电子信息专业英语-09.ppt

TechnicalEnglish,ForInformationScienceandElectronicEngineering,Unit9,Electronics:DigitalSignalsandSignalProcessing,PartI,DigitalSignalProcessing,3,NewWords,4,NewWords,5,NewWords,6,1,Digitalsignalprocessing(DSP)isthestudyofsignalsinadigitalrepresentationandtheprocessingmethodsofthesesignals.DSPandanalogsignalprocessingaresub-fieldsofsignalprocessing.DSPincludessub-fieldslikeaudioandspeechsignalprocessing,sonarandradarsignalprocessing,sensorarrayprocessing,spectralestimation,statisticalsignalprocessing,imageprocessing,signalprocessingforcommunications,biomedicalsignalprocessing,etc.,音频及语音信号处理、声纳和雷达信号处理、传感器阵列处理、谱估计、统计信号处理、图像处理、通信信号处理、生物医学信号处理,7,2,SincethegoalofDSPisusuallytomeasureorfiltercontinuousreal-worldanalogsignals,thefirststepisusuallytoconvertthesignalfromananalogtoadigitalform,byusingananalogtodigitalconverter.Often,therequiredoutputsignalisanotheranalogoutputsignal,whichrequiresadigitaltoanalogconverter.,数字信号处理的目标通常是测量连续的真实世界的模拟信号或对其滤波,8,3,ThealgorithmsrequiredforDSParesometimesperformedusingspecializedcomputers,whichmakeuseofspecializedmicroprocessorscalleddigitalsignalprocessors(alsoabbreviatedDSP).Theseprocesssignalsinrealtime,andaregenerallypurpose-designedapplication-specificintegratedcircuits(ASICs).1,这些（数字信号处理器）实时处理信号，通常是针对具体目的而设计的专用集成电路（ASIC）。,9,3,Whenflexibilityandrapiddevelopmentaremoreimportantthanunitcostsathighvolume,DSPalgorithmsmayalsobeimplementedusingfield-programmablegatearrays(FPGAs).2,当灵活性和快速开发比大批量生产的成本更重要时，DSP算法也可以用现场可编程门阵列来实现。,10,4DSPdomains,InDSP,engineersusuallystudydigitalsignalsinoneofthefollowingdomains:timedomain(one-dimensionalsignals),spatialdomain(multidimensionalsignals),frequencydomain,autocorrelationdomain,andwaveletdomains.Theychoosethedomaininwhichtoprocessasignalbymakinganinformedguess(orbytryingdifferentpossibilities)astowhichdomainbestrepresentstheessentialcharacteristicsofthesignal.3,他们按某些依据来猜测（或试验不同的可能性）那一个域能够最好地表示信号的本质特性来选择在其中进行信号处理的域。,11,4,Asequenceofsamplesfromameasuringdeviceproducesatimeorspatialdomainrepresentation,whereasadiscreteFouriertransformproducesthefrequencydomaininformation,thatis,thefrequencyspectrum.4Autocorrelationisdefinedasthecross-correlationofthesignalwithitselfovervaryingintervalsoftimeorspace.,从测量设备得到的样本序列产生（信号的）时域或空域表示，而离散Fourier变换则产生频域表示即频谱。,12,5Signalsampling,Withtheincreasinguseofcomputerstheusageandneedofdigitalsignalprocessinghasincreased.Inordertouseananalogsignalonacomputeritmustbedigitizedwithananalogtodigitalconverter(ADC).Samplingisusuallycarriedoutintwostages,discretizationandquantization.,采样通常分两步实现：离散化和量化,13,5,Inthediscretizationstage,thespaceofsignalsispartitionedintoequivalenceclassesanddiscretizationiscarriedoutbyreplacingthesignalwithrepresentativesignalofthecorrespondingequivalenceclass.5Inthequantizationstagetherepresentativesignalvaluesareapproximatedbyvaluesfromafiniteset.,在离散化阶段，信号空间被分割为相等的区间，用相应区间的代表性信号值代替信号本身。