Industrialrobothavingajointfreearm.docx
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Industrialrobothavingajointfreearm.docx
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Industrialrobothavingajointfreearm
Videodatatransmission
BACKGROUNDOFTHEINVENTION
Thisinventionrelatestovideodatatransmissionandisespeciallyusefulinthetransmissionofvideoinformationemployingconditionalreplenishment.
Thisapplicationisrelatedtocommonlyassignedco-pendingU.S.applicationtoClappSer.No.516,707filedJuly25,1983.
Atelevisionframetypicallycontainsabout200,000pictureelements,andifthedataaretobesentdigitallydesirably8bitsshouldbeprovidedtoconveythebrightnessinformationateachelement.Withaframerepetitionfrequencyof25persecondforamovingpicture,digitaltransmissionontheabovebasiswouldcallforatransmissionchannelcapacityof40Mbit/s.Thisrequirementcanbereducedconsiderablybymakinguseofthehighcorrelationbetweenoneframeandthenextwhicharisesbecauseusuallyonlyasmallportionofthepicturewillbemovingatanyonetime.Conditionalreplenishmentinvolvesthetransmissionofonlythechangestoanaccuracyofonly,say,16nonlinearlydistributedquantisinglevelsfromoneframetothenextandtheresultingdatareductionisnormallygreaterthantheadditionaldatawhichmustbesenttoidentifyaccuratelytheparticularareasoftheframetowhichthechangesrelate.Theareasareidentifiedbylinenumberandpictureelementaddressalongtheline;although9bitsarenecessarytoidentifythelinenumbercompletelyasavingismadebyusingonly3bitsgivingthenumbermodulo8.Afieldsynchronisingcodeissenttoidentifythefirstlineofafieldandsoarealllinenumberswhetherthereisachangeinthelineornot.Therateofdatatransmissionneededusingconditionalreplenishmentvariesconsiderablybecauseitdependsontheamountoftheframewhichismovingattheparticulartime,whereasitismuchmoreconvenienttohaveaconstantdatatransmissionrate.Inordertoovercomethisdifficultybufferstoresareprovidedatbothendsofthetransmissionchannel,anditisimportantthatthesebufferstoresneitherbecomeemptynoroverflow.Usingthistechniqueithasprovedpossibletotransmitamovingpicturesatisfactorilyovera2Mbit/schannel.
IfBE(t)isthenumberofbitsstoredintheencoderbuffer(termedtheencoderstate)atthetransmissionendofthechannelatatimetandBD(t)isthenumberofbitsstoredinthedecoderbufferattherecpetionendofthechannelattimet,itcanbeshownthatBE(t-Δt)+BD(t)=VRΔt
whereVRisthetransmissioncapacity(inbit/s)ofthechannelandisassumedtobeconstantandΔtisthetimedelaybetweendataenteringtheencoderbufferstoreandthemleavingthedecoderbufferstore.NormallyanoptimumvalueforΔtischosenwhichmakesthetotalofthedatastoredinthebuffersequaltohalfofthetotalavailablebuffermemory,andthecontrolofthedecoderbufferisbasedonthestateoftheencoderbufferatatimeΔtearlier.Thismeansthattheemptyingandoverflowingofthedecoderbuffercanbeanticipatedandappropriateactiontaken,eithertemporarilysuspendingdecodingordiscardingdata,sothatthecorruptionofthereproducedpictureresultingfromdatasuddenlybeingnotavailableorbeinglostcanbeavoided.
Sincethereplenishmentdataisderivedfromanimagescannedbyaconventionaltelevisionrasterandthereproducedimageisupdatedfromthereplenishmentdatalinebylineofasimilarraster,itfollowsthatthereadingofdatafromthedecoderbuffermustremainintrackwiththewritingofdataintotheencoderbufferifchangesinthefirstimagearetoappearaccuratelyinthereproducedimage.ThisformsanadditionalconstraintonthereadingfromthedecoderbufferandishandledbytransmittingtheencoderbufferstateBE(t)tothedecodersothatthedecoderbufferstateBD(t)canbepredictedonthebasisoftheequationgivenabove.
Inaddition,fourtechniquesareusedtoreducetheextremelyhighrateofvideodatawhichmightotherwiseoccurwhenthereisalotofpicturemovement.Thesefourtechniquesare:
(1)Thesensitivityofthemovementdetectorattheencoderisreducedastheencoderbufferfillincreasessothatthenumberofareasofdetectedpicturechangeisreduced.
(2)Fieldsub-samplingonatimebasisisintroduced.Inthistechniquealternatefieldsofinformationarediscardedandthedecoderisoperatedsoastointerplatethemissinginformationfromthetwoadjacenttransmittedfields.
(3)Sub-samplingonanelementbasisisintroducedsothatcertainmovingareaelementsarediscardedbytheencoderandinterpolatedbythedecoder.
(4)Iftheabovetechniquesfailtopreventimminentoverfilloftheencoderbuffer,thenallmovementofthetransmitterpictureisignoreduntiltheencoderbufferstatehasreturnedtoasafevalue.
