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Theimmensepopularityofcompactdigitalaudiodiscscanbepartiallyattributedtothehighsoundqualityandfreedomfromdeteriorationoftherecordingduetoagingorrepeatedplayingsorfromenvironmentalfactorssuchasdust.Itispossibletoenjoysimilaradvantagesfromthedigitalencodingofvideosignalsaswell.However,realtimecodinganddecodingofavideosignalrequiresmuchmoresignalbandwidththandoesanaudiosignal,currentlyavailabletransmittingandrecordingmediaprovidesinsufficientstoragecapacityforvideosignals.Itisnecessary,therefore,tousesomeformofsignalcompressionordigitalbitreductiontechniqueinordertorecorddigitally-encodedvideo.Anumberofsuchbitreductionorsignalcodingmethods,thatwillenabletherecordingoffull-motiondigitalimages,havebeenproposedinrecentyears.
Becausetheavailabledatarateinpresenttransmissionchannelsandstoragemediaislimited,theseknowncodingmethodsacttoreducethedatarateofthevideosignal.Forthepurposeofstoringdigitally-encodedfullmotionimagesonopticalstoragemedia,suchasCD-ROMorMOD(MagnetoOpticalDisc),forexample,amaximumvideodatarateofabout1.2Mbits/secondisallowed.
Oneexampleofasystemfordigitallycodinganddecodingvideosignalsisdisclosedinanarticleentitled"
SceneAdaptiveCoder"
W.H.Chen,W.K.Pratt,IEEETransactionsonCommunication,Vol.COM-32,No.3,issuedinMarch,1984inwhichsceneadaptivecoding/decodingofvideosignalsisdescribed.Thedescribedcodingtechniqueutilizesdiscretecosinetransform(DCT),avariablethresholdfortheDCTcoefficients,quantizationtechniques,Huffmancodingandabufferinthecoder.Thedecoderincludesabuffer,Huffmandecoder,thresholdaddingandinverseDCT.
Anothersuchcoding/decodingsystemisdisclosedin"
ComparisonBetweenProgressiveandInterlacedScanningforaFutureHDTVSystemwithDigitalDataRateReduction"
D.Westerkamp,H.Peters,Proceedingsofthe2ndInternationalWorkshopofSignalProcessingforHDTV,l'
Aquila,Feb.29-Mar.2,1988.Thedescribedsystemcontainsamotionadaptiveintrafield/interframecodingprocessor,inwhichthedigitizedvideodataistransformedbymeansofDCTtoobtainthequantizingcoefficients.
InthepreviouslydescribedsystemsthefullnessofthebuffercontrolsthethresholdandthequantizerstepsizefortheDCTcoefficients.Bothsystemsguaranteethatthebufferwillhavenooverfloworunderflowwithinagivenframe.Thiscausesavaryingquantizerstepsizeandaconsequentlyvaryingimagequalitywithineachframe.
InU.S.Pat.No.4,785,349,differentregionsofavideoframearecodedinadifferentmanner.Thismayalsocauseavaryingimagequalitywithineachframe.
Fromthedescriptionofthepreviouslydescribedsystems,itisknownthat,foragivencodingscheme,themoreablockofvideodataisquantized,thelessbitsarerequiredforcoding.Itisalsoknownthatthequantizationlevelmaybedeterminedbytheimage,orvideosignal,content.
TwocurrentlyknownmethodsmaybeusedtocomputethelevelofquantizationrequiredtocompressthedatarateofavideosignalwithinanavailablenumberofbitsN.Thefirstmethodcomputesathreshold/quantizationstepsizesothataframecanbecodedinapproximatelyNbits.Thequantizeristhenappliedwiththisuniformstepsizetothewholeframe.Thesecondmethodscansblocksofvideodatabyrowsandcolumnsinordertocomputealocalthreshold/quantizationstepsize.
Thefirstmethodallowsaspatiallyuniformquantizationstepsizetobeappliedtothewholeimage.However,computingtherightlevelofquantizationinordertocodetheimagetoascloseaspossibletoNbitsisverydifficult.Thesecondmethodallowsthecomputationofaquantizationstepsizeonablockbyblockbasis.However,thismethoddoesnotprovideauniformlevelofquantizationinthespatialdomain;
thereforetheimagequalitymaybedifferentfortheblocksofeachframe.
SUMMARYOFTHEINVENTION
Inaccordancewithanaspectofthepresentinvention,amethodofencodingdigitalimagesequencescomprisesthestepsofgeneratingblocksofpixelsfromaframeofdigitalvideoinformation.Theblocksofpixelsarescannedinapseudo-randommannerandtransformedtoobtaincorrespondingblocksofcoefficients.Theblocksofcoefficientsarestoredinabuffer.Eachblockofcoefficientsareindividuallyquantizedbytheuseofaquantizationlevelthatiscontrolledbythefullnessofthebuffer.
Theadvantagesofthepresentinventionwillbecomemoreapparentfromthefollowingdetaileddescriptionwhentakeninconjunctionwiththeaccompanyingdrawing.