,用有限集中的值来近似代表性的信号值,14,6,Inorderforasampledanalogsignaltobeexactlyreconstructed,theNyquist-Shannonsamplingtheoremmustbesatisfied.Thistheoremstatesthatthesamplingfrequencymustbegreaterthantwicethebandwidthofthesignal.,定理规定：采样频率必须大于两倍的信号带宽,15,6,Inpractice,thesamplingfrequencyisoftensignificantlymorethantwicetherequiredbandwidth.Themostcommonbandwidthscenariosare:DCBW(“baseband”);andfcBW,afrequencybandcenteredonacarrierfrequency(“directdemodulation”).,采样频率通常远大于信号带宽的两倍,16,7,Adigitaltoanalogconverter(DAC)isusedtoconvertthedigitalsignalbacktoanalog.Theuseofadigitalcomputerisakeyingredientintodigitalcontrolsystems.,17,8Timeandspacedomains,Themostcommonprocessingapproachinthetimeorspacedomainisenhancementoftheinputsignalthroughamethodcalledfiltering.Filteringgenerallyconsistsofsometransformationofanumberofsurroundingsamplesaroundthecurrentsampleoftheinputoroutputsignal.Therearevariouswaystocharacterizefilters;forexample:,滤波通常由在输入或输出信号当前样本周围的许多样本的某种变换组成,18,8,A“linear”filterisalineartransformationofinputsamples;otherfiltersare“non-linear.”Linearfilterssatisfythesuperpositioncondition,i.e.,ifaninputisaweightedlinearcombinationofdifferentsignals,theoutputisanequallyweightedlinearcombinationofthecorrespondingoutputsignals.6,线性滤波器满足叠加条件，就是说，如果输入是不同信号的加权线性组合，输出就是（各信号）相应输出的同样加权线性组合。,19,8,A“causal”filterusesonlyprevioussamplesoftheinputoroutputsignals;whilea“non-causal”filterusesfutureinputsamples.Anon-causalfiltercanusuallybechangedintoacausalfilterbyaddingadelaytoit.A“time-invariant”filterhasconstantpropertiesovertime;otherfilterssuchasadaptivefilterschangeintime.,“时不变”滤波器对时间具有不变的性质，诸如自适应滤波器等其它滤波器随时间而改变,20,8,Somefiltersare“stable”,othersare“unstable”.Astablefilterproducesanoutputthatconvergestoaconstantvaluewithtime,orremainsboundedwithinafiniteinterval.Anunstablefilterproducesoutputwhichdiverges.,稳定的滤波器产生的输出随时间收敛于一个不变的值，或在有限的时间间隔内保持有界,21,8,A“finiteimpulseresponse”(FIR)filterusesonlytheinputsignal,whilean“infiniteimpulseresponse”filter(IIR)usesboththeinputsignalandprevioussamplesoftheoutputsignal.FIRfiltersarealwaysstable,whileIIRfiltersmaybeunstable.,而无限脉冲响应（IIR）滤波器同时使用输入信号和以前的输出信号样本,22,9,MostfilterscanbedescribedinZ-domain(asupersetofthefrequencydomain)bytheirtransferfunctions.Afiltermayalsobedescribedasadifferenceequation,acollectionofzeroesandpolesor,ifitisanFIRfilter,animpulseresponseorstepresponse.7,滤波器也可以用差分方程或一组零极点表示，对于FIR滤波器还可以用冲击响应或阶跃响应表示。,23,9,TheoutputofanFIRfiltertoanygiveninputmaybecalculatedbyconvolvingtheinputsignalwiththeimpulseresponse.Filterscanalsoberepresentedbyblockdiagramswhichcanthenbeusedtoderiveasampleprocessingalgorithmtoimplementthefilterusinghardwareinstructions.,滤波器还可以用结构图来表示，它能用来推导样本处理算法，以便使用硬件指令实现滤波器,24,10Frequencydomain,SignalsareconvertedfromtimeorspacedomaintothefrequencydomainusuallythroughtheFouriertransform.TheFouriertransformconvertsthesignalinformationtoamagnitudeandphasecomponentofeachfrequency.OftentheFouriertransformisconvertedtothepowerspectrum,whichisthemagnitudeofeachfrequencycomponentsquared.,Fourier变换将信号信息变换成每个频率的幅度和相位成分,25,11,Themostcommonpurposeforanalysisofsignalsinthefrequencydomainisanalysisofsignalproperties.Theengineercanstudythespectrumtogetinformationofwhichfrequenciesarepresentintheinputsignalandwhicharemissing.,26,12,Therearesomecommonlyusedfrequencydomaintransformations.Forexample,thecepstrumconvertsasignaltothefrequencydomainthroughFouriertransform,takesthelogarithm,andthenappliesanotherFouriertransform.Thisemphasizesthefrequencycomponentswithsmallermagnitudewhileretainingtheorderofmagnitudesoffrequencycomponents.8,例如倒谱用Fourier变换将信号转换到频域，取对数，然后再作第二次Fourier变换。