电子信息与通信工程专业英语课件chapter5

电子信息与通信工程专业英语Chapter5DigitalSignalProcessing5.1SignalsandSystems1

Computersoperateondigitalsignals.Ascomputersproliferate,theneedforefficienthandlingofdigitalsignalsincreases.Furthermore,highspeedprocessingcapabilitiesofmoderncomputersattractapplicationsthatusedigitalsignals,whichfurtherdrivesthedevelopmentofdigitalsignaltechniques.Digitalsignalprocessing,orDSP,isessentialto2anenormousvarietyofoldandnewapplications.

AttheheartofDSPliethesignalstobeprocessed3.Signalsarevariationsthatcarryinformationfromoneplacetoanother.Theoutsideworld,forexample,providesvariationsinpressureorlightintensitythathumanscanperceive.Changesinpressureattheeardrumareheardassounds.Variationsinlightintensityattheretinaareseenasimages.Thesesignalsareanalogsignals.Theycantakeanyvaluefromacontinuumofvaluesandaredefinedateveryinstantoftime.Soundsareone-dimensionalanalogsignals:Thesize,oramplitude,ofpressurevariationschangeswithtime.Asanotherexample,thevoltageavailablefromanelectricaloutletinNorthAmericavariessmoothlyfromitsminimumtoitsmaximumandbacktoitsminimumagain,60timespersecond.

Inordertoprocesssignals,theymustfirstbecaptured.Soundsignals,forexample,areacquiredusingamicrophone,whichconvertsacousticsignalsintoelectricalsignals.Images,ontheotherhand,arecapturedusingananalogordigitalcamera.Inanalogcameras,lightsignalscontrolchemicalreactionsonaphotographicfilm.Indigitalcameras,lightsignalsfromobjectsinviewcreatechargepacketsthatareconvertedintoelectricalsignalsonatwo-dimensionalgrid.Theseelectricalsignalsare,likethelightsignalsthatproducedthem,analoginnature.Becausetheycarryinformationataninfinitenumberoflevelsandpointsintime,analogsignalsarenotsuitedtocomputerprocessing.Theymustbesampledandconvertedintodigitalformbeforetheycanbeprocessed.Digitalsignalsareperfectlysuitedforcomputerprocessingbecausetheyaredefinedatonlyafinitenumberoflevelsandpointsintime.

Bothanaloganddigitalsignalsarepresentinmostdigitalprocessingsystems.Analogsignalsattheinputtothesystemareconvertedtodigitalformforprocessing.Afterprocessing,signalsindigitalformareconvertedbacktoanalogformforoutput.Itistheprocessingstage,whereonlydigitalsignalsexist,thattheflexibilityandspeedofDSParerealized.Asystemanalyzes,combines,modifies,records,orplaysbacksignals.Animagecompressionsystem,forexample,recodesdigitalimagessotheyoccupylessmemoryspaceandthereforetakelesstimetotransmitthroughtheInternet.Aspeechrecognitionsystemisdesignedtounderstandhumanspeechautomatically.Adigitalfilter,totakeanotherexample,allowscertainsignalfrequenciestopassthroughthesystemwhileblockingothers.Fordigitalsignals,thepossibilitiesforprocessingarelimitless.AnyoperationthatispossiblefornumbersisalsopossiblefordigitalsignalsandwithspecializedDSPhardwareandsoftware,theoperationscanoccuratlightingspeeds.

Digitalsystemshavemanyadvantagesoveranalogsystem.Analogsystemsarecircuitsconstructedfromhardwarecomponentswhosepropertiescanvaryquitewidelywithinamanufacturer'stolerance4.Thepropertiescanalsochangewithtemperature,alteringthecircuit'sbehavior.Incontrast5,digitalsystemsbehaveinapredictable,repeatableway.Theirbehaviorisalmostcompletelyunaffectedbytheseproblems,becauseitismostlydeterminedbysoftware.Forthesamereason,digitalsystemsaremuchlessaffectedbynoisethananalogsystems.Inaddition,digitalsystemsaresmallerandconsumelesspowerthantheiranalogcounterparts.Perhapsthegreatestadvantageofdigitaltechnologyistheflexibilitythatallowsadesigntobemodifiedbysimplychangingafewlinesinaprogram.Mostanalogredesigninvolvesbuildingnewcircuitsfromscratch.

