基于MATLAB语音信号的A律13折线PCM编解码的设计与分析

本文格式为Word版，下载可任意编辑——基于MATLAB语音信号的A律13折线PCM编解码的设计与分析IntegratedPCMCodec

KULB.OHRIANDMICHAELJ.CALLAHAN,JR.,MEMBER,lEEE

Abstract–Aone-chipPCMcodeccircuithasbeenimplementedintheCMOSprocess.Thedesignusestwoseparatelineardigital-to-analogconverters,madewithchargeredistributiontechniques.

Experimentalresultsshowtbecircuittomeetacceptedrequirementsandoperatewithverylowpowerrequirements.SINGLE-CHIPCMOSPCMCODEC

THEprocessingofvoicesignalsinadigitalmannerisbecomingmorepopular.Thereareseveralcontendingmodulationschemesincludingpulse-widthmodulation

(PWM),pulse-amplitudemodulation(PAM),deltamodulation,andpulse-codemodulation(PCM).

AspecificPCMapproach[1]hasbecomethestandardinNorthAmerica.Therearetwoseparatetypesofapplicationsforthistime-divisionmultiplexscheme:transmissionanddigitalvoiceswitching.Transmissioninvolvessendingthevoicedataindigitalformfromonelocationtoanother.Sincethetransmitandreceivecircuitryareseparatedfromoneanother,somesortofsynchronizationisimpliedinthis

scheme.Applicationsofthistypeareconcentratorsandchannelbanks.

Switchinginvolvesonlytheconnectionofonevoicechanneltoanotherinadigitalmamner.Thisschemehasthetransmitandreceivecircuitryincloseproximitysothatclockingandsynchronizationmaybedonebyonecommoncircuit.ImportantapplicationsofthistypeareelectronicPABXanddigitalcentraloffices.

Inthesystemdefinedby[1],theanaloginputsignalissampledatan8kHzrate.Fig.1showsa1kHzinputsampledinthismanner.Ateachofthesesamplingtimes,theanaloginformationisconvertedintoan8-bitdigitalwordthatistransmittedinserialformatata1.544Mbit/srate.

Fig.2showstwenty-fourvoicechannelswhicharetimedivisionmultiplexedontoonepairofwires.(Forsimplicity,

onlysimplexoperationisshown.)Eachchannelisfirstbandlimitedtolessthan4kHzbytheXMITtilter,thensampled

andconvertedtoacompandeddigitalcode.The8-bitwordsaretransmittedseriallytoamultiplexer.Thebitstreamof1.544Mbits/sissenttoademultiplexerwheretheappropriateA~(lZ=l,2,“““24)channelisconnectedtotheBM(rrz=1,2,〞〞〞24)channel.Thisselectionisdonebythemaincomputer,

sometimesknownascommoncontrol.

Fortransmissionapplications,thedigitalPCMdatamightbetransmittedbetweentwocentraloffices.Forswitchingapplications,suchasforaPBX,thistechniqueallowsforconnectionoftwovoicepathsdigitally.Digitalswitchingobviatestheneedforlowon-resistanceanalogswitches.

ManuscriptreceivedJune5,1978;revisedSeptember11,1978.TheauthorsarewithMOSTEKCorporation,Carrollton,TX75006.

Theprocessselectedforthisdeviceismetal-gateCMOS.Thistechnologyallowsforverylowpowerdigitalcircuitryandeasieranalogdesignthansingle-polarityMOS.Thisprocessselectionrequirestheneedfortwosupplies,plusandminus5V.Tominimizepower,allthedigitallogicisoperatedfromthepositivesupplywithrespecttoground.Onlytheanalog

section(oneamplifierandonecomparator)operatesfromtheplusandminussupplies.CHIPARCHITECTURE

Fig.3showstheblockdiagramrepresentationofthecodeccircuit.Theimportantfeaturesoftheschemeusedarelistedbelow.

1)TwoindependentDAC’SforencodeanddecodefunctionsprovidesystemisolationnotachievableusingsharedDACapproach.ThecapacitivetwoDACapproachalsoeliminatesexternalsample/holdcapacitorsaswellasexternalfilterforautozeroingrequiredinsharedDACapproach.Thisminimizesexternalcomponentsrequired.2)CompletesignalingcompatibilitywithD3channelbankrequirements.

3)Thenumberofanalogcomponentsusedaretheminimumrequiredforsystem

implementation,namelytwo:onecomparatorandonlyoneopampontheentirecircuit.Minimizingthelinearcomponentshelpsreducesystemoperatingpowerwhichwastheoverridingconsiderationincircuitdesign.

UsingtheCMOSprocess,thedigitalportionsconsumepoweronlyduringtransitions.Thelinearsectionsconsumepowercontinuously.

4)Thedigitalcommandingsectionallowseasyconversionfromp-lawtoA-law.Synchronous/asynchronousdatainput/outputratesfrom128kHzto2.048MHzarepossiblewithnegligiblechangeinpowerdissipation.

MODESOFOPERATION

TheXMITandreceivefunctionarecompletelyindependentofeachotherandofthemasterclock.Thusthechipcanoperateinsynchronous/asynchronousmodeatvariousinput/outputclockrates.ThechiptimingdiagramisshowninFig.4andthereceiveandXMITmodesofoperationaredescribedindetailbelow.

A.ReceiveModeofOperation

Inthereceivemodeofoperation,theserialinputdataareshiftedintotheinputbufferatthereceiveclockrateduringtheperiodwhenreceivesyncishigh.Thetranslateddatafromthe8-to13-bitconverterarelatchedintothe13-bitreceivelatchwhichupdatestheoutputofthereceiveDACwith100

percentdutycycle.ThereceiveDACactsasasampleandholdandisbufferedbytheunity-gainopamptotheoutput.

Duringthesignalingframea7-bitdecodeisperformedandtheeighthdatabitislatchedintotheSigA/SigBoutputlatchasselectedbytheA/Bselect(RCV)input.Whentheeighthbitofawordisasignalingbit,itisassignedthevalueof1/2step.ThisresultsinalowerS/Dratiothanifitwerearbitrarilysettoeitheraoneorzero.ThecircuitofFig.5showstheimplementationof7-bit/8-bitdecode.Forframesonethroughfiveandseventhrougheleven,inputAis1.Bit8correspondstoQ7outputandduringdecodeBis1;thusoutputCisO,resultingin8-bitdecode.Forframessixandtwelve,inputAisOwhichresultsinbit8beingOandoutputC’,corresponding

to1/2stepoffsetcontrol,being1.Thisresultsina7-bitdecodewithaneffective1/2stepoffsetduringsignalingframesand$1-bitdecodeduringotherframes.

B.TransmitModeofOperation

Inthismodeofoperationtheanalogsignalissampledintheinputsample/holdwhichperformstheautozerofunctionsimultaneouslyasdescribedinthecircuitoperationsection.Followingtheholdmode,theencodingprocessiscompletedusingthesuccessiveapproximationtechnique.TheoperationoftheXMITDACissimilartotheoperationofthereceive

DACasdescribedearlier.

Aftertheencodeprocessiscompleted,theoutputoftheSARisloadedintotheoutputbuffer.Thedataaretransmitted

seriallyattheoutputclockrateduringtheperiodwhentheXMITsyncishigh.Duringthesignalingframe,signalinginformation(SigA/SigZ3)isinsertedin