Unit9DigitalSignalProcessingPassageABasicConceptsofDSPPassageBDigitalSignalProcessorsPassageCComparisonofDSPandASP

PassageABasicConceptsofDSP

Wedon’tspeakinadigitalsignal.Adigitalsignalisalanguageof1sand0sthatcanbeprocessedbymathematics.Wespeakinreal-world,analogsignals.Analogsignalsarerealworldsignalsthatweexperienceeveryday-sound,light,temperature,andpressure.Adigitalsignalisanumericalrepresentationoftheanalogsignal.Itmaybeeasierandmorecosteffectivetoprocessthesesignalsinthedigitalworld.Intherealworld,wecanconvertthesesignalsintodigitalsignalsthroughtheanalog-to-digitalconverter,processthesignals,andifneeded,bringthesignalsbackouttotheanalogworldthroughthe

digital-to-analogconverter.

1.Theessentialsofanalog-to-digitalanddigital-to-analogconversion

Thefirstessentialstepinanalog-to-digital(A/D)conversion(asshowninFigure9.1)istosampleananalogsignal.Thisstepisperformedbyasampleandholdcircuit,whichsamplesatregularintervalscalledsamplingintervals.Thelengthofthesamplingintervalisthesameasthesamplingperiod,andthereciprocalofthesamplingperiodisthesamplingfrequencyfs.AccordingtotheNyquisttheorem,asignalwithamaximumfrequencyofWHz(calledaband-limitedsignal)mustbesampledatleast2Wsamplespersecondtoensureaccuraterecording.

Whenthisminimumisnotrespected,distortioncalledaliasingoccurs.Aliasingcauseshighfrequencysignalstoappearaslowerfrequencysignals.Tobesurealiasingwillnotoccur,samplingisalwaysprecededbylowpassfiltering.Thelowpassfilter,calledtheanti-aliasingfilter,removesallfrequenciesabovehalftheselectedsamplingrate.

Figure9.1Analog-to-DigitalConversions

Afterabriefacquisitiontime,duringwhichasampleisacquired,thesampleandholdcircuitholdsthesamplesteadyfortheremainderofthesamplinginterval.ThisholdtimeisneededtoallowtimeforanA/Dconvertertogenerateadigitalcodethatbestcorrespondstotheanalogsample.

TheA/Dconverterchoosesaquantizationlevelforeachanalogsample.An

N-bitconverterchoosesamong2Npossiblequantizationlevels.Thelargerthenumberoflevels,thesmallerthequantizationerrors,calculatedasthedifferencebetweenthequantizedlevelandthetruesamplelevel.MostquantizationerrorsarelimitedinsizetohalfaquantizationstepQ.ThequantizationstepsizeiscalculatedasQ=R/2N,whereRisthefullscalerangeoftheanalogsignalandNisthenumberofbitsusedbytheconverter.Thestrengthofthesignalcomparedtothatofthequantizationerrorsismeasuredbydynamicrangeandsignal-to-noiseratio.

Adigitalsignalisrepresentedbyasetofverticallineswithcirclesatthetoptomarkthequantizationlevelsselectedforeachsample.ThebitrateforanA/DconverteristheNfs,wherefsisthesamplingrate.

Finally,eachdigitalsampleisassignedadigitalcode,whichcompletestheA/Dprocess.Theresultisadigitalbitstream.Itisthiscollectionofdigitalcodesthatis

processedindigitalsignalprocessing.

Tosummarize,A/Dcomprisesanti-aliasing,sampling,quantizationanddigitization.

Oncedigitalsignalprocessingiscomplete,digital-to-analog(D/A)conversion(asshowninFigure9.2)mustoccur.Thisprocessbeginsbyconvertingeachdigitalcodeintoananalogvoltagethatisproportionalinsizetothenumberrepresentedbythecode.Thisvoltageisheldsteadythroughzeroorderholduntilthenextcodeisavailable,one

samplingintervallater.Thiscreatesastaircase-likesignalthatcontainsfrequenciesabove

WHz.Thesesignalsareremovedwithasmoothinglowpassfilter,thelaststepinD/Aconversion.

