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英文翻译

郑州轻工业学院

本科毕业设计（论文）

英文翻译

题目MPEG-2视频压缩

学生姓名

专业班级电子信息科学与技术2004级1班

学号

院（系）计算机与通信工程学院

指导教师（职称）

完成时间2008年4月15日

英文原文

MPEG-2VIDEOCOMPRESSION

byP.N.Tudor.

ThispaperwontheIEEJLanghamThompsonPrize.

Electronics&CommunicationEngineeringJournal,December1995.

MPEG-2isanextensionoftheMPEG-1internationalstandardfordigitalcompressionofaudioandvideosignals.MPEG-1wasdesignedtocodeprogressivelyscannedvideoatbitratesuptoabout1.5Mbit/sforapplicationssuchasCD（compactdiscinteractive）.MPEG-2isdirectedatbroadcastformatsathigherdatarates;itprovidesextraalgorithmic'tools'forefficientlycodinginterlacedvideo,supportsawiderangeofbitratesandprovidesformultichannelsurroundsoundcoding.ThistutorialpaperintroducestheprinciplesusedforcompressingvideoaccordingtotheMPEG-2standard,outlinesthegeneralstructureofavideocoderanddecoder,anddescribesthesubsets（'profiles'）ofthetoolkitandthesetsofconstraintsonparametervalues（'levels'）definedtodate.

1VIDEOFUNDAMENTALS

TelevisionservicesinEuropecurrentlybroadcastvideoataframerateof25Hz.Eachframeconsistsoftwointerlacedfields,givingafieldrateof50Hz.Thefirstfieldofeachframecontainsonlytheoddnumberedlinesoftheframe（numberingthetopframelineasline1）.Thesecondfieldcontainsonlytheevennumberedlinesoftheframeandissampledinthevideocamera20msafterthefirstfield.Itisimportanttonotethatoneinterlacedframecontainsfieldsfromtwoinstantsintime.Americantelevisionissimilarlyinterlacedbutwithaframerateofjustunder30Hz.

Invideosystemsotherthantelevision,non-interlacedvideoiscommonplace（forexample,mostcomputersoutputnon-interlacedvideo）.Innon-interlacedvideo,allthelinesofaframearesampledatthesameinstantintime.Non-interlacedvideoisalsotermed'progressivelyscanned'or'sequentiallyscanned'video.

Thered,greenandblue（RGB）signalscomingfromacolourtelevisioncameracanbeequivalentlyexpressedasluminance（Y）andchrominance（UV）components.Thechrominancebandwidthmaybereducedrelativetotheluminancewithoutsignificantlyaffectingthepicturequality.Forstandarddefinitionvideo,CCIRrecommendationdefineshowthecomponent（YUV）videosignalscanbesampledanddigitisedtoformdiscretepixels.Theterms4:

2:

2and4:

2:

0areoftenusedtodescribethesamplingstructureofthedigitalpicture.4:

2:

2meansthechrominanceishorizontallysubsampledbyafactoroftworelativetotheluminance;4:

2:

0meansthechrominanceishorizontallyandverticallysubsampledbyafactoroftworelativetotheluminance.

MPEG-2iscapableofcompressingthebitrateofstandard-definition4:

2:

0videodowntoabout3-15Mbit/s.Atthelowerbitratesinthisrange,theimpairmentsintroducedbytheMPEG-2codinganddecodingprocessbecomeincreasinglyobjectionable.Fordigitalterrestrialtelevisionbroadcastingofstandard-definitionvideo,abitrateofaround6Mbit/sisthoughttobeagoodcompromisebetweenpicturequalityandtransmissionbandwidthefficiency.

2BITRATEREDUCTIONPRINCIPLES

Abitratereductionsystemoperatesbyremovingredundantinformationfromthesignalatthecoderpriortotransmissionandre-insertingitatthedecoder.Acoderanddecoderpairarereferredtoasa'codec'.Invideosignals,twodistinctkindsofredundancycanbeidentified.

Spatialandtemporalredundancy:

Pixelvaluesarenotindependent,butarecorrelatedwiththeirneighboursbothwithinthesameframeandacrossframes.So,tosomeextent,thevalueofapixelispredictablegiventhevaluesofneighbouringpixels.

Psychovisualredundancy:

Thehumaneyehasalimitedresponsetofinespatialdetail,andislesssensitivetodetailnearobjectedgesoraroundshot-changes.Consequently,controlledimpairmentsintroducedintothedecodedpicturebythebitratereductionprocessshouldnotbevisibletoahumanobserver.

TwokeytechniquesemployedinanMPEGcodecareintra-frameDiscreteCosineTransform（DCT）codingandmotion-compensatedinter-frameprediction.ThesetechniqueshavebeensuccessfullyappliedtovideobitratereductionpriortoMPEG,notablyfor625-linevideocontributionstandardsat34Mbit/sandvideoconferencesystemsatbitratesbelow2Mbit/s.

Intra-frameDCTcoding

DCT:

Atwo-dimensionalDCTisperformedonsmallblocks（8pixelsby8lines）ofeachcomponentofthepicturetoproduceblocksofDCTcoefficients（Fig.1）.ThemagnitudeofeachDCTcoefficientindicatesthecontributionofaparticularcombinationofhorizontalandverticalspatialfrequenciestotheoriginalpictureblock.ThecoefficientcorrespondingtozerohorizontalandverticalfrequencyiscalledtheDCcoefficient.

