《电子信息类专业英语 》课件-第9章

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.

根据信号频率和采样频率之间的关系的不同,可能出现“频谱反转”现象——基带频谱的形状和信号真实频谱的形状正好相反。

·beinvertedfrom...和……相反。

·所谓基带是指不加任何调制过程的数字信号传输。基带带宽全部用来传送数字脉冲信号,并可以通过时分复用的方式进行多路数字信号的传输。

［2］Ifauniversalmicroprocessorsolutionexistedwithwhicheverydesigncouldrealized,theelectronicsindustrywouldn’tbeaverycompetitiveplace.

假如存在可用以实现任何设计的通用微处理器的话,电子行业就不会竞争得如此激烈了。

·本句为虚拟语气。

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

而且,每次推出一个新产品都需要重新设计并经历所有制造流程。这样做不但造价昂贵,而且不利于迅速上市。

·time-to-market的意思为“上市时间”。

·foundry是指为第三方制造芯片的半导体制造商,可以指那些出售其富余生产能力的大型芯片制造企业,也可以是指专为其他公司制造芯片的企业。

［4］However,forapplicationsinwhichtheendproductmustprocessanswersinrealtime,ormustdosowhilepoweredbyconsumerbatteries,GPPscomparativelypoorrealtimeperformanceandhighpowerconsumptionallbutrulesthemout.

然而,在最终产品必须实时响应的应用中,或者必须在电池供电下实现实时响应的应用中,由于GPP实时性能较差、功耗大，因此就被排除在外了。

·allbut此处作rulethemout的状语,涵义为“几乎,差一点”。

·ruleout的意思是“将……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)数字信号处理是无数家用和商用系统中的关键部分,其应用领域与日俱增。因此,DSP正在成为技术专家和工程师专业知识的一个重要组成部分。

(2)数字信号处理可分为定点和浮点两大类。定点和浮点指的是器件内用于存储和操作数据的格式。定点数字信号处理通常用最少16位来表示每个数据,典型的浮点数字信号处理器用最少32位来存储每个数据。

(3)和其他科学和工程应用一样,数字信号处理器通常使用汇编语言或者C语言进行编程。然而,DSP程序和传统软件任务不同。首先,程序通常短得多,如100行而不是1万行。其次,执行速度往往是其应用的关键部分。

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.

传统的处理器遵循冯·诺依曼模型,该模型采用一个单一的共享存储器,同时存储程序指令和数据;而数字信号处理器使用的是哈佛结构或改进的哈佛结构,该结构包含多个程序和数据存储器以及访问这些存储器的多套总线。

·“冯·诺依曼结构”取名自美国杰出的数学家约翰·冯·诺依曼(JohnVon

Neumann,1903-1957)。他引导了20世纪初许多重大数学发现。他的主要成就包括:提出了存储程序计算机(storedprogramcomputer)的概念、对量子力学的数学公式化及在原子弹方面的工作。

·“哈佛结构”取名自20世纪40年代HowardAiken(1900-1973)领导的在哈佛大学(HarvardUniversity)做的研究工作。

［2］Thefirstdecisionisonwhethertochooseafixedpointorafloatingpointdevice.

首先要决定的是选择定点器件还是浮点器件。

·定点和浮点指的是器件内用于存储和操作数据的格式。通常，定点数字信号处理最少用16位来表示每个数据,典型的浮点数字信号处理器最少用32位来存储每个数据。

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.

信号(如声音、光或者电压)是携带信息的变化。模拟信号是现实世界中的信号。它们在每个时间点都有定义,其幅度的可能取值是无限的。模拟信号不太适合计算机处理。通过取样和量化可以将其转换成数字信号。另一方面,数字信号仅在抽样点有定义,而且其幅度仅能取有限的离散值。处理之后,数字信号被转换成模拟信号。

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