Figure9.2Digital-to-AnalogConversions

Theimagesofeachfrequencyfpresentinasampledsignalappear,throughsampling,attheinfinitenumberoffrequencies

kfs±fHz.WhenthesamplingrateislowerthantherequiredNyquistrate,thatisfs<2W,imagesofhighfrequencysignalserroneouslyappearinthebaseband(orNyquistrange)duetoaliasing.Whilethisundersamplingisnormallyavoided,itcanbeexploited.Forexample,signalswhosefrequenciesare

restrictedtoanarrowbandofhighfrequenciescanbesampledataratesimilartotwicethewidthofthebandinsteadoftwicethemaximumfrequency.

Alloftheimportantsignalcharacteristicscanbededucedfromthecopyofthespectrumthatappearsinthebasebandthroughsampling.Dependingontherelationshipbetweenthesignalfrequenciesandthesamplingrate,spectralinversionmaycausetheshapeofthespectruminthebasebandtobeinvertedfromthetruespectrumofthesignal.［1］

2.Technologiesfordigitalsignalprocessing

Ifauniversalmicroprocessorsolutionexistedwithwhicheverydesigncouldberealized,theelectronicsindustrywouldn’tbeaverycompetitiveplace.［2］

However,typicallyinmostelectronicdesigns,morethanoneprocessortechnologycanbeusedtoimplementtherequiredfunctions.Thetrickis,ofcourse,tochoosetheonethatbestdeliverstheperformance,size,powerconsumption,features,softwareandtoolstogetthejobdonefast-withoutbreakingthebudget.Afteralmosttwodecadesofdevelopment,digitalsignalprocessorscontinuetotaketheplaceofcompetitiveprocessors.Digitalsignalprocessorsare,afterall,atthecenterofsignalprocessing.

Adigitalsignalprocessor(DSP)isatypeofmicroprocessor-onethatisincrediblyfastandpowerful.ADSPisuniquebecauseitprocessesdatainrealtime.Thisreal-timecapabilitymakesaDSPperfectforapplicationsthatcannottolerateanydelays.Forexample,didyouevertalkonacellphonewheretwopeoplecouldn’ttalkatonce?Youhadtowaituntiltheotherpersonfinishedtalking.Ifyoubothspokesimultaneously,thesignalwascut-youdidn’theartheotherperson.Withtoday’sdigitalcellphones,whichuseDSP,youcantalknormally.TheDSPinsidecellphonesprocesssoundssorapidlyyouhearthemasquicklyasyoucanspeak-inrealtime.HerearejustsomeoftheadvantagesofdesigningwithDSPsoverothermicroprocessors:

·Single-cyclemultiply-accumulateoperations;

·Real-timeperformancesimulationandemulation;

·Flexibility;

·Reliability;

·Increasedsystemperformance;

·Reducedsystemcost.

However,therearesomeoftheotheralternativesavailablefordigitalsignalprocessing.HowtheycomparetoDSPs?

◆TheFPGAAlternative

Field-ProgrammableGateArrays(FPGA)havethecapabilityofbeingreconfigurablewithinasystem,whichcanbeabigadvantageinapplicationsthatneedmultipletrialversionswithindevelopment,offeringreasonablyfasttimetomarket.Theyalsooffergreaterrawperformanceperspecificoperationbecauseoftheresultingdedicatedlogiccircuit.However,FPGAsaresignificantlymoreexpensiveandtypicallyhavemuchhigherpowerdissipationthanDSPswithsimilarfunctionality.

Assuch,evenwhenFPGAsarethechosenperformancetechnologyindesignssuchaswirelessinfrastructure,DSPsaretypicallyusedinconjunctionwithFPGAstoprovidegreaterflexibility,betterprice/performanceratios,andlowersystempower.

◆TheASICAlternative

Application-specificICs(ASIC)canbetailoredtoperformspecificfunctionsextremely

well,andcanbemadequitepowerefficient.However,sinceASICSarenotfield-programmable,theirfunctionalitycannotbeiterativelychangedorupdatedwhileinproductdevelopment.Assuch,everynewversionoftheproductrequiresaredesignandtripsthroughthefoundry,anexpensiveproposition,andanimpedimenttorapidtime-to-market.［3］

ProgrammableDSPs,ontheotherhand,canbeupdatedwithoutchangingthesilicon,merelychangethesoftwareprogram,greatlyreducingdevelopmentcosts,and

availingaftermarketfeatureenhancementswithmerecodedownloads.Consequently,more

oftenthannot,whenyouseeASICsinrealtimesignalprocessingapplications,theyaretypicallyemployedasbusinterfaces,gluelogic,and/orfunctionalacceleratorsforaprogrammableDSP-basedsystem.