Thereductionofthesensitivityofthemovementdetectorattheencoderhastheeffectoffreezinglowcontrastchangeinformationwhichresultsinaneffectknownas"dirtywindow".Thiscanbetoleratedforshortperiods.Fieldsub-samplingproducesamovementjerkinessoverthewholefieldandtheprovisionofaninterpolatoratthedecodercanbeusedtoreducethejerkinessbyassumingthatallmovementistakingplaceatauniformspeed.Elementsub-samplingwhichinvolvesthetransmissionofthechangesrelatingtoalternateelements,forexample,enablestheamountofdatatransmittedwhenthereisalotofpicturemovementtobereducedtoaminimumwithoutintroducingthestopmovementtechniquereferredtoin(4)above.Adisadvantageofelementsub-samplingisthatinareasofpicturedetailinterpolationcanbeobjectionablebecausetheinterpolationassumesthatthebrightnessofaninterpolatedpointistheaverageofthetwoadjacentpointsinthesameline.Thisquiteclearlyisnottruewhentheinterpolatedpointisonaverticaledgebetweenlightanddarkareas.
SUMMARYOFTHEINVENTION
Itisanobjectofthepresentinventiontoavoidthisdisadvantageofelementsub-sampling.
Accordingtothepresentinventionthereisprovidedavideodatatransmissionsystemofthetypeinwhichapictureisrepresentedbydigitalvaluescorrespondingtothrbrightnessatelementsalongparallellinesacrossthepictureandonlytheelementswhichchangeinbrightnessfromoneframetothenextaretransmittedfromatransmittertoareceivertoupdateadigitalrepresentationofthepicturestoredinthereceiver,andinwhichinordertoreducetherateofdatatransmissionwhenmanypictureelementsareundergoingchangesinbrightnessthoseofcertainisolatedelementsarenottransmittedbutarecalculatedatthereceiverfromthebrightnesschangesofadjacentelements,whereintheselectionofeachofthecertainisolatedelementsisdependentontherenotbeinglargedifferencesbetweenthebrightnessesofadjacentornearadjacentelementsintheprecedinglineandclosetotheparticularcertainisolatedelement.
Thesizeofthedifferencebetweenthebrightnesschangesofadjacentornearadjacentelementsinalinedependsonwhetherornotthereisaverticallyextendingmovingedgeinthepicture.Alargedifferencewouldindicatethepresenceofsuchanedgewhereasasmalldifferencewouldindicateitsabsence.Aneffectofelementsubsampling,thatisnottransmittingthebrightnesschangesofcertainisolatedelementsbutcalculatingthemfromthoseoftheneighbouringelements,isthatinterpolationcandegradehighcontrastverticaledges.Thiseffectisavoidedbytheinventionbecausesub-samplingisnotusedwherethereisverticaldetail.
Theelementsubsamplingmayinvolvetransmittingthebrightnesschangesofalternateelementsonly,thisgivingthemaximumreductionindataratethatcanbeachievedbecauseallomittedelements,i.e.thosewhosebrightnesschangesarenottransmitted,musthaveatransmittedelementoneachsideforthesubsamplingnottogiverisetosubstantialhorizontalblurring.Preferablytheomittedelementsshouldlieindiagonalratherthanverticallines.
Thelineorfieldsynchronisingcodewordsmaycarryanindicationthatthevideodatahasbeensubjectedtoelementsubsamplingandthereceivermaybearrangedtotestthereceiveddatatoascertainwhetherornotamovingverticaledgewaslikelytohavebeendetectedbytheencoderproducingthedatafortransmissionandconsequentlywhetherornotthebrightnesschangeofaparticularelementisincludedinthereceiveddata.Thereforethereisrequirednospecificindicationthatanormallyomittedelementhasinfactbeentransmittedbecausethisinformationisinherentinthevideodata.
Unfortunately,whilstthesystemjustdescribedisquitesatisfactorywitherror-freedatatransmission,ittakesonlyanerrorinthebrightnesschangeofasingleelementforthereceivertodecideincorrectlyastowhetheranelementinthenextlinehasbeenomittedornot,whichinturnwillresultinthechangesrepresentedbeingallocatedtothewrongelementsandconsequentmistakesinthelaterlines.Inordertoovercomethisproblemthecodesusedtotransmitthebrightnesschangesofnormallyomittedelementsmayincludeanindicationthattheelementisonewhichwouldnormallybeomitted.Onewayofachievingthisistoreducethenumberofquantisinglevelsusedwhenintheelementsubsamplingmodeascomparedwithnormaltransmission,tousethecodesrepresentingthereducedsetofquantisinglevelsforthenormallytransmittedelements,andtousetheremainingcodesforanynormallyomittedelementswhicharetransmitted.Preferablythecodesusedfornormallytransmittedelementsandnormallyomittedelementswhicharetransmittedshouldbesufficientlydifferentforatransmissionerrornottoeffectcorruptionofacodeforonetypeofele
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