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外文翻译--电视信号.doc

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外文翻译--电视信号.doc

外文资料及译文原文TelevisionVideoSignalsAlthoughover50yearsold,thestandardtelevisionsignalisstilloneofthemostcommonwaytotransmitanimage.Figure8.3showshowthetelevisionsignalappearsonanoscilloscope.Thisiscalledcompositevideo,meaningthatthereareverticalandhorizontalsynchronizationsyncpulsesmixedwiththeactualpictureinformation.Thesepulsesareusedinthetelevisionreceivertosynchronizetheverticalandhorizontaldeflectioncircuitstomatchthevideobeingdisplayed.Eachsecondofstandardvideocontains30completeimages,commonlycalledframes,Avideoengineerwouldsaythateachframecontains525lines,thetelevisionjargonforwhatprogrammerscallrows.Thisnumberisalittledeceptivebecauseonly480to486oftheselinescontainvideoinformationtheremaining39to45linesarereservedforsyncpulsestokeepthetelevisionscircuitssynchronizedwiththevideosignal.Standardtelevisionusesaninterlacedformattoreduceflickerinthedisplayedimage.Thismeansthatalltheoddlinesofeachframearetransmittedfirst,followedbytheevenlines.Thegroupofoddlinesiscalledtheoddfield,andthegroupofevenlinesiscalledtheevenfield.Sinceeachframeconsistsoftwofields,thevideosignaltransmits60fieldspersecond.Eachfieldstartswithacomplexseriesofverticalsyncpulseslasting1.3milliseconds.Thisisfollowedbyeithertheevenoroddlinesofvideo.Eachlinelastsfor63.5microseconds,includinga10.2microsecondhorizontalsyncpulse,separatingonelinefromthenext.Withineachline,theanalogvoltagecorrespondstothegrayscaleoftheimage,withbrightervaluesbeinginthedirectionawayfromthesyncpulses.Thisplacethesyncbeyondtheblackrange.Invideojargon,thesyncpulsesaresaidtobeblackerthanblack..Thehardwareusedforanalogtodigitalconversionofvideosignalsiscalledaframegrabber.Thisisusuallyintheformofanelectronicscardthatplugsintoacomputer,andconnectstoacamerathroughacoaxialcable.Uponcommandfromsoftware,theframegrabberwaitsforthebeginningofthenextframe,asindicatedbytheverticalsyncpulses.Duringthefollowingtwofields,eachlineofvideoissampledmanytimes,typically512,640or720samplesperline,at8bitspersample.Thesesamplesarestoredinmemoryasonerowofthedigitalimage.Thiswayofacquiringadigitalimageresultsinanimportantdifferencebetweentheverticalandhorizontaldirections.Eachrowinthedigitalimagecorrespondstoonelineinthevideosignal,andthereforetoonerowofwellsintheCCD.Unfortunately,thecolumnsarenotsostraightforward.IntheCCD,eachrowcontainsbetweenabout400and800wellscolumns,dependingontheparticulardeviceused.WhenarowofwellsisreadfromtheCCD,theresultinglineofvideoisfilteredintoasmoothanalogsignal,suchasinFigure8.3.Inotherwords,thevideosignaldoesnotdependonhowmanycolumnsarepresentintheCCD.Theresolutioninthehorizontaldirectionislimitedbyhowrapidlytheanalogsignalisallowedtochange.Thisisusuallysetat3.2MHzforcolortelevision,resultinginarisetimeofabout100nanoseconds,i.e,about1/500thofthe53.2microsecondvideoline.Whenthevideosignalisdigitizedintheframegrabber,itisconvertedbackintocolumns,However,thesecolumnsinthedigitizedimagehavenorelationtothecolumnsintheCCD.Thenumberofcolumnsinthedigitalimagedependssolelyonhowmanytimestheframegrabbersampleseachlineofvideo.Forexample,aCCDmighthave800wellsperrow,whilethedigitizedimagemightonlyhave512pixelsi.e,columnsperrow.Thenumberofcolumnsinthedigitizedimageisalsoimportantforanotherreason.Thestandardtelevisionimagehasanaspectratioof4to3,i.e.,itisslightlywiderthanitishigh.Motionpictureshavethewideraspectratioof25to9.CCDsusedforscientificapplicationsoftenhaveanaspectratioof1to1,i.e,aperfectsquare.Inanyevent,theaspectratioofaCCDisfixedbytheplacementoftheelectrodes,andcannotbealtered.However,theaspectratioofthedigitizedimagedependsonthenumberofsamplesperline.Thisbecomesaproblemwhentheimageisdisplayed,eitheronavideomonitororinahardcopy.Iftheaspectratioisntproperlyreproduced,theimagelookssquashedhorizontallyorvertically.The525linevideosignaldescribedhereiscalledNTSCNationalTelevisionSystemsCommittee,astandarddefinedwaybackin1954.ThisisthesystemusedintheUnitedStatesandJapan.InEuropetherearetwosimilarstandardscalledPALPhaseAlternationbyLineandSECAMSequentialChrominanceAndMemory.Thebasicconceptsarethesame,justthenumbersaredifferent.BothPALandSECAMoperatewith25interlacedframespersecond,with625linesperframe.JustaswithNTSC,someoftheselinesoccurduringtheverticalsync,resultinginabout576linesthatcarrypictureinformation.Othermoresubtledifferencesrelatetohowcolorandsoundareaddedtothesignal.Themoststraightforwardwayoftransmittingcolortelevisionwouldbetohavethreeseparateanalogsignals,oneforeachofthethreecolorsthehumaneyecandetectred,greenandblue.Unfortunately,thehistoricaldevelopmentoftelevisiondidnotallowsuchasimplescheme.Thecolortelevisionsignalwasdevelopedtoallowexistingblackandwhitetelevisionsetstoremaininusewithoutmodification.Thiswasdonebyretainingthesamesignalforbrightnessinformation,butaddingaseparatesignalforcolorinformation.Invideojargon,thebrightnessiscalledtheluminancesignal,whilethecoloristhechrominancesignal.

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