梳状滤波器Word格式文档下载.docx
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梳状滤波器Word格式文档下载.docx
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Thisisaverybriefandsimplifieddescriptionofthecompositevideosignalformat.Thereismoreinformationaboutcompositevideosignalsintheaccompanyingarticle,VideoSignalFormats.
Acompositevideosignalisthesumofaluminance(brightness)signalandachrominance(color)signal.InvideoterminologytheyarereferredtoastheYandCsignalsrespectively.TheCsignalisaspeciallymodulatedcombinationoftwootherintermediatesignalscalledI&
Q,whichinturnwerecreatedfromtheoriginalRed,Green,andBlue(RGB)outputsofavideocameraorfilmscanner.
Televisionsignalscanbetransmittedovertheair,oroncablesystems,bymodulatinganRF(RadioFrequency)carriersignalwithaudioandcompositevideosignals.Thecompositevideoandnarrowbandwidthaudiosignalsareeasilyseparateduponreceptionbecausetheydon'
toverlapinthefrequencyspectrum.Inordertominimizethebandwidthusedbythecompositevideosignal,thefrequencyrangesoftheY&
Csignalsaredesignedtooverlap.ThispermitsmoreTVchannelstobebroadcastwithinthetotalfrequencyspectrumavailabletobroadcasttelevision.
TheproblemcreatedbyoverlappingtheY&
Csignal'
sfrequencyspectraisthatitisveryhardtoseparatethemcompletely.TheoriginaldesignersoftheU.S.NTSCtelevisionstandard(in1953)recognizedthislimitation,butbelieveditwasagoodtradeofftopreservefrequencyspaceandtoprovidecompatibilitywiththeexistingblackandwhiteTVsystem.
TheyverycleverlystandardizedonasystemwhichinterleavedtheYandCfrequencieswheretheirspectraoverlapped.TheYandCsignalenergyclustersaroundspecificfrequenciesrelatedtothehorizontalscanrate,fittingtogetherliketheteethoftwocombs.ThesignalsalternateinfrequencybetweenYclustersandCclusters.Thisisillustratedbyasmallsectionofthefrequencydistributionshownbelow.
Duringmovementofcertainimagesandpatterns,thefrequencyclusterscanbecomesmearedandwidenout.InsomecasestheY&
Cclusterswilloverlapandbecomeverydifficulttoseparate.Buttheinterleavingworksquitewellformostvideoimages.
TheoriginaldesignersoftheNTSCsystemprobablyexpectedtechnologytoprovideaneconomicalmeanstoeffectivelyseparatetheY/Csignalsmuchsoonerthanitactuallyhappened.Combfilters,thepreferredsolution,datebacktomuchearlierthanthe1953standards,buttheyweretooexpensivetouse.
Factsfortechnicallyobsessed-SomeofyoumayliketoknowthattheCsignalismodulatedontoa3.58MHz(approx)colorsub-carrier.Thesmallerthepicturedetail,thehigherarethefrequencycomponentsinthesignal.Itisalsousefultoknowthataframeofvideooccursabout30timespersecond.Aframeofvideoconsistsoftwofieldsofvideo,withafieldrateofabout60Hz.Thehorizontalscanlinesfromonefieldareinterlacedwiththescanlinesinthenextfield.Aftereachlineinthefield,theCsignal'
sphaseisshiftedby180degrees.Inotherwords,theCsignal(butnottheYsignal)isinvertedateveryotherline.Similarly,ineachsuccessiveframe,theCsignalfromthecorrespondinghorizontalscanline,isalsoinverted.
Onceagain,pleaserefertoVideoSignalFormats,ifyouwouldlikeamoredetailedexplanationofwhatthecompositevideosignalandtheseparationproblemisallabout.
WhatTypeofProductsHaveY/CSeparationFilters?
TVMonitors
ATVmonitorcanreceivevideoinformationinseveraldifferentsignalformats.RFinputsareprovidedforantennaorcableTVsignals.TheseareusuallythreadedFconnectorsorscrewterminals.CompositevideoinputsandoutputsaresuppliedwithRCAjacks.Y/Cvideo(alsocalledS-video)connectorsshouldbeprovidedonanymonitorworthyofconsiderationforHomeTheaterapplications.Y/Ccablesareterminatedwithspecial4-pinDINconnectorsthatmatetomatchingsockets.ProfessionalandindustrialgrademonitorsuseBNCconnectorsforbothcompositeandY/CconnectionsandmayalsoincludedirectRGBinputs.
Amonitormayreceiveacompositevideosignaldirectlyfromanothervideocomponent,oritcanrecoverthecompositevideofromtheRFsignaldeliveredbyacablesystem,orover-the-air.ThemonitormustemploysomeformofY/CseparationfiltertorecovertheYandCsignalinformationfromthecompositevideosignal.IfamonitorreceivesY/Cvideosignalsfromanothervideocomponent,thenthemonitor'
sY/Cseparationfilterwillbebypassed.