◆TheGPPAlternative

IncontrasttoASICsthatareoptimizedforspecificfunctions,general-purposemicroprocessors(GPPs)arebestsuitedforperformingabroadarrayoftasks.However,forapplicationsinwhichtheendproductmustprocessanswersinrealtime,ormustdosowhilepoweredbyconsumerbatteries,GPPscomparativelypoorrealtimeperformanceandhighpowerconsumptionallbutrulesthemout.[4]Moreandmore,theseprocessorsarebeingseenasthedinosaursoftheindustry,tooencumberedwithPCcompatibilityanddesktopfeaturestoadapttothechangingrealtimemarketplace.

Astheworldembracestinyhand-heldwireless-enabledproductsthatrequirepowerdissipationmeasuredinmilliwatts-notthewattsthattheseprocessorsconsume-DSPsaretheprogrammabletechnologyofchoice.ThattrendisboundtocontinueasdigitalInternetappliancesgetsmaller,fasterandmoreportable.

Notes

［1］Dependingontherelationshipbetweenthesignalfrequenciesandthesamplingrate,spectralinversionmaycausetheshapeofthespectruminthebasebandtobeinvertedfromthetruespectrumofthesignal.

根據(jù)信號頻率和采樣頻率之間的關(guān)系的不同,可能出現(xiàn)“頻譜反轉(zhuǎn)”現(xiàn)象——基帶頻譜的形狀和信號真實頻譜的形狀正好相反。

·beinvertedfrom...和……相反。

·所謂基帶是指不加任何調(diào)制過程的數(shù)字信號傳輸?；鶐捜坑脕韨魉蛿?shù)字脈沖信號,并可以通過時分復用的方式進行多路數(shù)字信號的傳輸。

［2］Ifauniversalmicroprocessorsolutionexistedwithwhicheverydesigncouldrealized,theelectronicsindustrywouldn’tbeaverycompetitiveplace.

假如存在可用以實現(xiàn)任何設計的通用微處理器的話,電子行業(yè)就不會競爭得如此激烈了。

·本句為虛擬語氣。

［3］Assuch,everynewversionoftheproductrequiresaredesignandtripsthroughthefoundry,anexpensiveproposition,andanimpedimenttorapidtime-to-market.

而且,每次推出一個新產(chǎn)品都需要重新設計并經(jīng)歷所有制造流程。這樣做不但造價昂貴,而且不利于迅速上市。

·time-to-market的意思為“上市時間”。

·foundry是指為第三方制造芯片的半導體制造商,可以指那些出售其富余生產(chǎn)能力的大型芯片制造企業(yè),也可以是指專為其他公司制造芯片的企業(yè)。

［4］However,forapplicationsinwhichtheendproductmustprocessanswersinrealtime,ormustdosowhilepoweredbyconsumerbatteries,GPPscomparativelypoorrealtimeperformanceandhighpowerconsumptionallbutrulesthemout.

然而,在最終產(chǎn)品必須實時響應的應用中,或者必須在電池供電下實現(xiàn)實時響應的應用中,由于GPP實時性能較差、功耗大，因此就被排除在外了。

·allbut此處作rulethemout的狀語,涵義為“幾乎,差一點”。

·ruleout的意思是“將……排除在外”。

Exercises

1.Fillintheblanks.

(1)Thefirstessentialstepin

conversionistosampleananalogsignal.

(2)Thelengthofthe

isthesameasthe

,andthereciprocalofthesamplingperiodisthesamplingfrequencyfs.

(3)Tosummarize,A/Dcomprises

,

,

and

.

(4)A

isatypeofmicroprocessor-onethatisincrediblyfastandpowerful.

(5)IncontrasttoASICsthatareoptimizedforspecificfunctions,

arebestsuitedforperformingabroadarrayoftasks.

(6)

havethecapabilityofbeingreconfigurablewithinasystem,whichcanbeabigadvantageinapplicationsthatneedmultipletrialversionswithindevelopment,offeringreasonablyfasttimetomarket.

2.TranslatethefollowingparagraphintoChinese.

