whx112减速机壳加工工艺及夹具设计-电视信号

原文

外文资 Althoughover50yearsold,thestandardevisionsignalisstilloneofthemostcommonwaytotransmitanimage.Figure8.3showshowtheevisionsignalappearsonanoscilloscope.Thisiscalledcomposite,meaningthatthereareverticalandhorizontalsynchronization(sync)pulsesmixedwiththeactualpictureinformation.Thesepulsesareusedinthe evisionreceivertosynchronizetheverticalandhorizontaldeflectioncircuitstomatchthe beingdisplayed.Eachsecondof contains30completeimages,commonlycalledframes,Aengineerwouldsaythateachframecontains525lines,the evisionjargonforwhatprogrammerscallrows.Thisnumberisalittledeceptivebecauseonly480to486oftheselinescontain information;theremaining39to45linesare forsyncpulsestokeepthe evision’scircuitssynchronizedwiththe evisionusesaninterlacedformattoreduceflickerinthedisplayedimage.Thismeansthatalltheoddlinesofeachframearetransmitted ,followedbytheevenlines.Thegroupofoddlinesiscalledtheoddfield,andthegroupofevenlinesiscalledtheevenfield.Sinceeachframeconsistsoftwofields,the transmits60fieldspersecond.Eachfieldstartswithacomplexseriesofverticalsyncpulseslasting1.3milliseconds.Thisisfollowedbyeithertheevenoroddlinesof Eachlinelastsfor63.5microseconds,includinga10.2microsecondhorizontalsyncpulse,separatingonelinefromthenext.Withineachline,the ogvoltagecorrespondstothegrayscaleoftheimage,withbrightervaluesbeinginthedirectionawayfromthesyncpulses.Thisplacethesynctheblackrange.In thesyncpulsesaresaidtobeblackerthanblack..Thehardwareusedfor og-to-digitalconversionof signalsiscalledaframegrabber.Thisisusuallyintheformofanelectronicscardthatplugsintoacomputer,andconnectstoacamerathroughacoaxialcable.Uponcommandfromsoftware,theframegrabberwaitsforthebeginningofthenextframe,asindicatedbytheverticalsyncpulses.Duringthefollowinofields,eachlineof issampledmanytimes,typically512,640or720samplesperline,at8bitspersample.Thesesamplesarestoredinmemoryasonerowofthedigitalimage.Thiswayofacquiringadigitalimageresultsinanimportantdifferencebetweentheverticalandhorizontaldirections.Eachrowinthedigitalimagecorrespondstoonelineinthe signal,andthereforetoonerowofwellsintheCCD.Unfortunay,thecolumnsarenotsostraightforward.IntheCCD,eachrowcontainsbetweenabout400and800wells(columns),dependingontheparticulardeviceused.WhenarowofwellsisreadfromtheCCD,theresultinglineofisfilteredintoasmooth signal,suchasinFigure8.3.Inotherwords,the signaldoesnotdependonhowmanycolumnsarepresentintheCCD.Theresolutioninthehorizontaldirectionislimitedbyhowrapidlythe ogsignalisallowedtochange.Thisisusuallysetat3.2MHzforcolor evision,resultinginarisetimeofabout100nanoseconds,i.e,about1/500thofthe53.2microsecond Whenthe signalisdigitizedintheframegrabber,itisconvertedbackintocolumns,However,thesecolumnsinthedigitizedimagehavenorelationtothecolumnsintheCCD.Thenumberofcolumnsinthedigitalimagedependssolelyonhowmanytimestheframegrabbersampleseachlineof .Forexample,aCCDmighthave800wellsperrow,whilethedigitizedimagemightonlyhave512pixels(i.e,columns)perrow.Thenumberofcolumnsinthedigitizedimageisalsoimportantforanotherreason.Thestandard evisionimagehasanaspectratioof4to3,i.e.,itisslightlywiderthanitishigh.Motionpictureshavethewideraspectratioof25to9.CCDsusedforscientificapplicationsoftenhaveanaspectratioof1to1,i.e, rfectsquare.Inanyevent,theaspectratioofaCCDisfixedbytheplacementoftheelectrodes,andcannotbealtered.However,theaspectratioofthedigitizedimagedependsonthenumberofsamplesperline.This esaproblemwhentheimageisdisplayed,eitheronamonitororina.Iftheaspectratioisn’tproperlyreproduced,theimagelookssquashedhorizontallyorvertically.The525line signaldescribedhereiscalledNTSC(National SystemsCommittee),astandarddefinedwaybackin1954.ThisisthesystemusedintheUnitedStatesandJapan.InEuropetherearetwosimilarstandardscalledPAL(PhaseAlternationbyLine)andSECAM(SequentialChrominanceAndMemory).Thebasicconceptsarethesame,justthenumbersaredifferent.BothPALandSECAMoperatewith25interlacedframespersecond,with625linesperframe.JustaswithNTSC,someoftheselinesoccurduringtheverticalsync,resultinginabout576linesthatcarrypictureinformation.Othermoresubtledifferencesrelatetohowcolorandsoundareaddedtothesignal.Themoststraightforwardwayoftransmittingcolor evisionwouldbetohavethree ogsignals,oneforeachofthethreecolorsthehumaneyecandetect:red,greenandblue.Unfortuna y,thehistoricaldevelopmentof evisiondidnotallowsuchasimplescheme.Thecolorevisionsignalwasdevelopedtoallowexistingblackandwhiteevisionsetstoremaininusewithoutmodification.Thiswasdonebyretainingthesamesignalforbrightnessinformation,butaddingaseparatesignalforcolorinformation.Injargon,thebrightnessiscalledtheluminancesignal,whilethecoloristhechrominancesignal.Thechrominancesignaliscontainedona3.58MHzcarrierwaveaddedtotheblackandwhite signal.Soundisaddedinthissameway,ona4.5MHzcarrierwave.Theevisionreceiverseparatesthesethreesignals,processesthemindividually,and binestheminthefinaldi