《通信工程专业英语（第二版）》课件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.检波；反调制；解调制amplituden.振幅；丰富，充足megahertzn.兆赫interferencen.干扰，冲突；干涉antennan.[电讯]天线conversionn.转换；变换WordsWordsTextWordsNotesDiscussionHomedigitizevt.[计]数字化binaryadj.[数]二进制的；二元的，二态的basebandn.基带preselectvt.预选wherebyadv.凭借；通过…；借以；与…一致modemn.调制解调器（等于modulator-demodulator）WordsTextWordsNotesDiscussionHomesuperpositionn.[数]叠加，重合quadraturen.正交；求积；弦convolutionaladj.卷积的；回旋的；脑回的trellisn.格子；格子结构；框架WordsTextWordsNotesDiscussionHome[1]analogmodulation：一般指调制信号和载波都是连续波的调制方式。它有调幅、调频和调相三种基本形式。TextWordsNotesDiscussionNotesHome[2]digitalmodulation：一般指调制信号是离散的，而载波是连续波的调制方式。它有四种基本形式：振幅键控、移频键控、移相键控和差分移相键控。TextWordsNotesDiscussionHome[3]AmplitudeShiftKeying（ASK）：即幅移键控，指的是振幅键控方式。这种调制方式是根据信号的不同，调节正弦波的幅度。幅度键控可以通过乘法器和开关电路来实现。TextWordsNotesDiscussionHome[4]Frequency-shiftkeying（FSK）：频移键控，指以数字信号控制载波频率变化的调制方式。根据已调波的相位连续与否，频移键控分为两类：相位不连续的频移键控和相位连续的频移键控。频移键控是信息传输中使用得较早的一种调制方式,它的主要优点是：实现起来较容易，抗噪声与抗衰减的性能较好。TextWordsNotesDiscussionHome[5]Phase-shiftkeying（PSK）：相位偏移调制，又称移相键控，是一种利用相位差异的信号来传送资料的调制方式。这种调制方式因此而得名。该传送信号必须为正交信号，其基底更须为单位化信号。TextWordsNotesDiscussionHome[6]quadratureamplitudemodulation（QAM）：正交振幅键控是一种将两种调幅信号（2ASK和2PSK）汇合到一个信道的方法，因此会双倍扩展有效带宽，正交调幅被用于脉冲调幅。TextWordsNotesDiscussionHome[7]trelliscodedmodulation（TCM）：网格编码调制,在传统的数字传输系统中，发送端和接收端的纠错与调制电路是两个独立的部分，而纠错编码会带来频带利用率的下降。为了提高频带的利用率，同时也希望在不增加信道传输带宽的前提下降低差错率，可以把编码和调制相结合统一进行设计，这就是所谓的网格编码调制。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.紧凑的，紧密的；简洁的audioadj.声音的；[声]音频的applicationn.应用；应用程序；应用软件fidelityn.保真度；忠诚；精确；尽责densityn.密度losslessadj.无损的WordsWordsTextWordsNotesDiscussionHomecontainern.集装箱；容器sinen.正弦verifyvt.核实；查证algorithmn.[计][数]算法，运算法则discreten.&adj.分立元件；离散的，不连续的nibblen.轻咬；啃；细咬WordsTextWordsNotesDiscussionHomenotablyadv.显著地；尤其interweavevt.（使）交织；（使）混杂formatn.&vt.格式；版式；使格式化WordsTextWordsNotesDiscussionHome[1]PulseDensityModulation（PDM