Sightsandsoundsfromtheworldaroundusareanalogsignals.Inordertoprocessthesesignals,sensorstunedtothesignals'characteristicsmustbeused.Sensorsexistforavastarrayofsignals.Microphonesarecommonsoundsensors.Changesinlightcanbecapturedbysemiconductordeviceslikephotodiodes,phototransistors,orCCD(charge-coupleddevice)chips,whoseabilitytocarrycurrentchangeswithincidentlight.Temperaturecanbemeasuredbythermistors,semiconductordeviceswhoseresistancechangeswithtemperature,orbythermocouples,consistingoftwodissimilarmetalsthatreactdifferentlytotemperatureanddevelopavoltagedifference.Accelerometersareusedtomeasureacceleration.Othersensorsincludestrainsensors,pressuresensors,andflowsensors.

Themostcommonoutputforallofthesesensorsisanelectricalsignal,voltageorcurrent,proportionaltothesignalbeingmeasured.Itisthisanalogelectricalsignalthatmustbeconvertedintodigitalform.Thevaluesthatthesignalmaytakebetweenitsminimumandmaximumvaluesarenotrestricted,andthesignalhasvaluesateveryinstantoftime.

Analog-to-digitalconversionoccursintwosteps.Thefirststepissampling.Samplinginstantsnormallyoccuratregularintervalscalledsamplingperiods.Ateachsamplingpoint,theanalogsignalissampled,andthevalueofthesignalisheldsteadyuntilthenextsamplingpoint.Thisprocessiscalledsampleandhold.Theverticaldashedlinesmarkthesamplingpoint.Samplingmustbefastenoughtocapturethemostrapidchangesinthesignalbeingsampled.Ifsamplingistooslow,importantsignalcharacteristicscanbelost,aproblemcalledaliasing.Notethattheactofsamplingtakesafiniteamountoftime,calledtheacquisitiontimeofthesampler.

Thesecondstepintheconversionbetweenananalogsignalandadigitalsignal,whichcanbeginatthecompletionofsampleacquisition,istoquantizeanddigitizetheanalogvalues.Theholdintervalnormallygivesampletimeforthis.Assoonaspossibleaftereachsamplinginstant,theconverterselectsthequantizationlevelthatapproximatesthesample-and-holdvalueascloselyaspossibleandthenassignsabinarydigitalcodethatidentifiesthequantizationlevel.Thiscompletestheanalog-to-digitalconversionprocess.Newwordsandphraseseardrum [ˈiəˌdrʌm] n. 鼓膜，耳膜variation [ˌvɛəriˈeiʃən] n. 变化，变动(的程度)；(数量、水平等的)变化，变更，变异retina [ˈretinə] n. 视网膜continuum [kənˈtinjuəm] n. 连续统一体instant [ˈinstənt] n. 片刻，顷刻，刹那，瞬间；某一时刻amplitude [ˈæmpliˌtu:d,-ˌtju:d] n. (声音、无线电波等的)振幅；幅度；广大，广阔；充足，丰富minimum [ˈminiməm] n. 最低限度，最小量；极小量maximum [ˈmæksiməm] adj.n. 最大值的，最大量的最大的量、体积、强度等Newwordsandphrasesacoustic [əˈku:stik] adj. 声音的，听觉的；原声的；自然声的predictable [priˈdiktəbl] adj. 可预言(预报)的，可预见的，可预料的affect [əˈfekt] vt. 影响；对……有影响；对……起作用involve [inˈvɔlv] vt. 使某事物成为必要条件或结果；需要；使参与，牵涉scratch [skrætʃ] vt.&vi. 抓，搔；删掉thermocouple [θə:məuˌkʌpl] n. 热电偶voltage [ˈvəultidʒ] n. 【电】电压，电压量，伏特数accelerometer n. 加速表，加速计，加速仪；过荷指示器vertical [ˈvə(r):tik(ə)l] adj. 垂直的，直立的，竖立的，纵的Newwordsandphrasesaliasing n. 折叠失真acquisition [ˌækwiˈziʃ(ə)n] n. 取得，获得；习得quantization [ˌkwɔntaiˈzeiʃən] n. 【物】量子化；分层approximate [əˈprɔksimət] adj. 近似的，大概的binary [ˈbainəri] adj. 二，双，复【化】二元的【数】二进制的NotesNotes1.本节涉及数字信号处理领域，标题可译为：信号与系统。2.