这就强调了幅度较小的频率成分同时保持了频率分量的数量级。,27,13Applications,ThemainapplicationsofDSPareaudiosignalprocessing,audiocompression,digitalimageprocessing,videocompression,speechprocessing,speechrecognition,digitalcommunications,radar,sonar,seismology,andbiomedicine.,28,13,Specificexamplesarespeechcompressionandtransmissionindigitalmobilephones,roommatchingequalizationofsoundinHiFiandsoundreinforcementapplications,weatherforecasting,economicforecasting,seismicdataprocessing,analysisandcontrolofindustrialprocesses,computer-generatedanimationsinmovies,medicalimagingsuchasCATscansandMRI,imagemanipulation,highfidelityloudspeakercrossoversandequalization,andaudioeffectsforusewithelectricguitaramplifiers.,29,14Implementation,DigitalsignalprocessingisoftenimplementedusingspecializedmicroprocessorssuchastheMC56000andtheTMS320.Theseoftenprocessdatausingfixed-pointarithmetic,althoughsomeversionsareavailablewhichusefloatingpointarithmeticandaremorepowerful.,它们通常使用定点算法处理数据，尽管也有一些使用浮点算法，运算能力更强大,30,14,ForfasterapplicationsFPGAsmightbeused.Beginningin2007,multicoreimplementationsofDSPshavestartedtoemerge.Forfasterapplicationswithvastusage,ASICsmightbedesignedspecifically.Forslowapplications,atraditionalslowerprocessorsuchasamicrocontrollercancope.,PartII,GeneralConceptsofDigitalSignalProcessing,32,NewWords,33,NewWords,34,1,Therehavebeentremendousdemandsintheuseofdigitalcomputersandspecial-purposedigitalcircuitryforperformingvariedsignalprocessingfunctionsthatwereoriginallyachievedwithanalogequipment.Thecontinuedevolutionofinexpensiveintegratedcircuitshasledtoavarietyofmicrocomputersandminicomputersthatcanbeusedforvarioussignalprocessingfunctions.,35,1,Itisnowpossibletobuildspecial-purposedigitalprocessorswithinmuchsmallersizeandlowercostconstraintsofsystemspreviouslyallanaloginnature.1,现在有可能在比以往全模拟系统小得多，而且成本也低得多的限制下构成专用数字处理器。,36,2,Wewillprovideageneraldiscussionofthebasicconceptsassociatedwithdigitalsignalprocessing.Todoso,itisappropriatetodiscusssomecommontermsandassumptions.Whereverpossible,thedefinitionsandterminologywillbeestablishedinaccordancewiththerecommendationsoftheIEEEGrouponAudioandElectroacoustics.,可能之处，定义和术语将依照IEEE音频和电声小组的推荐,37,3,Ananalogsignalisafunctionthatisdefinedoveracontinuousrangeoftimeandinwhichtheamplitudemayassumeacontinuousrangeofvalues.Commonexamplesarethesinusoidalfunction,thestepfunction,theoutputofamicrophone,etc.Thetermanalogapparentlyoriginatedfromthefieldofanalogcomputation,inwhichvoltagesandcurrentsareusedtorepresentphysicalvariables,butithasbeenextendedinusage.,模拟信号是定义在连续时间上的函数，其幅度取值是连续的,38,4,Continuous-timesignalisafunctionthatisdefinedoveracontinuousrangeoftime,butinwhichtheamplitudemayeitherhaveacontinuousrangeofvaluesorafinitenumberofpossiblevalues.Inthiscontext,ananalogsignalcouldbeconsideredasaspecialcaseofacontinuous-timesignal.Inpractice,however,thetermsanalogandcontinuous-timeareinterchangedcasuallyinusageandareoftenusedtomeanthesamething.,连续时间信号是定义在连续时间上的函数，但是它的幅度可能是连续值也可以是有限的可能值,39,4,Becauseoftheassociationofthetermanalogwithphysicalanalogies,preferencehasbeenestablishedforthetermcontinuous-time.Nevertheless,therewillbecasesinwhichthetermanalogwillbeusedforclarity,particularlywhereitrelatestothetermdigital.2,由于“模拟”一词与物理类比的关联，已经确立了优先使用“连续时间”这一术语。不过有时为了清楚起见也用“模拟”一词，特别是与“数字”相联系时。,40,5,Thetermquantizationdescribestheprocessofrepresentingavariablebyasetofdistinctvalues.Aquantizedvariableisonethatmayassumeonlydistinctvalues.,经量化的变量只能取离散值,41,6,Adiscrete-timesignalisafunctionthatisdefinedonlyataparticularsetofvaluesoftime.Thismeansthattheindependentvariable,time,isquantized.Iftheamplitudeofadiscrete-timesignalispermittedtoassumeacontinuousrangeofvalues,thefunctionissaidtobeasampled-datasignal.Asampled-datasignalcouldarisefromsamplingananalogsignalatdiscretevaluesoftime.,离散时间信号是定义在某些特定时间值上的函数,42,7,Adigitalsignalisafunctioninwhichbothtimeandamplitudearequantized.Adigitalsignalmayalwaysberepresentedbyasequenceofnumbersinwhicheachnumberhasafinitenumberofdigits.,43,8,Thetermsdiscrete-timeanddigitalareofteninterchangedinpracticeandareoftenusedtomeanthesamething.Agreatdealofthetheoryunderlyingdiscrete-timesignalsisapplicabletopurelydigitalsignals,soitisnotalwaysnecessarytomakerigiddistinctions.Thetermdiscrete-timewillmoreoftenbeusedinpursuingtheoreticaldevelopments,andthetermdigitalwillmoreoftenbeusedindescribinghardwareorsoftwarerealizations.,许多基于离散时间信号的定理适用于纯数字信号，因此没有必要总是对两者作严格的区分,44,9,Asystemcanbedescribedbyanyoftheprecedingtermsaccordingtothetypeofhardwareorsoftwareemployedandthetypeofsignalspresent.Thus,referencecanbemadetoanalogsystems,continuous-timesystems,discrete-timesystems,digitalsystems,etc.,根据使用的硬件或软件的类型和当前信号的类型，一个系统可以用任意的前述术语来描述,45,10,Alinearsystemisoneinwhichtheprincipleofsuperpositionapplies.Alinearsystemcanbedescribedbylineardifferentialordifferenceequations.Atime-invariantlinearsystemisoneinwhichtheparametersarefixedanddonotvarywithtime.,线性系统可以用线性的微分或差分方程来描述,46,11,Alumpedsystemisonethatiscomposedoffinitenonzeroelementssatisfyingordinarydifferentialordifferenceequationrelationships,asopposedtoadistributedsystem,satisfyingpartialdifferentialequationrelationships.3,集总系统是由有限非零元素构成，满足常微分（或差分）方程的系统，与满足偏微分方程的分布式系统相对应。,47,12,Thestandardformfornumericalprocessingofadigitalsignalisthebinarynumbersystem.Thebinarynumbersystemmakesuseonlyofthevalues0and1torepresentallpossiblenumbers.,48,12,Thenumberoflevelsmthatcanberepresentedbyanumberhavingnbinarydigits(bits)isgivenbyConversely,ifmisthenumberofpossiblelevelsrequired,thenumberofbitsrequiredisthesmallestintegergreaterthanorequaltolog2m.4,可用n位二进制（n比特）表示的等级数m由m=2n给出。反过来，如果m是要求的等级数，所需的比特数是大于等于log2m的最小整数。