外文翻译--电视信号.doc

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1、外文资料及译文原文：TelevisionVideoSignalsAlthoughover50yearsold,thestandardtelevisionsignalisstilloneofthemostcommonwaytotransmitanimage.Figure8.3showshowthetelevisionsignalappearsonanoscilloscope.Thisiscalledcompositevideo,meaningthatthereareverticalandhorizontalsynchronization(sync)pulsesmixedwiththeactual

2、pictureinformation.Thesepulsesareusedinthetelevisionreceivertosynchronizetheverticalandhorizontaldeflectioncircuitstomatchthevideobeingdisplayed.Eachsecondofstandardvideocontains30completeimages,commonlycalledframes,Avideoengineerwouldsaythateachframecontains525lines,thetelevisionjargonforwhatprogra

3、mmerscallrows.Thisnumberisalittledeceptivebecauseonly480to486oftheselinescontainvideoinformation；theremaining39to45linesarereservedforsyncpulsestokeepthetelevisionscircuitssynchronizedwiththevideosignal.Standardtelevisionusesaninterlacedformattoreduceflickerinthedisplayedimage.Thismeansthatalltheodd

4、linesofeachframearetransmittedfirst,followedbytheevenlines.Thegroupofoddlinesiscalledtheoddfield,andthegroupofevenlinesiscalledtheevenfield.Sinceeachframeconsistsoftwofields,thevideosignaltransmits60fieldspersecond.Eachfieldstartswithacomplexseriesofverticalsyncpulseslasting1.3milliseconds.Thisisfol

5、lowedbyeithertheevenoroddlinesofvideo.Eachlinelastsfor63.5microseconds,includinga10.2microsecondhorizontalsyncpulse,separatingonelinefromthenext.Withineachline,theanalogvoltagecorrespondstothegrayscaleoftheimage,withbrightervaluesbeinginthedirectionawayfromthesyncpulses.Thisplacethesyncbeyondtheblac

6、krange.Invideojargon,thesyncpulsesaresaidtobeblackerthanblack.Thehardwareusedforanalog-to-digitalconversionofvideosignalsiscalledaframegrabber.Thisisusuallyintheformofanelectronicscardthatplugsintoacomputer,andconnectstoacamerathroughacoaxialcable.Uponcommandfromsoftware,theframegrabberwaitsforthebe

7、ginningofthenextframe,asindicatedbytheverticalsyncpulses.Duringthefollowingtwofields,eachlineofvideoissampledmanytimes,typically512,640or720samplesperline,at8bitspersample.Thesesamplesarestoredinmemoryasonerowofthedigitalimage.Thiswayofacquiringadigitalimageresultsinanimportantdifferencebetweentheve

8、rticalandhorizontaldirections.Eachrowinthedigitalimagecorrespondstoonelineinthevideosignal,andthereforetoonerowofwellsintheCCD.Unfortunately,thecolumnsarenotsostraightforward.IntheCCD,eachrowcontainsbetweenabout400and800wells(columns),dependingontheparticulardeviceused.WhenarowofwellsisreadfromtheCC

9、D,theresultinglineofvideoisfilteredintoasmoothanalogsignal,suchasinFigure8.3.Inotherwords,thevideosignaldoesnotdependonhowmanycolumnsarepresentintheCCD.Theresolutioninthehorizontaldirectionislimitedbyhowrapidlytheanalogsignalisallowedtochange.Thisisusuallysetat3.2MHzforcolortelevision,resultinginari

10、setimeofabout100nanoseconds,i.e,about1/500thofthe53.2microsecondvideoline.Whenthevideosignalisdigitizedintheframegrabber,itisconvertedbackintocolumns,However,thesecolumnsinthedigitizedimagehavenorelationtothecolumnsintheCCD.Thenumberofcolumnsinthedigitalimagedependssolelyonhowmanytimestheframegrabbe

11、rsampleseachlineofvideo.Forexample,aCCDmighthave800wellsperrow,whilethedigitizedimagemightonlyhave512pixels(i.e,columns)perrow.Thenumberofcolumnsinthedigitizedimageisalsoimportantforanotherreason.Thestandardtelevisionimagehasanaspectratioof4to3,i.e.,itisslightlywiderthanitishigh.Motionpictureshaveth

12、ewideraspectratioof25to9.CCDsusedforscientificapplicationsoftenhaveanaspectratioof1to1,i.e,aperfectsquare.Inanyevent,theaspectratioofaCCDisfixedbytheplacementoftheelectrodes,andcannotbealtered.However,theaspectratioofthedigitizedimagedependsonthenumberofsamplesperline.Thisbecomesaproblemwhentheimage

13、isdisplayed,eitheronavideomonitororinahardcopy.Iftheaspectratioisntproperlyreproduced,theimagelookssquashedhorizontallyorvertically.The525linevideosignaldescribedhereiscalledNTSC(NationalTelevisionSystemsCommittee),astandarddefinedwaybackin1954.ThisisthesystemusedintheUnitedStatesandJapan.InEuropeth

14、erearetwosimilarstandardscalledPAL(PhaseAlternationbyLine)andSECAM(SequentialChrominanceAndMemory).Thebasicconceptsarethesame,justthenumbersaredifferent.BothPALandSECAMoperatewith25interlacedframespersecond,with625linesperframe.JustaswithNTSC,someoftheselinesoccurduringtheverticalsync,resultinginabo

15、ut576linesthatcarrypictureinformation.Othermoresubtledifferencesrelatetohowcolorandsoundareaddedtothesignal.Themoststraightforwardwayoftransmittingcolortelevisionwouldbetohavethreeseparateanalogsignals,oneforeachofthethreecolorsthehumaneyecandetect:red,greenandblue.Unfortunately,thehistoricaldevelop

16、mentoftelevisiondidnotallowsuchasimplescheme.Thecolortelevisionsignalwasdevelopedtoallowexistingblackandwhitetelevisionsetstoremaininusewithoutmodification.Thiswasdonebyretainingthesamesignalforbrightnessinformation,butaddingaseparatesignalforcolorinformation.Invideojargon,thebrightnessiscalledtheluminancesignal,whilethecoloristhechrominancesignal.