BRIEFDESCRIPTIONOFTHEDRAWING
Inthedrawing,FIG.1isablockdiagramofoneembodimentofacoderconstructedinaccordancewithanaspectofthepresentinvention;
FIG.2isablockdiagramofoneembodimentofadecoderconstructedinaccordancewithanaspectofthepresentinvention;
FIG.3isarepresentationofrandomlyscannedblocksofpixelswithinavideoimage;
FIG.4illustratesanexampleofaquantizationmatrixforablockofpixels;
FIG.5illustratesarepresentativepixelscanningmatrix;
and
FIG.6isagraphillustratingtheoperationofaquantizerinaccordancewiththepresentinvention.
DETAILEDDESCRIPTION
Inordertoobtainavideoimageofmediumresolution,apixelmatrixoftheorderof704by240pixelsisnecessary.ThevideoinformationisillustrativelyencodedsuchthattheluminancecomponentYisencodedforeachofthepixels,whereasthecolorcomponentsUandVarecodedintheformofmacro-pixels,withamacro-pixelbeingmadeupof4luminancepixels.
Eachvideoframeisdividedintomacro-blocksof16by16luminancepixels.Eachmacro-blockiscomposedof4blocksof8by8pixelsfortheluminancecomponentYandoneblockof8by8macro-pixelsforeachofUandV.Asinglenon-encodedimagecomprising704by240pixelsconsistsof(704/16)×
(240/16)=660macroblocks,whichequals(660×
6)or3960blocks,equivalentto(3960×
64)or253440bytes.
Aframeofthissizewouldoccupyabout250KByteinanuncodedformat,whereastheavailablebandwidthof1.2Mbits/secwouldonlyallowabout(1.2Mbit/s)/(8×
30/s)=5KByteperframe,whichcorrespondstolessthan1/2bitperpixel.Acompressionrateofabout1/50isthereforerequiredtobeachieved.
InFIG.1adigitalimagesource11isconnectedtoarandomaccessmemory(RAM)12whichstoresthevideoinformationofaframeidentifiedasn.RAM12receivesblockaddresseswithinacurrentimagefromapseudo-randomblockaddressgenerator10.Eachblockofpixelsisthensenttoasubtractor141andtoamotioncompensationcircuit17.
Inmanyinstances,thevideoinformationcontainedinthepreviousframen-1isverysimilartothevideoinformationcontainedinthepresentorcurrentframen,onlythedifferencesbetweenframesn-1andnareencoded.Thistechniqueiscalledinterframecoding(delta-coding)andresultsinarelativelylowdatarate.
However,insomecasesitisnecessarytoencodeaframebyitself,suchaswhenthereisa"
cut"
orscenetransitionintheimagesequence.Thistechniqueiscalledintraframecoding.Itisalsonecessarytouseintraframecodingtoallownon-sequentialdecodingwhenitisdesirabletostartdecodingfromvariouspointswithinanimagesequencestoredonaCD,suchaswouldberequiredforrandomaccesstovideoinformationonadisc.Intraframecodingdoesresultsinahigherdatarate,however.
Whenthereismotionintheimagesequence,interframecodingmayyieldpoorqualitycodedimages.Inthosesituations,themotionintheimagesequenceshouldbedetected.These"
motion"
differencesbetweencorrespondingelementsinframesn-1andnarethenusedtominimizethecodingerroroftheinterframecoding,therebyleadingtoimprovedimagequality.Thepurposeofmotioncompensationistobuildanimageascloseaspossibletoframenbyusingthedetectedmotionelementsfromframen-1.
Thisprocessisperformedinthemotioncompensationcircuit17onapixelblockbasis.AblockB(n,i,j)fromframen,locatedatframecoordinates(i,j),forexample,isevaluatedbycopyingtheblockB'
(n-1,i+x,j+y)locatedatframecoordinates(i+x,j+y)inaframememory(RAM)142whichcontainsframen-1.AmotioncompensationvectorV(n,x,y),representingtheapparentmotionoftheimageatframecoordinates(i,j)onthescreen,isgenerated.Whenusing8by8pixelblocksformotioncompensation,themotioncompensationvectorsmayhaveaprecisionof±
1pixelandarangeof±
16pixels.
Onceagoodapproximationofframenhasbeenobtainedfromframen-1byusingmotioncompensation,theresidualpixeldifferencesremaintobeencoded.Anestimationofthedifferencebetweencorrespondingblocksofframesn-1andnisdeterminedandadecisionismadewithrespecttoeachblockastowhethermotioncompensatedinterframecodingorintraframecodingyieldsbetterimagequalityinconnectionwithagivendatarate.Thisdecisionismadebyerrorevaluationcircuit18bywayofevaluationofaquadraticerrorfunction.Theequationofthequadraticerroris:
##EQU1##whereiiandjjarecoordinateswithintheblocksB(n,i,j)andB'
(n-l,i+x,j+y),respectively,ofpixelsPandP'
respectively.E(B,B'
)isanestimationofthedifferencebetweenblockB(n,i,j)offramenandblockB'
(n-l,i+x,j+y)offramen-1.B(n,i,j)isthetargetblock(notcoded)andB'
(n-l,i+x,j+y)isablockobtainedfromframen-1afterinterframecodingTheencodingo