,m=2n,49,13,Theprocessbywhichdigitalsignalprocessingisachievedwillbeillustratedbyasimplifiedsysteminwhichthesignalisassumedtovaryfrom0to7voltsandinwhich8possiblelevels(at1Vincrements)areusedforthebinarynumbers.5,实现数字处理的过程将用一个简化系统来说明，假定信号在0V到7V之间变化，以1V为增量，用8种可能的值表示成二进制数。,50,13,AblockdiagramisshowninFigure9.1,andsomewaveformsofinterestareshowninFigure9.2.Thesignalisfirstpassedthroughacontinuous-timepresamplingfilterwhosefunctionwillbediscussedlater.,51,13,ThesignalisthenreadatintervalsofTsecondsbyasampler.Thesesamplesmustthenbequantizedtooneofthestandardlevels.Althoughtherearedifferentstrategiesemployedinthequantizationprocess,onecommonapproach,whichwillbeassumedhere,isthatasampleisassignedtothenearestlevel.Thus,asampleofvalue4.2Vwouldbequantizedto4V,andasampleofvalue4.6Vwouldbequantizedto5V.,52,14,ThisprocessforthesignalgivenisillustratedinFigure9.2.Thepulsesrepresentingthesignalhavebeenmadeverynarrowtoillustratethefactthatothersignalsmaybeinserted,ormultiplexed,intheemptyspace.Thesepulsesmaythenberepresentedasbinarynumbers.Inorderthatthesenumberscouldbeseenonthefigure,eachhasbeenshownovermuchofthespaceinagiveninterval.,为了使这些数字可以从图中看到，每组都显示在给定间隔的空档处,53,14,Inpractice,ifothersignalsaretobeinserted,thepulsesrepresentingthebitsofthebinarynumberscouldbemadeveryshort.Agivenbinarynumbercouldthenbereadinaveryshortintervalatthebeginningofasamplingperiod,thusleavingmostofthetimeavailableforothersignals.,一个给定的二进制数就可以在采样周期开始的很短间隔内读到，这样就给其它信号留出了大部分的可用时间,54,15,Theprocessbywhichananalogsampleisquantizedandconvertedtoabinarynumberiscalledanalog-to-digital(A/D)conversion.Ingeneral,thedynamicrangeofthesignalmustbecompatiblewiththatoftheA/Dconverteremployed,andthenumberofbitsemployedmustbesufficientfortherequiredaccuracy.,信号的动态范围要和所用的A/D转换器相一致，为了达到所要求的精确度，要使用足够的比特数,55,16,Thesignalcannowbeprocessedbythetypeofunitappropriatefortheapplicationintended.Thisunitmaybeageneral-purposecomputerorminicomputer,oritmaybeaspecialunitdesignedspecificallyforthispurpose.Atanyrate,itiscomposedofsomecombinationofstandarddigitalcircuitscapableofperformingthevariousarithmeticfunctionsofaddition,subtraction,multiplication,etc.Inaddition,ithaslogicandstoragecapability.,56,17,Attheoutputoftheprocessor,thedigitalsignalcanbeconvertedtoanalogformagain.Thisisachievedbytheprocessofdigital-to-analog(D/A)conversion.Inthisstep,thebinarynumbersarefirstsuccessivelyconvertedbacktocontinuous-timepulses.Thegapsbetweenthepulsesarethenfilledinbyareconstructionfilter.,57,17,Thisfiltermayconsistofaholdingcircuit,whichisaspecialcircuitdesignedtoholdthevalueofapulsebetweensuccessivesamplevalues.Insomecases,theholdingcircuitmaybedesignedtointerpolatetheoutputsignalbetweensuccessivepointsaccordingtosomeprescribedcurve-fittingstrategy.6Inadditiontoaholdingcircuit,abasiccontinuous-timefiltermaybeemployedtoprovideadditionalsmoothingbetweenpoints.,在某些情况下，可设计保持电路，将输出信号在连续样点之间按照设定的曲线拟合方法进行内插。