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電子信息與通信工程專業(yè)英語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 [?i??dr?m] n. 鼓膜,耳膜variation [?v??ri?ei??n] n. 變化,變動(的程度);(數(shù)量、水平等的)變化,變更,變異retina [?retin?] n. 視網(wǎng)膜continuum [k?n?tinju?m] n. 連續(xù)統(tǒng)一體instant [?inst?nt] n. 片刻,頃刻,剎那,瞬間;某一時刻amplitude [??mpli?tu:d,-?tju:d] n. (聲音、無線電波等的)振幅;幅度;廣大,廣闊;充足,豐富minimum [?minim?m] n. 最低限度,最小量;極小量maximum [?m?ksim?m] adj.n. 最大值的,最大量的最大的量、體積、強度等Newwordsandphrasesacoustic [??ku:stik] adj. 聲音的,聽覺的;原聲的;自然聲的predictable [pri?dikt?bl] adj. 可預言(預報)的,可預見的,可預料的affect [??fekt] vt. 影響;對……有影響;對……起作用involve [in?v?lv] vt. 使某事物成為必要條件或結(jié)果;需要;使參與,牽涉scratch [skr?t?] vt.&vi. 抓,搔;刪掉thermocouple [θ?:m?u?k?pl] n. 熱電偶voltage [?v?ultid?] n. 【電】電壓,電壓量,伏特數(shù)accelerometer n. 加速表,加速計,加速儀;過荷指示器vertical [?v?(r):tik(?)l] adj. 垂直的,直立的,豎立的,縱的Newwordsandphrasesaliasing n. 折疊失真acquisition [??kwi?zi?(?)n] n. 取得,獲得;習得quantization [?kw?ntai?zei??n] n. 【物】量子化;分層approximate [??pr?ksim?t] adj. 近似的,大概的binary [?bain?ri] adj. 二,雙,復【化】二元的【數(shù)】二進制的NotesNotes1.本節(jié)涉及數(shù)字信號處理領域,標題可譯為:信號與系統(tǒng)。2.

isessentialto對……是必要的;此句譯為:數(shù)字信號處理(或簡稱DSP),對于許多應用來講都是必需的。例句:Passwordsarethefirstlineofdefenseagainstintruders,sosettingstrongpasswordsisessentialtothesecurityofyoursystem.密碼是抵御入侵者的第一道防線,因此設置強密碼對于系統(tǒng)安全是絕對必要的。3.

liev.(抽象事物)存在,在于,有(……關系)。此句譯為:DSP內(nèi)部存在著要進行處理的信號。4.翻譯技巧:拆句法,即把一個長而復雜的句子拆譯成若干個較短、較簡單的句子,通常用于英譯漢中。此句譯為:模擬系統(tǒng)是由元器件搭建而成的電路,而元器件的特性在制造誤差允許的范圍內(nèi)差異很大。5.

incontrast,相比之下。此句譯為:相比之下,數(shù)字系統(tǒng)的工作具有可預測性和可重復性。例句:Incontrast,Americanworkershavenochoicebutto"contribute"toSocialSecurity,whethertheylikethedealornot.與之相比,美國的工人沒有選擇,不管他們愿意不愿意,他們只能“持續(xù)地”把錢投入社會保險。。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 [pit?] v. 扔,投,拋,擲spectrum [?spektr?m] n. 【物】譜,光譜;波譜;能譜,質(zhì)譜magnitude [?m?ɡnitju:d] n. 大小;積;廣大,巨大;偉大,重大,重要性;量domain [d?u?mein] n. (學問,活動等的)領域;范圍choir [?kwai?(r)] n. (教堂、學校的)唱詩班soprano [s??prɑ:n?u] n. 【樂】女高音,高音部;女高音歌手;唱最高音者equation [i?kwei?(?)n] n. 【數(shù)】方程式,等式enormous [i?n?:(r)m?s] adj. 巨大的;大量的;龐大的extremely [iks?tri:mli] adv. 極端地,非常地negligible [?neɡlid??b(?)l] adj. 可以忽視的;不足取的;很小的,微不足道的Notes1.本節(jié)涉及數(shù)字信號處理領域,標題可譯為:數(shù)字信號及其頻譜。2.

ontheotherhand從另一方面來說,相反,反之。同義詞匯:onthecontrary。低頻信號是常量或隨時間變化較慢的信號,在頻譜的左端,而高頻信號隨時間變化較快,出現(xiàn)在頻譜的右端。3.

lowpassfilter低通濾波器;highpassfilters高通濾波器;bandpassfilter帶通濾波器;bandstopfilter帶阻濾波器。此句譯為:低通濾波器使低頻成分通過,阻礙高頻成分;高通濾波器則相反。帶通濾波器允許一定頻帶內(nèi)的頻率通過,而帶阻濾波器則允許一定頻帶外的所有頻率通過。4.

