梳状滤波器.docx

梳状滤波器.docx

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梳状滤波器

Y/CSeparationBasics

ThePurposeofthisGuide

Thepurposeofacombfilteristoseparateacompositevideosignal'sluminance（Y）informationfromitscolor（C）information.ThereareseveralwaystoperformY/Cseparationandcombfiltersarethemostcommon.Therearedifferenttypesofcombfiltersandtheyvarywidelyinperformance.Thedifferencesaredirectlyobservableinthevideoartifactsthatexistafterseparation.InthisarticleIwillexplainwhatartifactstoexpectformeachtypeofY/Cseparationfilterandwheretolookforthem.Thebestwaytounderstandtheadvantagesofthedifferentcombfiltertechnologiesistostartwithalittleinformationaboutthecompositevideosignal.

TheCompositeVideoSignal

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