isessentialto对……是必要的；此句译为：数字信号处理(或简称DSP)，对于许多应用来讲都是必需的。例句：Passwordsarethefirstlineofdefenseagainstintruders,sosettingstrongpasswordsisessentialtothesecurityofyoursystem.密码是抵御入侵者的第一道防线，因此设置强密码对于系统安全是绝对必要的。3.

liev.(抽象事物)存在，在于，有(……关系)。此句译为：DSP内部存在着要进行处理的信号。4.翻译技巧：拆句法，即把一个长而复杂的句子拆译成若干个较短、较简单的句子，通常用于英译汉中。此句译为：模拟系统是由元器件搭建而成的电路，而元器件的特性在制造误差允许的范围内差异很大。5.

incontrast，相比之下。此句译为：相比之下，数字系统的工作具有可预测性和可重复性。例句：Incontrast,Americanworkershavenochoicebutto"contribute"toSocialSecurity,whethertheylikethedealornot.与之相比，美国的工人没有选择，不管他们愿意不愿意，他们只能“持续地”把钱投入社会保险。。5.2DigitalSignalandTheirSpectra1Anyofthesignalscanbeconvertedintodigitalform.InDSP,itisfrequentlyimportanttoknowwhatfrequenciesarepresentinasignal,inadditiontoitstime-basedbehavior.Itiscommonknowledge,forexample,thatfemalevoicesaregenerallyhigherpitchedthanmalevoices,butitistheprecisefrequencyelementspresentinaspeechsoundthatareimportantforautomaticspeechrecognition.Aspectrumisapresentationofthefrequencyelementsthatarepresentinasignal.Theimportanceofafrequencyelementinthesignalisshownbythemagnitudeofthespectrumatthatfrequency.Thespectrumisplottedasmagnitudeagainstfrequencyand,assuch,isafrequencydomainpresentationofinformationaboutthesignal.ItismostoftencalculatedusinganFFT(fastFouriertransform).Lowfrequencysignalsareconstantorchangeslowlyovertime.Theyappearattheleftendofthespectrum.Highfrequencysignals,ontheotherhand2,changerapidlywithtime.Theyappearattherightendofthespectrum.Digitalfiltersprovideaconvenientmeanstochangethenatureofasignal.Themostcommonfilterschangethefrequencycharacteristicsofasignal,lettingsomefrequenciesinthesignalpasswhileblockingothers.Alowpassfilter,forexample,letslowfrequenciesthroughwhileblockinghighfrequencies.Ahighpassfilterdoesjusttheopposite.Abandpassfilterallowsabandoffrequenciestopass,whileabandstopfilterallowsallfrequenciesoutsideabandtopass3.Ifalowpassfilterwereappliedtothesoundsofasingingchoir,thefilterwouldtendtoextractthebassvoicesandblockthesopranovoices.Ahighpassfilterwouldpassthesopranovoiceswhileblocking,orattenuating,thebassvoices.Digitalfiltersarenothingmorethanequationsdefinedbyalistoffiltercoefficients4.Theseequationsformpartofadigitalfilterprogramthatacceptsrawdataasinputandproducesfiltereddataasoutput.Thebeautyofsuchprogramsisthatfilterredesigndoesnotrequireahardwarechange:Changingfilterbehaviorisplishedbysimplychangingthelistoffiltercoefficients.Digitalfilterprogramscanbeimplementedonanyprocessor,buttheyareimplementedmostefficientlyonhardwarethatisspeciallydesignedtoperformfilteringandotherDSPoperationsatenormousspeeds5.ThisDSPhardwareenablesextremelycomplextaskstobeperformedinnegligibleamountsoftime,whichisthereasonitissooftenfoundatthecoreofcomputation-intensiveapplications.Newwordsandphrasespitch [pitʃ] v. 扔，投，抛，掷spectrum [ˈspektrəm] n. 【物】谱，光谱；波谱；能谱，质谱magnitude [ˈmæɡnitju:d] n. 大小；积；广大，巨大；伟大，重大，重要性；量domain [dəuˈmein] n. (学问，活动等的)领域；范围choir [ˈkwaiə(r)] n. (教堂、学校的)唱诗班soprano [səˈprɑ:nəu] n. 【乐】女高音，高音部；女高音歌手；唱最高音者equation [iˈkweiʒ(ə)n] n. 【数】方程式，等式enormous [iˈnɔ:(r)məs] adj. 巨大的；大量的；庞大的extremely [iksˈtri:mli] adv. 极端地，非常地negligible [ˈneɡlidʒəb(ə)l] adj. 可以忽视的；不足取的；很小的，微不足道的Notes1.本节涉及数字信号处理领域，标题可译为：数字信号及其频谱。2.

