(diào)幅被用于脈沖調(diào)幅。TextWordsNotesDiscussionHome[7]trelliscodedmodulation（TCM）：網(wǎng)格編碼調(diào)制,在傳統(tǒng)的數(shù)字傳輸系統(tǒng)中，發(fā)送端和接收端的糾錯(cuò)與調(diào)制電路是兩個(gè)獨(dú)立的部分，而糾錯(cuò)編碼會(huì)帶來(lái)頻帶利用率的下降。為了提高頻帶的利用率，同時(shí)也希望在不增加信道傳輸帶寬的前提下降低差錯(cuò)率，可以把編碼和調(diào)制相結(jié)合統(tǒng)一進(jìn)行設(shè)計(jì)，這就是所謂的網(wǎng)格編碼調(diào)制。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.Whatarethemajorreasonsforapplicationofmodulationtoinformationsignals?2.Whatarethefrequentlyemployedmethodsofmodulatinganalogsignals?3.Doyouknowanyotheradvancedformsofdigitalmodulation?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.Whatarethemajorreasonsforapplicationofmodulationtoinformationsignals?Thethreemajorreasonsareasfollows:First,modulationmethodshavetobeappliedtotheinformationsignalsinorderfor“frequencytotranslate”thesignalsintotherangeoffrequenciesthatarepermittedbythechannel.Second,eachuser’sinformationsignalneedstobemodulatedontoanassignedcarrierofaspecificfrequencytopreventmutualinterference.Third,theapplicationmodulationtoinformationsignalscontributestothetranslationofthevoicefrequencytoahigherfrequency,whichallowstheuseofamuchsmallerantenna.TextWordsNotesDiscussionHome2.Whatarethefrequentlyemployedmethodsofmodulatinganalogsignals?Twocommonlyusedmethodsareusedtomodulateanalogsignals.Onetechnique,calledamplitudemodulation,variestheamplitudeofafixed-frequencycarrierwaveinproportiontotheinformationsignal.Theothertechnique,calledfrequencymodulation,variesthefrequencyofafixed-amplitudecarrierwaveinproportiontotheinformationsignal.TextWordsNotesDiscussionHome3.Doyouknowanyotheradvancedformsofdigitalmodulation?Thereareanothertwoadvancedmethodsofdigitalmodulation:quadratureamplitudemodulation(QAM)andtrellis-codedmodulation(TCM).QAMsignalsactuallytransmittwoamplitude-modulatedsignalsinphasequadrature(i.e.,90°apart),sothatfourormorebitsarerepresentedbyeachshiftofthecombinedsignal,whilethelattercombinesconvolutionalcodeswithQAM.TextWordsNotesDiscussionHomeThankyou!Unit11TextBPulse-codeModulationPulse-codeModulationPulse-codemodulation1(PCM)isamethodusedtodigitallyrepresentsampledanalogsignals.Itisthestandardformofdigitalaudioincomputers,compactdiscs,digitaltelephonyandotherdigitalaudioapplications.InaPCMstream,theamplitudeoftheanalogsignalissampledregularlyatuniformintervals,andeachsampleisquantizedtothenearestvaluewithinarangeofdigitalsteps.Linearpulse-codemodulation(LPCM)isaspecifictypeofPCMwherethequantizationlevelsarelinearlyuniform.ThisisincontrasttoPCMencodingswherequantizationlevelsvaryasafunctionofamplitude(aswiththeA-lawalgorithmortheμ-lawalgorithm).ThoughPCMisamoregeneralterm,itisoftenusedtodescribedataencodedasLPCM.TextTextWordsNotesDiscussionHomeAPCMstreamhastwobasicpropertiesthatdeterminethestream'sfidelitytotheoriginalanalogsignal:thesamplingrate,whichisthenumberoftimespersecondthatsamplesaretaken;andthebitdepth,whichdeterminesthenumberofpossibledigitalvaluesthatcanbeusedtorepresenteachsample.TextTextWordsNotesDiscussionHome1.ImplementationsPCMisthemethodofencodinggenerallyusedforuncompressedaudio,althoughthereareothermethodssuchaspulse-densitymodulation[1](usedalsoonSuperAudioCD).The4ESSswitchintroducedtime-divisionswitchingintotheUStelephonesystemin1976,basedonmediumscaleintegratedcircuittechnology.LPCMisusedforthelosslessencodingofaudiodataintheCompactdiscRedBookstandard(informallyalsoknownasAudioCD),introducedin1982.TextTextWordsNotesDiscussionHomeAES3(specifiedin1985,uponwhichS/PDIFisbased)isaparticularformatusingLPCM.OnPCs,PCMandLPCMoftenrefertotheformatusedinWAV(definedin1991)andAIFFaudiocontainerformats(definedin1988).LPCMdatamayalsobestoredinotherformatssuchasAU,rawaudioformat(header-lessfile)andvariousmultimediacontainerformats.TextTextWordsNotesDiscussionHomeLPCMhasbeendefinedasapartoftheDVD(since1995)andBlu-ray(since2006)standards.Itisalsodefinedasapartofvariousdigitalvideoandaudiostorageformats(e.g.DVsince1995,AVCHD[2]since2006).LPCMisusedbyHDMI(definedin2002),asingle-cabledigitalaudio/videoconnectorinterfacefortransmittinguncompresseddigitaldata.