Thefirststepinconvertingananalogsignalissampling.Thisstepisaccomplished

usingasampleandholdcircuit,whichgrabsasamplefromthesignalandholdsitsteadyuntilthenextsamplingtime.Samplesarenormallycollectedatregulartimeintervals,calledsamplingperiods.Iftherateofthesamplingisnotadequate,distortioncalledaliasingcanoccur.TheoutputfromthesampleandholdcircuitispassedtoanA/Dconverter,whichchoosesaquantizationlevelthatisclosesttotheactualamplitudeofthesignal.

ThisisthesecondstepinA/Dconversion.Thenumberoflevelstochoosefromdependsonthenumberofbitsusedbytheconverter:forNbits,2Nlevelsarepossible.Sinceanalog

amplitudescannotingeneralberepresentedperfectlyinadigitalsystem,quantizationerrorsoccur.

3.TranslatethefollowingsentencesintoEnglish.

(1)數(shù)字信號處理是無數(shù)家用和商用系統(tǒng)中的關(guān)鍵部分,其應用領(lǐng)域與日俱增。因此,DSP正在成為技術(shù)專家和工程師專業(yè)知識的一個重要組成部分。

(2)數(shù)字信號處理可分為定點和浮點兩大類。定點和浮點指的是器件內(nèi)用于存儲和操作數(shù)據(jù)的格式。定點數(shù)字信號處理通常用最少16位來表示每個數(shù)據(jù),典型的浮點數(shù)字信號處理器用最少32位來存儲每個數(shù)據(jù)。

(3)和其他科學和工程應用一樣,數(shù)字信號處理器通常使用匯編語言或者C語言進行編程。然而,DSP程序和傳統(tǒng)軟件任務不同。首先,程序通常短得多,如100行而不是1萬行。其次,執(zhí)行速度往往是其應用的關(guān)鍵部分。

PassageBDigitalSignalProcessors

Digitalsignalprocessingtaskscanbeperformedbyallprocessors.Specializeddigitalsignalprocessors(DSPs),however,performthesetasksmostefficientlyandmostquickly.WhiletraditionalprocessorsfollowtheVonNeumannarchitecturemodel(asshowninFigure9.3),whichassumesasharedsinglememorytobeusedforbothprograminstructionsanddata,DSPsusetheHarvardormodifiedHarvardarchitecture(asshownin

Figure9.4),whichincludesmultipleprogramanddatamemories,

alongwithmultiplebusestoaccessthem.[1]Thisarrangementmeansthatmuchlesswaitingisrequiredwhen

instructionsor

numbersarefetchedfrommemory.Infactatleastoneofeachcanbefetchedsimultaneously.Suchoverlappingoftasksiscalledpipelining.Inadditiontomultiplememoriesandbuses,allDSPshavefastmultipliers,accumulators,andshifters,andmanyhavehardwaresupportforcircularbuffers.Addressgeneratorscanspeedupaccessestomemorylocationsreferencedbyregisters.

Figure9.3VonNeumannArchitectureFigure9.4HarvardArchitecture

DSPsareavailableintwomajorclasses:fixedpointandfloatingpoint.Thefixedpointclassrepresentsrealnumbersinafixednumberofbits.Thepositionofthebinarypoint(similartothedecimalpoint)canbecontrolledbytheprogrammer,anddeterminestherangeofnumbersthatcanberepresented.Astherangeincreases,though,theavailableprecisiongoesdown,sincefewerbitslietotherightofthebinarypoint.In16bits,theformats16.0,15.1,14.2,13.3,12.4,11.5,10.6,9.7,8.8,7.9,6.10,5.11,

4.12,3.13,2.14,and1.15arepossible.Thedynamicrange,calculatedas20log(FullScaleRange/SmallestResolvableDifference),remainsthesameforall16-bitformats,

20log216=96.3dB.

FloatingpointDSPsrepresentrealnumbersusingamantissaandanexponent,similartoscientificnotation:Manycombinemantissaandexponentintoa32-bitnumber.The

dynamicrangeforfloatingpointdevicesiscalculatedfromthelargestandsmallestmultipliers2E,whereEistheexponent.Thus,forarepresentationthatuses24bitsforthemantissaand8bitsforthesignedexponent,thedynamicrangeis20lg(2127/2-128)=1535.3dB.Alargedynamicrangemeansthesystemhasgreatpowertorepresentawiderangeofinput

signals,fromverysmalltoverylarge.