ontheotherhand从另一方面来说，相反，反之。同义词汇：onthecontrary。低频信号是常量或随时间变化较慢的信号，在频谱的左端，而高频信号随时间变化较快，出现在频谱的右端。3.

lowpassfilter低通滤波器；highpassfilters高通滤波器；bandpassfilter带通滤波器；bandstopfilter带阻滤波器。此句译为：低通滤波器使低频成分通过，阻碍高频成分；高通滤波器则相反。带通滤波器允许一定频带内的频率通过，而带阻滤波器则允许一定频带外的所有频率通过。4.

nothingmorethan，只不过。同义词汇：just,nothingbut。此句译为：数字滤波器只不过是由一系列滤波器系数定义的方程。5.转折语句，后一句为宾语从句。数字滤波程序可在任何处理器上实现，但在专门设计用来进行高速滤波或其他DSP处理的硬件上实现最有效。5.3ApplicationsofFiberDSP1DSPhasmanyexcitingapplicationsinawiderangeoffields,andthenumberofapplicationsgrowseveryyear.Whattheapplicationshaveincommonisthattheyaredefinedprimarilybysoftware,thatis2,byalistofprograminstructionsthatexecuteonaparticularhardwareplatform.Inotherwords,theverysameDSPhardwaremaybethebasisforawidevarietyofapplications.Amongthemostaccessiblearethoseapplicationsthatoperateonfamiliarone-dimensionalsignalslikespeechormusic,ortwo-dimensionalsignalslikeimages.

Largevocabulariesofcontinuousspeechcanberecognizedusinginformationfromdigitalspeechsignals.Mostoften,speechrecognitionmethodsarebasedonaspectralanalysisofspeechsounds.Thoughthisanalysiscanbetime-consuming3,fastDSPhardwaremakesreal-timerecognitionareality.Thelargerthepopulationofusersofaspeechrecognizer,themorevariablethespeechandthemoredifficultthetask.Greatervocabularysizealsomakesdiscriminationbetweenwordsmorechallenging.TheAT&Ttelephonecompanyhas,forseveralyears,usedaword-spottingtechniqueforitsdirectoryassistancesystem.Wordspottinghelpsoperatorsrespondmorequicklybysearchingfornumbersandsimplewordsembeddedincontinuousspeechandthencollatingbasicinformationaboutarequest.Automaticvoicerecognitionforradiocontrolmakeshands-freeradiousepossibleinjetcockpits,andsimilartechnologyallowshands-freeuseofcellularphonesinautomobiles.Incomplementarytechnology,advancedspeechsynthesisprogramsgeneratenatural-soundingspeechforautomobilewarnings,telephoneinformationmessagesandtelephonebanking.Indeed,advancedmodelsofhumanspeechdevelopedforspeechsynthesizershavebeenusedtoproduceDSP-basedhearingaidsthataretunedtocompensatepreciselyforanindividual'sspecifichearingdeficiencies.DSPalsomakescontributionsintheareaofmusicandothersounds.Oldrecordingsofmusicandaudiotrackscanbecleanedupbyremovingclickingandbuzzingsoundsinthebackground.Also,thesoundsofmusicalinstrumentscanbecloselycopiedforelectronicsynthesizers.Combinationsofactualrecordingsandmathematicalmodelsproducehighqualitysynthesisformanyinstruments,includingpiano,violin,andflute.FantasticeffectsarepossiblewhenDSPisappliedtoimagery.Imagescanbecombined,aswhenForrestGumpwasdigitallyaddedtoafilmofPresidentNixon.Backgroundspecklingcanberemovedbyaveragingseveralpicturesofthesamescene.Movementcanbedetectedbysubtractingoneimagefromanotherofthesamescene,highlightingtheplaceswherethetwoimagesdiffer.Colormapscanbealtered,andcontrastimproved.