OnceYandCsignalsareavailable,theCsignalisdemodulatedbackintoI&
Qsignals.ThentheR,G,BsignalsarerecoveredbydecodingtheY,I,&
Qsignalsinaspecialmatrix.TheR,G,BsignalsareusedtodrivetheCRT'
sthreeseparateelectronbeams,oneforeachcolor.
LaserdiscPlayers
VideoinformationisstoredasRFmodulatedcompositevideoonlaserdiscs.Thelaserdiscplayerrecoversthecompositevideosignalandprovidescompositevideooutputs.NearlyalllaserdiscplayersalsoincludeaY/CseparationfiltertogenerateY/Coutputsignals.
Itisacommonmistaketothinkthatonlycompositevideoconnectionsshouldbeusedbetweenalaserdiscplayerandamonitor,simplybecausethelaserdiscsignalisstoredascompositevideo.Thisseemslogical,sinceitavoidspassingthesignalthoughthelaserdiscplayer'
sY/Cseparationfilter.Butthislogicisfaultybecauseeventuallythecompositesignalwillstillhavetobeseparatedbythemonitor'
sY/Cseparationfilter.
Itisessentialtodeterminewhichcomponent,thelaserdiscplayerorthemonitor,hasthesuperiorY/Cseparationfilterandprovideaconnectionthatbypassestheinferiorfilter.Ifthemonitorhasthebetterfilter,thenthecompositevideooutputshouldbeused.Ifthelaserdiscplayerhasthebetterfilter,thentheY/Cconnectionshouldbeused.Theinformationintheremainderofthisguidewillhelpidentifywhichproducthasthesuperiorfilter.
VCR'
s&
Camcorders
AllconsumerVCR'
sandcamcordersrecordvideoinseparateYandCfrequencybands,notinthecompositevideoformat.ThisistrueregardlessofwhetherornotthecomponentincludesY/Cinputsandoutputs.Beforethesignalscanberecorded,theVCRmustseparatecompositevideoinputsignalsintoY/Cvideo.
Inthisrespect,connectinganLDplayertoaVCRcreatesthesamekindofinterconnectdecisionsasconnectingtheLDplayertoamonitor.Again,itisimperativetodeterminewhichcomponenthasthesuperiorY/Cseparationfilterandchoosetheinterconnectformataccordingly.
Ofcourse,ifyoumustconnectalaserdiscplayertoaVHSVCRthedecisionwillbemadeforyou,sincealmostnoVHSVCR'
shaveY/Cinputoroutputconnectors.(Butthenwhywouldyoueverwanttodosuchathing,anyway?
)
SinceVCR'
srecordvideoinseparateYandCfrequencybands,theVCR'
sY/Coutputshouldalwaysbeusedtoconnecttoamonitor,oranotherVCRfordubbing.ThisavoidssummingtheY/CsignalsintheoutputstageoftheVCRtocreateacompositesignal,andsplittingthemagainwithaY/Cseparationfilterintheotherproduct.
DVDPlayers
DVDplayersdonotstorevideointhecompositeformatandthereforetheydonothavecombfilters.Buttheycangenerateacompositevideooutputforcompatibilitywitholderorlow-endTVmonitors.ThecompositevideooutputofaDVDplayershouldonlybeusedasalastresort.Usetheanalogcomponentvideooutputsifyouhaveequipmentwithcompatiblecomponentvideoinputs.Otherwise,usetheY/Cvideooutput.ThisisexplainedinmoredetailintheaccompanyingarticleonVideoSignalFormats,whichdiscussestheDVD'
scomponentvideoformat.
SimpleAnalogFilters
Description
EarlyTVmonitorsandVCR'
susedsimpleanalogfiltersforY/Cseparation.Thistechniqueisonlyusedbytheleastexpensiveproductstoday.Butdon'
tskipthissection.ItwillintroducethebasicartifactscreatedbyincompleteY/Cseparation,andestablishamethodologythatcanbeusedfortestingandidentificationofthedifferentseparationfiltertechnologies.Itisfundamentaltounderstandingthecombfilterdiscussionsthatfollow.
InthismostbasicY/Cseparationtechnology,thecompositevideoissplitintoYandCsignalsbysimpleanalogfilters.TheYsignalisderivedbyfilteringoutthecolorinformationfromcompositevideowithalowpassornotchfilter.Thisworksbecausethecolorinformationisinafrequencybandcenteredatabout3.58MHzandextendsdowntoabout2.1-2.3MHz.(TheaccompanyingarticleonVideoSignalFormatsexplainsmoredetailsabouttheYandCsignal'
soverlappingfrequencyranges.)Dependingontheproduct,theYfilterwassettorejectfrequenciesaboveabout2.5MHz.AlthoughthiseffectivelyfiltersmostoftheCsignaloutoftheluminancesignal,italsoremovesthehigherfrequencyYsignalcomponents.Thislossofbandwidthreducesthehorizontalresolutionoftheluminancesignal,andfinedetailsinthepicturearelost.
IftheCinformationisnotcompletelyfilteredoutoftheYsignal,itcreatesinterferen
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