AssemblylanguageisthecommandlanguageforDSPs.DSPsoftenhavespecializedinstructionsthatmakeprogrammingforcommonDSPtasksmoreconvenientandmore

efficient.Forexample,mostDSPsoffermulti-functioninstructionsthatexploittheirparallelarchitecture.Otherconstructsthatarefrequentlyofferedareefficientloopingschemes,sincesomanyDSPoperationsinvolveagreatdealofrepetition.

ChoosingaDSPforaparticularapplicationisnotalwayseasy.Thefirstdecisionisonwhethertochooseafixedpointorafloatingpointdevice.［2］

Generally,fixedpointdevicesarecheaperandquicker,butfloatingpointdevicesaremoreconvenienttoprogramandmoresuitedtocalculation-intensivealgorithms.Second,thedatawidthoftheDSPdetermineshowaccuratelyitcanrepresentnumbers.Speedisanotherissue,notonlyhowmanycyclesoccurineachsecond,butalsohowmanyinstructionsexecuteineachcycleandhowmuchworkeachoftheseinstructionsaccomplishes.

OnewaytoassesstheminimumrequirementsfortheDSPistoestimatehowmanyinstructionsmustbeexecutedforeachreceivedsample.Whenthisnumberismultipliedbythesamplingfrequency,theminimumrequirednumberofinstructionspersecondisobtained.

ThespecifichardwareandsoftwarefeaturesofferedbyaparticularDSPcanmakeonechoicebetterthananother,ascantheamountofon-chipmemoryavailable.SometimesDSPsarechosenbecausewell-matchedsupportinghardware,particularlyA/DandD/Aconverters,isobtainable.Frequently,thequalityandconvenienceofthesoftwaretools,forbothlowlevelandhighlevelprogramminglanguages,arealsomajorfactors,asistheavailabilityofthirdpartysoftware.Asalways,costisafactor.Infact,quiteoften,theDSPthatisfastestandoffersthemostfeatures,butalsofitsthebudget,istheoneselected.

DSPscanbepurchasedinthreeforms,asacore,asaprocessor,andasaboardlevelproduct.InDSP,theterm“core”referstothesectionoftheprocessorwherethekeytasksarecarriedout,includingthedataregisters,multiplier,ALU,addressgenerator,andprogramsequencer.Acompleteprocessorrequirescombiningthecorewithmemoryandinterfacestotheoutsideworld.Whilethecoreandtheseperipheralsectionsare

designedseparately,theywillbefabricatedonthesamepieceofsilicon,makingthe

processorasingleintegratedcircuit.

SupposeyoubuildcellulartelephonesandwanttoincludeaDSPinthedesign.YouwillprobablywanttopurchasetheDSPasaprocessor,thatis,anintegratedcircuitthatcontainsthecore,memoryandotherinternalfeatures.ToincorporatethisICinyourproduct,youhavetodesignaprintedcircuitboardwhereitwillbesolderedinnexttoyourotherelectronics.ThisisthemostcommonwaythatDSPsareused.

Now,supposethecompanyyouworkformanufacturesitsownintegratedcircuits.Inthiscase,youmightnotwanttheentireprocessor,justthedesignofthecore.Aftercompletingtheappropriatelicensingagreement,youcanstartmakingchipsthatarehighlycustomizedtoyourparticularapplication.Thisgivesyoutheflexibilityofselectinghowmuchmemoryisincluded,howthechipreceivesandtransmitsdata,howitispackaged,andsoon.CustomdevicesofthistypeareanincreasinglyimportantsegmentoftheDSPmarketplace.

ThereareseveraldozencompaniesthatwillsellyouDSPsalreadymountedonaprintedcircuitboard.ThesehavesuchfeaturesasyourDSPsalreadymountedonaextramemory,A/DandD/Aconverters,EPROMsockets,multipleprocessorsonthesameboard,andsoon.Whilesomeoftheseboardsareintendedtobeusedasstandalonecomputers,mostareconfiguredtobepluggedintoahost,suchasapersonalcomputer.CompaniesthatmakethesetypesofboardsarecalledThirdPartyDevelopers.ThebestwaytofindthemistoaskthemanufactureroftheDSPyouwanttouse.LookattheDSPmanufacturer’swebsite;ifyoudon’tfindalistthere,sendthemane-mail.Theywillbemorethanhappytotellyouwhoareusingtheirproductsandhowtocontactthem.