Incommunications,DSPplaysseveralimportantroles,particularlyinsupportofcellularphones,digitalmodems,andaudioandvideotransmissiontechnologies.DigitalcellulartelephoneuseDSPfortwomaintasks:tocodespeechascompactlyaspossiblewhilemaintainingintelligibility,andtotransmitcodedspeechreliablytoareceiveracrossawirelessradiofrequencylink4.Voicecodingalgorithms,calledvocoders,matchvoicedatatoamodelofspeechthatrequiresonlyafewparameters.Theseparametersaretransmittedtothereceiverinsteadofthespeechsignal,wheretheparametersareappliedtothesamemodelagaintorecoverareasonablecopyoftheoriginalvoicedata.Useofvocodersdecreasesdramaticallythesizeofthetransmissionsneededtosupportanindividualcall,providingacommensurateincreaseintheefficiencyofthecellularnetwork.

Sinceradiofrequencylinkscannotsupportthetransmissionofdigitaldatadirectly,theymustemployamodulator.Inonecommonscheme,quadratureamplitudemodulation(QAM)5,severalbitsofdigitaldataatatimedeterminetheamplitudeandphaseofasinewavethatistransmittedintheirplace.Atthereceiver,theamplitudeandphaseofthereceivedsignalaredetectedbyademodulatorbeforetheoriginaldigitaldatamaybeidentified.Thetwomainobstaclestocleantransmissionofacallaresignalfadingandnoise.Severalstrategiesareusedtomitigatetheireffects.

