DSP（数值信号处理）芯片应用(1)

DSPAPPLICATIONS9.aSECTION9DSPAPPLICATIONS
HighPerformanceModemsforPlainOldTelephoneService(POTS)
RemoteAccessServer(RAS)Modems
ADSL(AssymetricDigitalSubscriberLine)
DigitalCellularTelephones
GSMHandsetUsingSoftFone™BasebandProcessorandOthello™Radio
AnalogCellularBasestations
DigitalCellularBasestations
MotorControl
CodecsandDSPsinVoicebandandAudioApplications
ASigma-DeltaADCwithProgrammableDigitalFilterDSPAPPLICATIONS9.bDSPAPPLICATIONS9.1SECTION9DSPAPPLICATIONSWaltKesterHIGHPERFORMANCEMODEMSFORPLAINOLDTELEPHONESERVICE(POTS)Modems(Modulator/Demodulator)arewidelyusedtotransmitandreceivedigitaldatausinganalogmodulationoverthePlainOldTelephoneService(POTS)networkaswellasprivatelines.Althoughthedatatobetransmittedisdigital,thetelephonechannelisdesignedtocarryvoicesignalshavingabandwidthofapproximately300to3300Hz.Thetelephonetransmissionchannelsuffersfromdelaydistortion,noise,crosstalk,impedancemismatches,near-endandfar-endechoes,andotherimperfections.Whilecertainlevelsofthesesignaldegradationsareperfectlyacceptableforvoicecommunication,theycancausehigherrorratesindigitaldatatransmission.Thefundamentalpurposeofthetransmitterportionofthemodemistopreparethedigitaldatafortransmissionovertheanalogvoiceline.Thepurposeofthereceiverportionofthemodemistoreceivethesignalwhichcontainstheanalogrepresentationofthedata,andreconstructtheoriginaldigitaldataatanacceptableerrorrate.Highperformancemodemsmakeuseofdigitaltechniquestoperformsuchfunctionsasmodulation,demodulation,errordetectionandcorrection,equalization,andechocancellation.Ablockdiagramofanordinarytelephonechannel(oftenreferredtoasplainoldtelephoneservice–orPOTS)isshowninFigure9.1.Mostvoicebandtelephoneconnectionsinvolveseveralconnectionsthroughthetelephonenetwork.The2-wiretwistedpairsubscriberlineavailableatmostsitesisgenerallyconvertedtoa4-wiresignalatthetelephonecentraloffice:twowiresfortransmit,andtwowiresforreceive.Thesignalisconvertedbacktoa2-wiresignalatthefar-endsubscriberline.The2-to4-wireinterfaceisimplementedwithacircuitcalledahybrid.Thehybridintentionallyinsertsimpedancemismatchestopreventoscillationsonthe4-wiretrunkline.Themismatchforcesaportionofthetransmittedsignaltobereflectedorechoedbacktothetransmitter.Thisechocancorruptdatathetransmitterreceivesfromthefar-endmodem.Half-duplexmodemsarecapableofpassingsignalsineitherdirectionona2-wireline,butnotsimultaneously.Full-duplexmodemsoperateona2-wirelineandcantransmitandreceivedatasimultaneously.Full-duplexoperationrequirestheabilitytoseparateareceivesignalfromthereflection(echo)ofthetransmittedsignal.Thisisaccomplishedbyassigningthesignalsinthetwodirectionsdifferentfrequencybandsseparatedbyfiltering,orbyechocancellinginwhichalocallysynthesizedreplicaofthereflectedtransmittedsignalissubtractedfromthecompositereceivesignal.DSPAPPLICATIONS9.2Figure9.1Therearetwotypesofechoinatypicalvoicebandtelephoneconnection.Thefirstechoisthereflectionfromthenear-endhybrid,andthesecondechoisfromthefar-endhybrid.Inlongdistancetelephonetransmissions,thetransmittedsignalishetrodynedtoandfromacarrierfrequency.Sincelocaloscillatorsinthenetworkarenotexactlymatched,thecarrierfrequencyofthefar-endechomaybeoffsetfromthefrequencyofthetransmittedcarriersignal.Inmodernapplicationsthisshiftcanaffectthedegreetowhichtheechosignalcanbecanceled.Itisthereforedesirablefortheechocancellertocompensateforthisfrequencyoffset.Fortransmissionoverthetelephonevoicenetwork,thedigitalsignalismodulatedontoanaudiosinewavecarrier,producingamodulatedtonesignal.Thefrequencyofthecarrierischosentobewellwithinthetelephoneband.Thetransmittingmodemmodulatestheaudiocarrierwiththetransmitdatasignal,andthereceivingmodemdemodulatesthetonetorecoverthereceivedatasignal.Thebasebanddatasignalmaybeusedtomodulatetheamplitude,thefrequency,orthephaseoftheaudiocarrier,dependingonthedataraterequired.Thesethreetypesofmodulationareknownasamplitudeshiftkeying(ASK),frequencyshiftkeying(FSK),andphaseshiftkeying(PSK).Initssimplestformthemodulatedcarriertakesononeoftwostates-thatis,oneoftwoamplitudes,oneoftwofrequencies,oroneoftwophases.Thetwostatesrepresentalogic0oralogic1.Low-tomedium-speeddatalinksusuallyuseFSKupto1,200bits/s.MultiphasePSKareusedfor2,400bits/sand4,800bits/slinks.PSKutilizesbandwidthmoreefficientlythanFSKbutismorecostlytoimplement.ASKisleastefficientandisANALOGMODEMUSINGPLAINOLDTELEPHONESERVICE(POTS)ANALOGCHANNELHYBRIDHYBRID+×TRANSMITCHANNELRECEIVECHANNEL×+TRANSMITTERRECEIVERRECEIVERTRANSMITTERNOISEFREQUENCYSHIFTNOISEFREQUENCYSHIFTFAR-ENDECHONEAR-ENDECHOFAR-ENDMODEMNEAR-ENDMODEMFOUR-WIRETRUNK222222DSPAPPLICATIONS9.3usedonlyforverylowspeedlinks(lessthan100bits/s).For9,600bits/supto33,600bits/sacombinationofPSKandASKisused,knownasQuadratureAmplitudeModulation(QAM).TheInternationalTelegraphandTelephoneConsultativeCommittee(CCITTinFrance)hasestablishedstandardsandrecommendationsformodemswhicharegiveninFigure9.2.Figure9.2ThegoalindesigninghighperformancemodemsistoachievethehighestdatatransferratepossibleoverthePOTSnetworkandavoidtheexpenseofusingdedicatedconditionedpriva