KeepinmindthatthedistinctionbetweenDSPsandothermicroprocessorsisnot

alwaysaclearline.Forinstance,lookathowInteldescribestheMMXtechnologyadditiontoitsPentiumprocessor:“Intelengineershaveadded57powerfulnewinstructionsspecificallydesignedtomanipulateandprocessvideo,audioandgraphicaldataefficiently.Theseinstructionsareorientedtothehighlyparallel,repetitivesequencesoftenfoundinmultimediaoperations.”

Inthefuture,wewillundoubtedlyseemoreDSP-likefunctionsmergedintotraditionalmicroprocessorsandmicrocontrollers.TheInternetandothermultimediaapplicationsareastrongdrivingforceforthesechanges.Theseapplicationsareexpandingsorapidly,intwentyyearsitisverypossiblethatthedigitalsignalprocessormaybethe“traditional”microprocessor.

Notes

［1］WhiletraditionalprocessorsfollowtheVonNeumannarchitecturemodel,whichassumesasharedsinglememorytobeusedforbothprograminstructionsanddata,DSPsusetheHarvardormodifiedHarvardarchitecture,whichincludesmultipleprogramanddatamemories,alongwithmultiplebusestoaccessthem.

傳統(tǒng)的處理器遵循馮·諾依曼模型,該模型采用一個單一的共享存儲器,同時存儲程序指令和數(shù)據(jù);而數(shù)字信號處理器使用的是哈佛結(jié)構(gòu)或改進的哈佛結(jié)構(gòu),該結(jié)構(gòu)包含多個程序和數(shù)據(jù)存儲器以及訪問這些存儲器的多套總線。

·“馮·諾依曼結(jié)構(gòu)”取名自美國杰出的數(shù)學家約翰·馮·諾依曼(JohnVon

Neumann,1903-1957)。他引導了20世紀初許多重大數(shù)學發(fā)現(xiàn)。他的主要成就包括:提出了存儲程序計算機(storedprogramcomputer)的概念、對量子力學的數(shù)學公式化及在原子彈方面的工作。

·“哈佛結(jié)構(gòu)”取名自20世紀40年代HowardAiken(1900-1973)領(lǐng)導的在哈佛大學(HarvardUniversity)做的研究工作。

［2］Thefirstdecisionisonwhethertochooseafixedpointorafloatingpointdevice.

首先要決定的是選擇定點器件還是浮點器件。

·定點和浮點指的是器件內(nèi)用于存儲和操作數(shù)據(jù)的格式。通常，定點數(shù)字信號處理最少用16位來表示每個數(shù)據(jù),典型的浮點數(shù)字信號處理器最少用32位來存儲每個數(shù)據(jù)。

Exercises

1.Fillintheblanks.

(1)Thefirstdecisionisonwhethertochoosea

pointora

pointdevice.

(2)ThespecifichardwareandsoftwarefeaturesofferedbyaparticularDSPcanmakeonechoicebetterthananother,ascantheamountof

available.

(3)SometimesDSPsarechosenbecause

supportinghardware,particularly

A/DandD/Aconverters,isobtainable.

(4)DSPscanbepurchasedinthreeforms,

,

,

and

.

(5)Companiesthatmakethesetypesofboardsare

called

.

(6)Acompleteprocessorrequirescombiningthecorewith

and

totheoutsideworld.

2.TranslatethefollowingparagraphsintoChinese.

(1)Theprimaryreasonforprocessingreal-worldsignalsistoextractinformationfromthem.Thisinformationnormallyexistsintheformofsignalamplitude(absoluteorrelative),frequencyorspectralcontent,phase,ortimingrelationshipswithrespecttoothersignals.Oncethedesiredinformationisextractedfromthesignal,itmaybeusedinanumberofways.

(2)Anotherrequirementforsignalprocessingistocompressthefrequencycontentofthesignal(withoutlosingsignificantinformation)thenformatandtransmittheinformationatlowerdatarates,therebyachievingareductioninrequiredchannelbandwidth.Highspeedmodemsandadaptivepulsecodemodulationsystems(ADPCM)makeextensiveuseofdatareductionalgorithms,asdodigitalmobileradiosystems,MPEGrecordingandplayback,andHighDefinitionTelevision(HDTV).