Forexample,errorcodesthatcanbecheckedbythereceiverareappendedtosomeportionsofthetransmitteddata.Also,parametersfromdifferentblocksofspeechareinterleavedwithoneanothertoreducethechancethattheintegrityofanentireblockwillbedestroyedbyasingleburstofnoise.Asaresultofnoiseandfading,thereceivedsignalmaybequiteunlikethetransmittedsequence.Toassistinsignalrecovery,aknownpatternofbitsissometimessentfromthetransmittertothereceivertoallowthereceivertoestimatethecharacteristicsofthelinkandthereforehavethegreatestchanceofidentifyingthebitsinthereceivedsignalcorrectly.Modemtechnologypermitcomputerstocommunicateusingordinarytelephonelinesortelevisioncable.Manyofthealgorithmsthatpermitdatatobecarriedatveryhighratesonthesemediarelyoncomplexcalculationsthatmustbecompletedquicklytokeepupwithingdata.Digitaldatamustfirstbecodedforwiredtransmissioninamannerthatgiveshighthroughputandreliabledetection.Telephonelineimpairmentslikenoise,fading,andcrosstalkcan,however,introducederrors.Tosaveretransmissions,errorcodescanbeaddedtothetransmitteddatathatpermitthereceivernotonlytodetectbutalsotocorrecterrors.Asforcellularphones,digitalmodemssometimesexchangeknownsignalstomonitorthequalityofthecommunicationschannel.Somemodemscanevenadjusttheirtransmissionratesinaccordancewiththequalityoftheline,tohelperrorstoaminimum.Inadditiontocodinganderrorcontrol,modemscanapplycompressiontoreducetheamountofdatatobetransmitted.Inonecompressionscheme,forexample,shortcodesaresubstitutedforthemostcommonbitpatterns,codesthatarereplacedbytheoriginalpatternsatthereceiver.Forsecuretransmissions,somemodemsprovideencryptioncapabilityaswell.Whethertransmissioniswiredorwireless,thegrowthoftheInternethasoccasionedanenormousincreaseinthemovementofaudioandvideofiles.Tosavespaceandalsotransmissiontime,compressionstrategieshavedevelopedfarbeyondsimplesubstitutionalcodes.VideocompressionschemeslikeMPEG(movingpictureexpertsgroup)5,forexample,relyonthefactthatneighboringframesofavideosequencehaveverysimilarcontent.Insteadofcodingallpixelsinallframes,thedifferencesbetweenframesarecoded.MPEGalsoincorporatesanaudiocompressionstandard,MP3,whichexploitsthecharacteristicsofthehumanauditorysystem.TheresultsofDSPcanbeadmiredandenjoyedwithoutanyknowledgeoftheunderlyingmathematics.Activeanalysis,filtering,ortransformationofsignalsdoes,however,requiresomeunderstandingofthemaththatdrivesDSP.Newwordsandphrasesaccessible [əkˈsesəb(ə)l] adj. 可以进入的；容易理解的；能接近的cockpit [ˈkɔkˌpit] na. 战场；(戏院的)正厅；(军舰内的)伤兵室complementary [ˌkɔmpliˈment(ə)ri] adj. 补充的；补足的；互补的【生】互配(力)的synthesizer [ˈsinθəsaizə(r)] n. 合成者，合成物【电】合成器，综合器hearingaid 助听器obstacle [ˈɔbstək(ə)l] n. 障碍(物)，妨害，阻碍，干扰exchange [iksˈtʃeindʒ] n. 交换，互换accordance [əˈkɔ:(r)d(ə)ns] n. 一致，协调Newwordsandphrasessubstitutional [ˌsʌbstiˈtju:ʃənəI] adj. 替代的；取代的execute [ˈeksiˌkju:t] v. 实行，实施，执行；履行；贯彻，完成dimensional [diˈmenʃənəl] adj. 尺寸的；空间的quadrature [ˈkwɔdrətʃə] n. 【物】正交；转象差；九十度相位差flute [flu:t] n. 【乐】长笛；长笛吹奏者；(风琴的)长笛音fantastic [fænˈtæstik] adj. 空想的，异想天开的speckle [ˈspek(ə)l] n. 斑点；斑纹v.加斑点；玷污；点缀intelligibility [inˈtelidʒibiliti] n. 可理解性；可理解的事物vocoder [vəuˈKəudə] n. 〈美〉语音编码机Newwordsandphrasescommensurate [kəˈmenʃərət] adj. 相称的，相当的；能通约的；同量的，同样大小的dramatically [drəˈmætikli] adv. 显著地；戏剧地vocabulary [vəuˈkæbjuləri] n. 词汇，单词集identify [aiˈdentifai] v. 验明(正身)，鉴别，辨认，识别calculation [ˌkælkjuˈleiʃ(ə)n] n. 计算，计算法；推定，预测encryption [inˈkripʃən] n. 【计】加密Notes1.本节涉及数字信号处理领域，标题可译为：光纤数字信号处理的应用。2.

thatis,插入语，即；就是；换句话说；同义词汇：thatistosay,inotherwords。executeon，运行。此句可译为：更确切地说这些应用的共同之处就是它们主要由软件来确定，即由特定硬件平台上运行的一系列程序指令来确定。3.

time-consuming费时，花费大量时间的。构词类型：名词＋动词-ing构成形容词。例如：peace-loving热爱和平的，下文中word-spotting译为字识别的。此句译为：尽管这种分析很费时，但快速DSP硬件可实现实时识别。4.动词不定式作表语。实际上省略了Thetwomaintasksareto…,andto…。此句译为：数字蜂窝电话中，DSP主要完成两方面的任务：在保证语音可识别的基础上，对语音进行尽可能的压缩编码；在无线传输中可靠地传输该编码的语音。5.

quadratureamplitudemodulation(QAM)正交幅度调制。movingpictureexpertsgroup运动图像专家组。参考译文5.1信号与系统计算机所使用的是数字信号。随着计算机应用的普及，对数字信号进行高效处理的需求日益迫切，并且现代计算机的高速处理能力使数字信号得到广泛应用，进一步促进了数字信号技术的发展。数字信号处理(或简称DSP)对于许多应用来讲都是必需的。DSP内部存在着要进行处理的信号。信号是将信息从一处携带到另一处的变化。例如，外界具有人们可感受到的压力或光强度的变化，人们所听到的声音就是耳膜感觉到的压力变化，所看到的图像就是视网膜感受到的光强度变化。这些信号都是模拟信号，他们在任意时刻都有值，且可取连续值范围内的任意值。声音是一维模拟信号，压力变化的大小会随时间改变，还有，北美地区电线上的输出电压会在其最大值和最小值之间平滑变化，每秒60次。