\RF64containerformat(definedin2007)usesLPCMandalsoallowsnon-PCMbitstreamstorage:variouscompressionformatscontainedintheRF64fileasdatabursts(DolbyE,DolbyAC3,DTS,MPEG-1/MPEG-2Audio)canbe"disguised"asPCMlinear.TextTextWordsNotesDiscussionHome2.ModulationsInthediagram,asinewave(redcurve)issampledandquantizedforPCM.Thesinewaveissampledatregularintervals,shownasverticallines.Foreachsample,oneoftheavailablevalues(onthey-axis)ischosenbysomealgorithm.Thisproducesafullydiscreterepresentationoftheinputsignal(bluepoints)thatcanbeeasilyencodedasdigitaldataforstorageormanipulation.Forthesinewaveexampleatright,wecanverifythatthequantizedvaluesatthesamplingmomentsare8,9,11,13,14,15,15,15,14,etc.Encodingthesevaluesasbinarynumberswouldresultinthefollowingsetofnibbles:1000(2^3×1+2^2×0+2^1×0+2^0×0=8+0+0+0=8),1000,1001,1011,1101,1110,1111,1111,1111,1110,etc.Thesedigitalvaluescouldthenbefurtherprocessedoranalyzedbyadigitalsignalprocessor.SeveralPCMstreamscouldalsobemultiplexedintoalargeraggregatedatastream,generallyfortransmissionofmultiplestreamsoverasinglephysicallink.Onetechniqueiscalledtime-divisionmultiplexing(TDM)andiswidelyused,notablyinthemodernpublictelephonesystem.TextTextWordsNotesDiscussionHomeThePCMprocessiscommonlyimplementedonasingleintegratedcircuitgenerallyreferredtoasananalog-to-digitalconverter(ADC).TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHome3.DemodulationTorecovertheoriginalsignalfromthesampleddata,a"demodulator"canapplytheprocedureofmodulationinreverse.Aftereachsamplingperiod,thedemodulatorreadsthenextvalueandshiftstheoutputsignaltothenewvalue.Asaresultofthesetransitions,thesignalhasasignificantamountofhigh-frequencyenergycausedbyaliasing.Toremovetheseundesirablefrequenciesandleavetheoriginalsignal,thedemodulatorpassesthesignalthroughanalogfiltersthatsuppressenergyoutsidetheexpectedfrequencyrange(greaterthantheNyquist[3]frequency).ThesamplingtheoremshowsPCMdevicescanoperatewithoutintroducingdistortionswithintheirdesignedfrequencybandsiftheyprovideasamplingfrequencytwicethatoftheinputsignal.Forexample,intelephony,theusablevoicefrequencybandrangesfromapproximately300Hzto3400Hz.Therefore,pertheNyquist–Shannonsamplingtheorem,thesamplingfrequency(8kHz)mustbeatleasttwicethevoicefrequency(4kHz)foreffectivereconstructionofthevoicesignal.TextTextWordsNotesDiscussionHomeTheelectronicsinvolvedinproducinganaccurateanalogsignalfromthediscretedataaresimilartothoseusedforgeneratingthedigitalsignal.ThesedevicesareDigital-to-analogconverters(DACs).Theyproduceavoltageorcurrent(dependingontype)thatrepresentsthevaluepresentedontheirdigitalinputs.Thisoutputwouldthengenerallybefilteredandamplifiedforuse.TextTextWordsNotesDiscussionHome4.StandardsamplingprecisionandratesCommonsampledepthsforLPCMare8,16,20or24bitspersample.LPCMencodesasinglesoundchannel.SupportformultichannelaudiodependsonfileformatandreliesoninterweavingorsynchronizationofLPCMstreams.Whiletwochannels(stereo)isthemostcommonformat,somecansupportupto8audiochannels(7.1surround).TextTextWordsNotesDiscussionHomeCommonsamplingfrequenciesare48kHzasusedwithDVDformatvideos,or44.1kHzasusedinCompactdiscs.Samplingfrequenciesof96kHzor192kHzcanbeusedonsomenewerequipment,withthehighervalueequatingto6.144megabitpersecondfortwochannelsat16-bitpersamplevalue,butthebenefitshavebeendebated.ThebitratelimitforLPCMaudioonDVD-Videoisalso6.144Mbit/s,allowing8channels(7.1surround)×48kHz×16-bitpersample=6,144kbit/s.ThereisaL32bitPCM,andtherearemanysoundcardsthatsupportit.TextTextWordsNotesDiscussionHomecompactadj.緊湊的，緊密的；簡(jiǎn)潔的audioadj.聲音的；[聲]音頻的applicationn.應(yīng)用；應(yīng)用程序；應(yīng)用軟件fidelityn.保真度；忠誠(chéng)；精確；盡責(zé)densityn.密度losslessadj.無(wú)損的WordsWordsTextWordsNotesDiscussionHomecontainern.集裝箱；容器sinen.正弦verifyvt.核實(shí)；查證algorithmn.[計(jì)][數(shù)]算法，運(yùn)算法則discreten.&adj.分立元件；離散的，不連續(xù)的nibblen.輕咬；啃；細(xì)咬WordsTextWordsNotesDiscussionHomenotablyadv.顯著地；尤其interweavevt.（使）交織；（使）混雜formatn.&vt.格式；版式；使格式化WordsTextWordsNotesDiscussionHome[1]PulseDensityModulation（PDM