Fig.1Thediscretecosinetransform（DCT）.

PixelvalueandDCTcoefficientmagnitudearerepresentedbydotsize.

TheDCTdoesn'tdirectlyreducethenumberofbitsrequiredtorepresenttheblock.Infactforan8x8blockof8bitpixels,theDCTproducesan8x8blockof11bitcoefficients（therangeofcoefficientvaluesislargerthantherangeofpixelvalues.）Thereductioninthenumberofbitsfollowsfromtheobservationthat,fortypicalblocksfromnaturalimages,thedistributionofcoefficientsisnon-uniform.Thetransformtendstoconcentratetheenergyintothelow-frequencycoefficientsandmanyoftheothercoefficientsarenear-zero.Thebitratereductionisachievedbynottransmittingthenear-zerocoefficientsandbyquantisingandcodingtheremainingcoefficientsasdescribedbelow.Thenon-uniformcoefficientdistributionisaresultofthespatialredundancypresentintheoriginalimageblock.

Quantisation:

ThefunctionofthecoderistotransmittheDCTblocktothedecoder,inabitrateefficientmanner,sothatitcanperformtheinversetransformtoreconstructtheimage.IthasbeenobservedthatthenumericalprecisionoftheDCTcoefficientsmaybereducedwhilestillmaintaininggoodimagequalityatthedecoder.Quantisationisusedtoreducethenumberofpossiblevaluestobetransmitted,reducingtherequirednumberofbits.

Coding:

TheserialisationandcodingofthequantisedDCTcoefficientsexploitsthelikelyclusteringofenergyintothelow-frequencycoefficientsandthefrequentoccurrenceofzero-valuecoefficients.TheblockisscannedinadiagonalzigzagpatternstartingattheDCcoefficienttoproducealistofquantisedcoefficientvalues,orderedaccordingtothescanpattern.

Thelistofvaluesproducedbyscanningisentropycodedusingavariable-lengthcode（VLC）.EachVLCcodeworddenotesarunofzerosfollowedbyanon-zerocoefficientofaparticularlevel.VLCcodingrecognisesthatshortrunsofzerosaremorelikelythanlongonesandsmallcoefficientsaremorelikelythanlargeones.TheVLCallocatescodewordswhichhavedifferentlengthsdependingupontheprobabilitywithwhichtheyareexpectedtooccur.Toenablethedecodertodistinguishwhereonecodeendsandthenextbegins,theVLChasthepropertythatnocompletecodeisaprefixofanyother.

Fig.1showsthezigzagscanningprocess,usingthescanpatterncommontobothMPEG-1andMPEG-2.MPEG-2hasanadditional'alternate'scanpatternintendedforscanningthequantisedcoefficientsresultingfrominterlacedsourcepictures.

Toillustratethevariable-lengthcodingprocess,considerthefollowingexamplelistofvaluesproducedbyscanningthequantisedcoefficientsfromatransformedblock:

12,6,6,0,4,3,0,0,0...0

Thefirststepistogroupthevaluesintorunsof（zeroormore）zerosfollowedbyanon-zerovalue.Additionally,thefinalrunofzerosisreplacedwithanendofblock（EOB）marker.Usingparenthesestoshowthegroups,thisgives:

（12）,（6）,（6）,（0,4）,（3）EOB

Thesecondstepistogeneratethevariablelengthcodewordscorrespondingtoeachgroup（arunofzerosfollowedbyanon-zerovalue）andtheEOBmarker.Table1showsanextractoftheDCTcoefficientVLCtablecommontobothMPEG-1andMPEG-2.MPEG-2hasanadditional'intra'VLCoptimisedforcodingintrablocks（seeSection4）.UsingthevariablelengthcodefromTable1andaddingspacesandcommasforreadability,thefinalcodedrepresentationoftheexampleblockis:

00000000110100,001000010,001000010,00000011000,001010,10

Table1:

ExtractfromtheMPEG-2DCTcoefficientVLCtable.

Lengthofrunofzeros

Valueofnon-zero

coefficient

Variable-lengthcodeword

0

12

00000000110100

0

6

001000010

1

4

00000011000

0

3

001010

EOB

-

10

Motion-compensatedinter-frameprediction

Thistechniqueexploitstemporalredundancybyattemptingtopredicttheframetobecodedfromaprevious'reference'frame.Thepredictioncannotbebasedonasourcepicturebecausethepredictionhastoberepeatableinthedecoder,wherethesourcepicturesarenotavailable（thedecodedpicturesarenotidenticaltothesourcepicturesbecausethebitratereductionprocessintroducessmalldistortionsintothedecodedpicture.）Consequently,thecodercontainsalocaldecoderwhichreconstructspicturesexactlyastheywouldbeinthedecoder,fromwhichpredictionscanbeformed.

3MPEG-2DETAILS

Codecstructure

InanMPEG-2system,theDCTandmotion-compensatedinterframepredictionarecombined,asshowninFig.2.Thecodersubtractsthemotion-compensatedpredictionfromthesourcepicturetoforma'predictionerror'picture.ThepredictionerroristransformedwiththeDCT,thecoefficientsarequantisedandthesequantisedvaluescodedusingaVLC.Thecodedluminanceandchrominancepredictionerroriscombinedwith'sideinformation'requiredbythede