要对信号进行处理，首先必须获取信号。例如，声音信号可通过麦克风将声信号转变为电信号。而图像可通过模拟或数字照相机来获得，在模拟照相机中，由光信号控制胶片上的化学反应；在数字照相机中，由视线物体的光信号产生电荷并将其转变为二维栅格上的电信号，这些电信号与产生它们的光信号不一样，自然也是模拟信号。模拟信号可在任意时刻取值，并且具有无限多个电平，不适合计算机处理。所以在处理前，必须对它们进行采样并将其转换为数字形式。数字信号只在有限的时间点上取值，并且具有有限个电平，所以非常适合计算机处理。在大多数数字信号处理系统中数字信号和模拟信号二者都会出现。系统输入端的模拟信号会被转换为数字形式进行处理，处理后，数字形式的信号再被转换为模拟信号进行输出。灵活快速的DSP是在只存在数字信号的处理阶段实现的。通过DSP系统可以对信号进行分析、合成、修正、记录或播放。例如，通过图像压缩系统可以对数字图像进行再编码，以使图像占用的内存空间较少，并以较短的时间在网间传输。语音识别系统用来自动识别人的声音。再如，数字滤波器允许某些信号频率通过系统，而其他信号频率被滤除。总之，数字信号的处理方式多种多样。任何对数字可行的运算，同样适用于数字信号，使用专用的DSP软硬件，可进行快速运算。数字系统与模拟系统相比有许多优点。模拟系统是由元器件搭建而成的电路，而元器件的特性在制造误差允许的范围内差异很大，且特性还会随温度改变，从而改变了电路的性能。相比之下，数字系统的工作具有可预测性和可重复性。由于数字系统主要取决于软件，所以它的性能几乎不受以上因素的影响。因此，数字系统比模拟系统有更好的抗噪声性能。此外，数字系统比模拟系统电路体积小、功耗低。不过，数字技术的最大优势是它的灵活性较大，可通过简单地更改程序中的一些语句来修改设计，而对于大多数模拟系统而言，如果要重新设计，则往往需要重新搭建电路。人们身边的声音和图像都是模拟信号。为了处理这些信号，就要用到相应的传感器。各种信号的传感器是不同的。麦克风是最普通的声音传感器；光的变化可通过半导体器件记录，包括二极管、光电晶体管以及CCD(电荷耦合器件)芯片，它们的载流能力会随着入射光而变化；温度可通过阻值随温度变化的热敏电阻、半导体器件测得，或利用热电偶测量，热电偶由两种不同的金属制成，两种不同的金属对温度的反应不同，从而形成电压；加速计用来测量加速度。其他传感器还有应力传感器、压力传感器和流量传感器。这些传感器的输出通常是与被测信号成正比例的电信号(电压或电流)。模拟电信号必须转换成数字的形式。此信号可取其最小值和最大值之间的任意值，且在任意时刻均有值。模数转换分两步进行。第一步是采样，采样时刻通常处在固定的间隔点上，这个间隔称为采样周期。在每一个采样点对模拟信号进行采样，且将该采样值保持到下一个采样点的过程称为采样保持。采样要足够快，以捕捉被采样信号中的最快变化；如果采样太慢，将丢失重要的信号特征，出现混叠。注意，采样也要占用有限的时间，称之为采样器的采集时间。模数转换的第二步是对模拟值进行量化和数字化，这在采样完成后即可进行。在采样保持期间有足够的时间完成这一步。每个采样结束后，转换器会尽快选择与采样保持电平最接近的量化电平，然后分配一个二进制数字代码来标识这个量化电平，至此就完成了模数转换过程。5.2数字信号及其频谱

任何信号均可转换为数字形式。在DSP中，除了基于时间的变化外，通常还需要知道信号的频率。例如，众所周知，女性的音调通常比男性的高，但对于自动语音识别系统，语音中的准确频率分量非常重要。频谱是对信号中所含频率分