通信专业文献及翻译基于JPEG技术融合DSP和FPGA的可纠错图像传输系统.docx
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通信专业文献及翻译基于JPEG技术融合DSP和FPGA的可纠错图像传输系统.docx
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通信专业文献及翻译基于JPEG技术融合DSP和FPGA的可纠错图像传输系统
英文原文
MixedDSP/FPGAimplementationofanerror-resilientimagetransmissionsystembasedonJPEG2000
MarcoGrangetto,EnricoMagli,MaurizioMartina,FabrizioVacca
Abstract
Thispaperdescribesademonstratorofanerror-resilientimagecommunicationsystemoverwirelesspacketnetworks,basedonthenovelJPEG2000standard.Inparticular,thedecoderimplementationisaddressed,whichisthemostcriticaltaskintermsofcomplexityandpowerconsumption,inviewofuseonawirelessportableterminalforcellularapplications.ThesystemimplementationisbasedonamixedDSP/FPGAarchitecture,whichallowstoparallelizesomecomputationaltasks,thusleadingtoefficientsystemoperation.
1Introduction
Nowadays,thereisagrowinginterestintheend-to-endtransmissionofimages,especiallymotivatedbytheshort-termdeploymentofnextgenerationmobilecommunicationservices(UMTS-IMT2000).However,transmissioninanetworked,tetherlessenvironmentprovidesbothopportunitiesandchallenges.Thewirelesscontextimpliesthatthedatamayundergobiterrorsandpacketlosses,makingitnecessarytoforeseeerrorrecoverymodalities.Itistherebynecessarythatimagecommunicationtechniquesareprovidedwiththeabilitytorecover,oratleastconceal,theeffectofsuchlosses.TheforthcomingJPEG2000imagecompressionstandardhasbeendesignedtomatchtheserequirements,andembedssomeerrordetectionandconcealmenttools.
Thispaperaddressesthedevelopmentofademonstratorofanerror-resilientJPEG2000decoderimplementationforimagecommunicationoveralossypacketnetwork.TherobustnesstopacketerasuresisachievedbycombiningtheflexibilityoftheJPEG2000frameworkwiththepowerfulnessofsource-channeladaptive,optimizedReed-Solomoncodes.Thedecoderimplementationisparticularlysignificantinthecontextofwirelessportableterminalsfornext-generationcellularsystems,wherethelimitedpowerbudgetandavailabledimensionsimposesevereconstraintsonthedesignofamultimediaprocessingsystem.
2Systemoverview
Inthefollowingweprovideabriefdescriptionofthefunctionalunitsoftheimplementedsystem.
2.1JPEG2000imagecompression
JPEG2000isthenovelISOstandardforstillimagecoding,andisintendedtoprovideinnovativesolutionsaccordingtothenewtrendsinmultimediatechnologies.Atthetimeofthiswriting,thestandardisinadvancedpublicationstage;theFinalCommitteeDraftisthemostrecentJPEG2000descriptionpubliclyavailable,whichourimplementationconformsto.JPEG2000notonlyyieldssuperiorperformancewithrespecttoexistingstandardsintermsofcompressioncapabilityandsubjectivequality,butalsonumerousadditionalfunctionalities,suchaslosslessandlossycompression,progressivetransmission,anderrorresilience.ThearchitectureoftheJPEG2000isbasedonthetransformcodingapproach.Animagemaybedividedintoseveralsub-images(tiles),toreducememoryandcomputingrequirements.Abiorthogonaldiscretewavelettransform(DWT)isfirstappliedtoeachtile,whoseoutputisaseriesofversionsofthetileatdifferentresolutionlevels(subbands);then,thetransformcoefficientsarequantized,independentlyforeachsubband,withanembeddeddead-zonequantizer.Eachsubbandofthewaveletdecompositionisdividedintorectangularblocks(code-blocks),whicharein-dependentlyencodedwithEBCOT(EmbeddedBlockCodingwithOptimizedTruncation);thislatterisbasedonabit-planeapproach(i.e.themostsignificantbitsofthesubbandcoefficientsaretransmittedfirst),contextmodelingandarithmeticcoding.ThebitstreamoutputbyEBCOTisorganizedbytherateallocatorintoasequenceoflayers,eachlayercontainingcontributionsfromeachcode-block;thetruncationpointsassociatedwitheachlayerareoptimizedintheratedistortionsense.ThefinalJPEG2000bitstreamconsistsofamainheader,followedbyoneormoresectionscorrespondingtoindividualtiles.Eachtilecomprisesatileheaderandalayeredrepresentationoftheincludedcode-blocks,organizedintopackets.Inordertoformaprogressivebitstream,i.e.onethatcanbeonlypartiallydecodedwithminimalpenalty,thelayersareformedandorderedinsuchawaythatthemostimportantinformationisplacedatthebeginningofthebitstream.TheJPEG2000decoderperformsexactlythesamesteps(exceptforrateallocation),inreverseorder:
syntaxparsing,codeblockdecodingbyEBCOT,inversequantization,inverseDWT,andtilemosaicking;thisissketchedintheright-hand-sideboxofFig.1.
