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HSYNC#
(VCTL1)
VCLKIN
VSYNC#
(VCTL2)
64 Clocks
64 Clocks
3.4.5ShortandLongFieldDetect
3.5VideoInputFiltering
VideoInputFiltering
ThefieldindicatormethodusestheFIDinputdirectlytodeterminethecurrentfield.ThisisusefulforY/C
datastreamsthatdonothaveembeddedEAVandSAVcodes.TheFIDinputissampledatthestartof
eachfield.IfFID=0,thenfield1isstarting;ifFID=1,thenfield2isstarting.Thestartofeachfieldis
definedbytheVRSTbitinVCxCTLandiseitherthestartorendofverticalblankingasdeterminedbythe
VBLNKinput.TheFINVbitmaybeusedinthismethodinsystemswheretheFIDinputhastheopposite
polarityorwherethefieldidentificationchangelagsthestartofthefield.
ThefielddetectmethodusesHYSNCandVSYNCbasedfielddetectlogic.ThisisusedforBT.656orY/C
systemsthatprovideonlyHSYNCandVSYNC.ThefielddetectlogicsamplesthestateoftheHSYNC
inputontheVSYNCactiveedge.IfHSYNCisactiveontheactiveVSYNCedge,thenfield1isdetected;if
HSYNCisinactiveontheactiveVSYNCedge,thenfield2isdetected.Becauseofslighttimingvariations,
theVSYNCtransitionmaynotcoincideexactlywiththeHSYNCtransition.Thedetectionlogicshould
implementa±64clockdetectionwindowaroundHSYNC.IfbothHSYNCandVSYNCleadingedgesoccur
within64cyclesofeachother,thenfield1isdetected;otherwise,field2isassumed.Thisisshownin
Figure3-7foractive-lowsyncsignals.
Figure3-7.Field1DetectionTiming
TheshortandlongfielddetectlogicisusedtonotifytheDSPwhenacapturedfieldshorterorlongerthan
expected.DetectionisenabledbytheSFDEandLFDEbitsinVCxCTL.TheSFDandLFDbitsinVPIS
indicatewhenashortorlongfieldoccurredandtriggeraninterrupttotheDSPifenabled.
Ifaverticalblankingperiodisdetectedbeforetheendofthecapturefield,ashortfieldisdetected.IfEAV
isusedforverticalsync(EXC=0),thenashortfieldisdetectedwhenanEAVwithV=1occursonor
beforeVCOUNT=VCYSTOPn.IftheVCTL2inputisusedforverticalsync(EXC=1),thenashortfieldis
detectedifaVCTL2activeedgeoccursbeforeVCOUNT=(VCYSTOPn).
Ifaverticalblankingperiodoccursmorethan1linepasttheendofthecapturefield,alongfieldis
detected.AlongfieldisdetectedwhenVCOUNT=VCYSTOPn+1.(Alongfieldisonlydetectedwhen
theVRSTbitinVCxCTLisclearedto0;whenVRST=1,alongfieldisalwaysdetected.)Longfield
detectioncannotbeusedifthecapturewindowisaverticalsubsetofthefieldthatcropslinesatthe
bottom.Suchawindowwouldalwaysresultinalongfielddetection.IfVCTL2isusedforverticalsync,
thentheVCTL2signalmustrepresentVBLNK(verticalblank)forproperlongfielddetect.IfVCTL2isa
VSYNC(verticalsync)input,thenalongfieldisalwaysdetected.(EvenifVCYSTOPnissettothelast
activeline,VCOUNTusuallyincrementspastVCYSTOPn+1whileitcountstheverticalfrontporchlines
thatoccurpriortoVSYNCactive.)LongfielddetectionisonlyavailablewhenVRSTisconfiguredtobe
resetatthestartofverticalblanking(VRST=0inVCX_CTL).
Thevideoinputfilterperformssimplehardwarescalingandre-samplingonincoming8-bitBT.656or8-bit
Y/Cdata.Filteringhardwareisalwaysdisabledduringrawdatacapturemodes.Forproperfilteroperation,
thechannel'sEXCbitinVCxCTLmustbeclearedto0(embeddedtimingreferencecodesused)andthe
CAPENinputmustnotgoinactiveduringtheactivevideowindow.
SPRUEM1–May2007VideoCapturePort57
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