3.TranslatethefollowingparagraphintoEnglish.

信號(如聲音、光或者電壓)是攜帶信息的變化。模擬信號是現(xiàn)實世界中的信號。它們在每個時間點都有定義,其幅度的可能取值是無限的。模擬信號不太適合計算機處理。通過取樣和量化可以將其轉(zhuǎn)換成數(shù)字信號。另一方面,數(shù)字信號僅在抽樣點有定義,而且其幅度僅能取有限的離散值。處理之后,數(shù)字信號被轉(zhuǎn)換成模擬信號。

PassageCComparisonofDSPandASP

Signalsmaybeprocessedusinganalogtechniques(analogsignalprocessing,orASP),digitaltechniques(digitalsignalprocessing,orDSP),oracombinationofanaloganddigitaltechniques(mixedsignalprocessing,orMSP).Insomecases,thechoiceoftechniquesisclear;inothers,thereisnoclearcutchoice,andsecond-orderconsiderationsmaybeusedtomakethefinaldecision.

WithrespecttoDSP,thefactorthatdistinguishesitfromtraditionalcomputeranalysisofdataisitsspeedandefficiencyinperformingsophisticateddigitalprocessingfunctionssuchasfiltering,FFTanalysis,anddatacompressioninrealtime.

Thetermmixedsignalprocessingimpliesthatbothanaloganddigitalprocessingisdoneaspartofthesystem.Thesystemmaybeimplementedintheformofaprintedcircuitboardorasingleintegratedcircuitchip.Inthecontextofthisbroaddefinition,ADCsandDACsareconsideredtobemixedsignalprocessors,sincebothanaloganddigitalfunctionsareimplementedineach.RecentadvancesinVeryLargeScaleIntegration(VLSI)processingtechnologyallowcomplexdigitalprocessingaswellasanalogprocessingtobeperformedonthesamechip.TheverynatureofDSPitselfimpliesthatthesefunctionscanbeperformedinreal-time.

ASPvs.DSPToday’sengineerfacesachallengeinselectingthepropermixofanaloganddigitaltechniquestosolvethesignalprocessingtaskathand.Itisimpossibletoprocessreal-worldanalogsignalsusingpurelydigitaltechniques,sinceallsensors(microphones,thermocouples,straingages,piezoelectriccrystals,diskdriveheads,etc.)areanalogsensors.Therefore,somesortofsignalconditioningcircuitryisrequiredinordertopreparethesensoroutputforfurthersignalprocessing,whetheritbeanalogordigital.Signalconditioningcircuitsare,inreality,analogsignalprocessors,performingsuchfunctionsasmultiplication(gain),isolation(instrumentationamplifiersandisolationamplifiers),

detectioninthepresenceofnoise(highcommon-modeinstrumentationamplifiersetc.),dynamicrangecompression(logamps,LOGDACs,andprogrammablegainamplifiers),andfiltering(bothpassiveandactive).SeveralmethodsofaccomplishingsignalprocessingareshowninFigure9.5.Thetopportionofthefigureshowsthepurelyanalogapproach.ThelatterpartsofthefigureshowtheDSPapproach.NotethatoncethedecisionhasbeenmadetouseDSPtechniques,thenextdecisionmustbewheretoplacetheADCinthesignalpath.

Figure9.5SomeMethodsofAccomplishingSignalProcessing

Ingeneral,astheADCismovedclosertotheactualsensor,moreoftheanalogsignalconditioningburdenisnowplacedontheADC.TheaddedADCcomplexitymaytaketheformofincreasedsamplingrate,widerdynamicrange,higherresolution,inputnoise

rejection,inputfilteringandprogrammablegainamplifiers(PGAs),on-chipvoltagereferences,etc.,allofwhichaddfunctionalityandsimplifythesystem.Withtoday’shigh-resolution/highsamplingratedataconvertertechnology,significantprogresshasbeenmadeinintegratingmoreandmoreoftheconditioningcircuitrywithintheADC/DACitself.

Inthemeasurementarea,forinstance,24-bitADCsareavailablewithbuilt-inprogrammablegainamplifiers(PGAs)whichallowfull-scalebridgesignalsof10mVtobedigitized

directlywithnofurtherconditioning.Atvoicebandandaudiofrequencies,completecoder-decoders(CodecsorAnal