2.2AdaptiveReed-Solomonpacketprotection
AlthoughJPEG2000embodiesadvancederrorconcealmenttechniquestomitigatetheeffectoferrors,itdoesneithercontain,norspecifyanyerrorcorrectionmethod,inordertorecoverlostpackets.Ontheotherhand,packetlossesarelikelytooccurinanetworkpotentiallysubjecttocongestion,asisoftenthecaseinpractice.Inordertoovercomethisproblem,atechniquehasbeenrecentlyproposed,calledUnequalLossProtection(ULP),andbasedonthejointuseofRScodesandpacketinterleaving,asshownintheleft-hand-sideofFig.1.Letusconsideramaximumrateallocatedtotheimagetransmission,e.g.NpacketsofsizeL;thesourcebitstreamisrowwiseinsertedintheinterleavingmatrix,followedbyaproperamountofparitysymbols,sayTiforthei-throw.Thepacketsarereadonthecolumnsoftheinterleaver.Theallocationproblemconsistsinfindingtheoptimalpartitioningbetweensourceandcodesymbolsforeachrowoftheinterleaver,soastomaximizethequalityofserviceatthereceiver;seeforimplementationdetails.Atthedecoder,duetotheerrorcorrectioncapabilityofRScodes,thei-throwcanbeexactlyrecoveredprovidedthatthenumberofpacketerasureshasbeenlessthanTi.
Figure1Systemarchitecture
2.3Proposedsystem
Inthispaperwepresentademonstratorofacompletedecoderforimagecommunicationoveralossypacketnetwork.Thedecoderconsistsof1)ULPdecoder,followedby2)JPEG2000decoder.ThegoalistodemonstratethatsuchtaskscanbeeffectivelyperformedonamodernDSP,satisfyingreal-timeoperationrequirements.ThisobjectivehasbeenaccomplishedbyusingamixedDSP/FPGAarchitecture.Inparticular,theimplementationhasbeendevelopedusingaTexasInstruments(TI)TMS320C6201DSPboardastargetdevice.Theadditionalbitstreamprotectionprotocol,basedonRScodes,hasbeenimplementedonaVirtexXCVl000FPGAdevicefromXILINX,whichisabletoconcurrentlyexchangedatawiththeDSP.AsisshowninSect.4,thedevelopedsystemisabletoperformveryfastdecodingofstillimages,ataratecompatiblewithreal-timevideoapplications;thus,anextensiontoMotionJPEG2000videocodingcanbeenvisaged,withminormodificationsofthecurrentdemonstrator.
3Systemarchitecture
ResortingtomixedDSP/FPGAbasedarchitecturesallowstoachievevery-highperformancesystems,withexcellentpropertiesofreconfigurability.PreliminarypartitioningstudieshaveputintoevidencethatwavelettransformandEBCOTarethemostdemandingtasksinJPEG2000decodingprocess.However,sincemanyextensionsofthestandardarestillpossible,anall-DSPimplementationoffersanexcellentdegreeofreconfigurability.Ontheotherhand,FPGAiswellsuitedtotheimplementationofaReedSolomoncore,neededtogranterrorresilienceofthecommunicationsystem.Itisworthnoticingthat,withanULP,theRSdecoderneedstobere-adaptedtoworkwithwordsofbitswhichcankeepdifferentamountsofinformationsymbolsand,asaconsequence,differentamountsofprotectionsymbols.Theneedforchangingtheseparameters"on-the-fly"isperfectlyfulfilledbyanFPGA,bysimplyloadingnewvaluesintosomeconfigurationregisters.Moreover,thehighmemorybandwidthneededtoquicklyde-packetizeandde-codethereceivedbitstreammakestheuseofaReedSolomonFPGAimplementationveryattractive.TheJPEG2000decodermodule,entirelyimplementedonDSP,iscomposedbyfourmainblocks:
syntaxparser,entropydecoder(EBCOT),uniformscalardequantizer,andinversewavelettransform.Moreover,twoadditionaltasks,devotedtocommunicationmanagementbetweenDSP,FPGAandapersonalcomputer,havebeenintroduced.
3.1Syntaxparser
TheparseristhefunctionalblockthatinterfacestheJPEG2000decoderwiththeRSdecoder.ItretrievesRSdecodedpackets,andextractsfromthecompressedJPEG2000bitstreamalltherelevantinformationneededtoperformimagereconstruction.Firstly,thebitstreammainheaderisread,whichcontainsinformationontheparametersusedduringtheencodingprocess(e.g.imagesize,waveletfilterused,numberofdecompositionlevels,quantizationthresholds,andsoon).Afterthat,tileheadersareread,whichprovideinformationspecifictoeachimagetile.Finally,eachpacketcontainedinthebitstreamisread,andthedataandparametersofeachcodeblockareextracted,andfedasinputstotheEBCOTdecoder.
3.2EBCOT
Rightafterthebitstreamsyntaxparser,thesubsequentstageintheJPEG2000decompressionchainistheentropydecoder(EBCOT).Fromanalgorithmicpointofview,EBCOTisablock-basedbitplaneencoderfollowedbyareducedcomplexityarithmeticcoder(MQ).Itsubdivideseachwaveletsubbandintoadisjointsetofrectangularblocks,calledcode-blocks.Thenthecompressionalgorithmisindependentlyappliedtoeverycode-block.Thesamplesofeverycodeblockarearrangedintoso-calledbitplanes.Todecodeacode-block,EBCOTalwaysstartsfromthemostsignificantbitplanes,andthenmovestowardstheleastsignificantones.Thecompressedinformationofeverycode-blockisth
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