,58,18,Afundamentalquestionthatmayariseiswhetherornotsomeinformationhasbeenlostintheprocess.Afterall,thesignalhasbeensampledonlyatdiscreteintervalsoftime;istheresomethingthatmightbemissedintheinterveningtimeintervals?Furthermore,intheprocessofquantization,theactualamplitudeisreplacedbytheneareststandardlevel,whichmeansthatthereisapossibleerrorinamplitude.,信号仅仅在离散的时间间隔处被采样；在介于时间间隔内是否有一些信息丢失了呢,59,19,Inregardtothesamplingquestion,itwillbeshownthat,ifthesignalisbandlimited,andifthesamplingrateisgreaterthanorequaltotwicethehighestfrequency,thesignalcantheoreticallyberecoveredfromitsdiscretesamples.7,关于采样的问题，我们将表明，如果信号带宽有限，并且采样率大于等于最高频率的两倍，理论上信号就能从离散的样本恢复。,60,19,Thiscorrespondstoaminimumoftwosamplespercycleatthehighestfrequency.Inpractice,thissamplingrateisusuallychosentobesomewhathigherthantheminimumrate(say,threeorfourtimesthehighestfrequency)inordertoensurepracticalimplementation.Forexample,ifthehighestfrequencyoftheanalogsignalis5kHz,thetheoreticalminimumsamplingrateis10,000samplespersecond,andapracticalsystemwouldemployaratesomewhathigher.,61,19,Theinputcontinuous-timesignalisoftenpassedthroughalow-passanalogpresamplingfiltertoensurethatthehighestfrequencyiswithintheboundsforwhichthesignalcanberecovered.8,常将输入的连续时间信号通过一个低通模拟预采样滤波器，以确保最高频率落在信号能够完全恢复的界限之内。,62,20,Ifasignalisnotsampledatasufficientlyhighrate,aphenomenonknownasaliasingresults.Thisconceptresultsinafrequencybeingmistakenforanentirelydifferentfrequencyuponrecovery.Forexample,supposeasignalwithfrequenciesrangingfromdcto5kHzissampledatarateof6kHz,whichisclearlytoolowtoensurerecovery.Ifrecoveryisattempted,acomponentoftheoriginalsignalat5kHznowappearstobeat1kHz,resultinginanerroneoussignal.,混叠会导致在恢复时一个频率被误作为完全不同的频率,63,20,Acommonexampleofthisphenomenonisonewewillcallthewagonwheeleffect,probablynoticedbythereaderinwesternmoviesasthephenomenoninwhichthewheelsappeartoberotatingbackwards.9,一个普通例子就是我们称之为“车轮效应”的现象，就是读者在西部电影中也许注意到的车轮看起来向后转动的情况。,64,20,Sinceeachindividualframeofafilmisequivalenttoadiscretesamplingoperation,iftherateofspokespassingagivenangleistoolargeforagivenmovieframerate,thewheelsappeartobeturningeitherbackwardsorataveryslowspeed.10,因为影片的每一帧相当于一次离散的采样，如果相对于电影帧频而言轮辐越过的给定角度过大，轮子看起来就会向后转动或以很慢的速度转动。,65,20,Theeffectofapresamplingfilterremovesthepossibilitythataspurioussignalwhosefrequencyistoohighforthesystemwillbemistakenforoneintheproperfrequencyrange.11,采样前预滤波消除了这种对系统而言频率过高的伪信号被错误地当作适当频率范围内的另一信号的可能性。,66,21,Withrespecttothequantizationerror,itcanbeseenthattheerrorcanbemadeassmallasonechoosesifthenumberofbitscanbemadearbitrarilylarge.Ofcourse,thereisapracticalmaximumlimit,soitisnecessarytotoleratesomeerrorfromthisphenomenon.Evenincontinuous-timesystems,theremaybenoisepresentwhichwouldintroduceuncertaintyintheactualmagnitude.Infact,theuncertaintypresentinthedigitalsamplingprocessiscalledquantizationnoise.,如果使用任意多的比特数，可使误差任意小,67,22,LetEmaxandEminrepresentthemaximumandminimumvaluesofthesignal,andletqrepresenttheverticaldistancebetweensuccessivequantumlevels.Usingnandmaspreviouslydefined,wehave,(9-2),68,22,Assumingthatasamplebetweentwosuccessivequantumlevelsisassignedtothenearestquantumlevel,thepeakquantizationnoiseandpeakper