nothingmorethan,只不過。同義詞匯:just,nothingbut。此句譯為:數(shù)字濾波器只不過是由一系列濾波器系數(shù)定義的方程。5.轉(zhuǎn)折語句,后一句為賓語從句。數(shù)字濾波程序可在任何處理器上實現(xiàn),但在專門設計用來進行高速濾波或其他DSP處理的硬件上實現(xiàn)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 [?k?ses?b(?)l] adj. 可以進入的;容易理解的;能接近的cockpit [?k?k?pit] na. 戰(zhàn)場;(戲院的)正廳;(軍艦內(nèi)的)傷兵室complementary [?k?mpli?ment(?)ri] adj. 補充的;補足的;互補的【生】互配(力)的synthesizer [?sinθ?saiz?(r)] n. 合成者,合成物【電】合成器,綜合器hearingaid 助聽器obstacle [??bst?k(?)l] n. 障礙(物),妨害,阻礙,干擾exchange [iks?t?eind?] n. 交換,互換accordance [??k?:(r)d(?)ns] n. 一致,協(xié)調(diào)Newwordsandphrasessubstitutional [?s?bsti?tju:??n?I] adj. 替代的;取代的execute [?eksi?kju:t] v. 實行,實施,執(zhí)行;履行;貫徹,完成dimensional [di?men??n?l] adj. 尺寸的;空間的quadrature [?kw?dr?t??] n. 【物】正交;轉(zhuǎn)象差;九十度相位差flute [flu:t] n. 【樂】長笛;長笛吹奏者;(風琴的)長笛音fantastic [f?n?t?stik] adj. 空想的,異想天開的speckle [?spek(?)l] n. 斑點;斑紋v.加斑點;玷污;點綴intelligibility [in?telid?ibiliti] n. 可理解性;可理解的事物vocoder [v?u?K?ud?] n. 〈美〉語音編碼機Newwordsandphrasescommensurate [k??men??r?t] adj. 相稱的,相當?shù)?;能通約的;同量的,同樣大小的dramatically [dr??m?tikli] adv. 顯著地;戲劇地vocabulary [v?u?k?bjul?ri] n. 詞匯,單詞集identify [ai?dentifai] v. 驗明(正身),鑒別,辨認,識別calculation [?k?lkju?lei?(?)n] n. 計算,計算法;推定,預測encryption [in?krip??n] n. 【計】加密Notes1.本節(jié)涉及數(shù)字信號處理領域,標題可譯為:光纖數(shù)字信號處理的應用。2.

thatis,插入語,即;就是;換句話說;同義詞匯:thatistosay,inotherwords。executeon,運行。此句可譯為:更確切地說這些應用的共同之處就是它們主要由軟件來確定,即由特定硬件平臺上運行的一系列程序指令來確定。3.

time-consuming費時,花費大量時間的。構(gòu)詞類型:名詞+動詞-ing構(gòu)成形容詞。例如:peace-loving熱愛和平的,下文中word-spotting譯為字識別的。此句譯為:盡管這種分析很費時,但快速DSP硬件可實現(xiàn)實時識別。4.動詞不定式作表語。實際上省略了Thetwomaintasksareto…,andto…。此句譯為:數(shù)字蜂窩電話中,DSP主要完成兩方面的任務:在保證語音可識別的基礎上,對語音進行盡可能的壓縮編碼;在無線傳輸中可靠地傳輸該編碼的語音。5.

quadratureamplitudemodulation(QAM)正交幅度調(diào)制。movingpictureexpertsgroup運動圖像專家組。參考譯文5.1信號與系統(tǒng)計算機所使用的是數(shù)字信號。隨著計算機應用的普及,對數(shù)字信號進行高效處理的需求日益迫切,并且現(xiàn)代計算機的高速處理能力使數(shù)字信號得到廣泛應用,進一步促進了數(shù)字信號技術的發(fā)展。數(shù)字信號處理(或簡稱DSP)對于許多應用來講都是必需的。DSP內(nèi)部存在著要進行處理的信號。信號是將信息從一處攜帶到另一處的變化。例如,外界具有人們可感受到的壓力或光強度的變化,人們所聽到的聲音就是耳膜感覺到的壓力變化,所看到的圖像就是視網(wǎng)膜感受到的光強度變化。這些信號都是模擬信號,他們在任意時刻都有值,且可取連續(xù)值范圍內(nèi)的任意值。聲音是一維模擬信號,壓力變化的大小會隨時間改變,還有,北美地區(qū)電線上的輸出電壓會在其最大值和最小值之間平滑變化,每秒60次。

要對信號進行處理,首先必須獲取信號。例如,聲音信號可通過麥克風將聲信號轉(zhuǎn)變?yōu)殡娦盘?。而圖像可通過模擬或數(shù)字照相機來獲得,在模擬照相機中,由光信號控制膠片上的化學反應;在數(shù)字照相機中,由視線物體的光信號產(chǎn)生電荷并將其轉(zhuǎn)變?yōu)槎S柵格上的電信號,這些電信號與產(chǎn)生它們的光信號不一樣,自然也是模擬信號。模擬信號可在任意時刻取值,并且具有無限多個電平,不適合計算機處理。所以在處理前,必須對它們進行采樣并將其轉(zhuǎn)換為數(shù)字形式。數(shù)字信號只在有限的時間點上取值,并且具有有限個電平,所以非常適合計算機處理。在大多數(shù)數(shù)字信號處理系統(tǒng)中數(shù)字信號和模擬信號二者都會出現(xiàn)。系統(tǒng)輸入端的模擬信號會被轉(zhuǎn)換為數(shù)字形式進行處理,處理后,數(shù)字形式的信號再被轉(zhuǎn)換為模擬信號進行輸出。靈活快速的DSP是在只存在數(shù)字信號的處理階段實現(xiàn)的。通過DSP系統(tǒng)可以對信號進行分析、合成、修正、記錄或播放。例如,通過圖像壓縮系統(tǒng)可以對數(shù)字圖像進行再編碼,以使圖像占用的內(nèi)存空間較少,并以較短的時間在網(wǎng)間傳輸。語音識別系統(tǒng)用來自動識別人的聲音。再如,數(shù)字濾波器允許某些信號頻率通過系統(tǒng),而其他信號頻率被濾除??傊瑪?shù)字信號的處理方式多種多樣。任何對數(shù)字可行的運算,同樣適用于數(shù)字信號,使用專用的DSP軟硬件,可進行快速運算。數(shù)字系統(tǒng)與模擬系統(tǒng)相比有許多優(yōu)點。模擬系統(tǒng)是由元器件搭建而成的電路,而元器件的特性在制造誤差允許的范圍內(nèi)差異很大,且特性還會隨溫度改變,從而改變了電路的性能。相比之下,數(shù)字系統(tǒng)的工作具有可預測性和可重復性。由于數(shù)字系統(tǒng)主要取決于軟件,所以它的性能幾乎不受以上因素的影響。因此,數(shù)字系統(tǒng)比模擬系統(tǒng)有更好的抗噪聲性能。此外,數(shù)字系統(tǒng)比模擬系統(tǒng)電路體積小、功耗低。不過,數(shù)字技術的最大優(yōu)勢是它的靈活性較大,可通過簡單地更改程序中的一些語句來修改設計,而對于大多數(shù)模擬系統(tǒng)而言,如果要重新設計,則往往需要重新搭建電路。人們身邊的聲音和圖像都是模擬信號。為了處理這些信號,就要用到相應的傳感器。各種信號的傳感器是不同的。麥克風是最普通的聲音傳感器;光的變化可通過半導體器件記錄,包括二極管、光電晶體管以及CCD(電荷耦合器件)芯片,它們的載流能力會隨著入射光而變化;溫度可通過阻值隨溫度變化的熱敏電阻、半導體器件測得,或利用熱電偶測量,熱電偶由兩種不同的金屬制成,兩種不同的金屬對溫度的反應不同,從而形成電壓;加速計用來測量加速度。其他傳感器還有應力傳感器、壓力傳感器和流量傳感器。這些傳感器的輸出通常是與被測信號成正比例的電信號(電壓或電流)。模擬電信號必須轉(zhuǎn)換成數(shù)字的形式。此信號可取其最小值和最大值之間的任意值,且在任意時刻均有值。模數(shù)轉(zhuǎn)換分兩步進行。第一步是采樣,采樣時刻通常處在固定的間隔點上,這個間隔稱為采樣周期。在每一個采樣點對模擬信號進行采樣,且將該采樣值保持到下一個采樣點的過程稱為采樣保持。采樣要足夠快,以捕捉被采樣信號中的最快變化;如果采樣太慢,將丟失重要的信號特征,出現(xiàn)混疊。注意,采樣也要占用有限的時間,稱之為采樣器的采集時間。模數(shù)轉(zhuǎn)換的第二步是對模擬值進行量化和數(shù)字化,這在采樣完成后即可進行。在采樣保持期間有足夠的時間完成這一步。每個采樣結(jié)束后,轉(zhuǎn)換器會盡快選擇與采樣保持電平最接近的量化電平,然后分配一個二進制數(shù)字代碼來標識這個量化電平,至此就完成了模數(shù)轉(zhuǎn)換過程。5.2數(shù)字信號及其頻譜

任何信號均可轉(zhuǎn)換為數(shù)字形式。在DSP中,除了基于時間的變化外,通常還需要知道信號的頻率。例如,眾所周知,女性的音調(diào)通常比男性的高,但對于自動語音識別系統(tǒng),語音中的準確頻率分量非常重要。頻譜是對信號中所含頻率分

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