VGA视频接口转PAL制信号的电路图

./VGA视频接口转PAL制信号的电路图

我国许多飞机使用过类似彩色电视的PAL制显示器,在显示图形分辨率要求不是很苛刻的情况下,这些设备完全可以作为机载计算机的显示终端。但一般计算机的显示接口并不直接支持PAL制,需要在VGA接口和PAL制之间进行适当的转换。美国模拟器件公司的AD725就具有此项功能,图1是具体的转换电路。AD725支持PAL制和NTSC制式,PAL即逐行倒相正交平衡调幅制,是我国电视采用的标准;NTSC是全国电视系统委员会制式,是美用的标准。VGA接口主要包括2个同步信号和3个色彩信号,即水平同步<行同步>信号HSync,垂直同步<帧同步>信号VSync,以及R<红>、G<绿>、B<蓝>三色强度信号。其中,HSync和VSync是TTL逻辑电平,RGB是模拟信号。RGB输出经过交流耦合连接到AD725相应的输入,在靠近AD725附近要对模拟信号端接75的电阻,其电压最大峰-峰值为714mV。为增强抑制噪音,需要较大的输入阻容,每个输入端都接有0.1F的串联电容。HSync和VSync逻辑电平输入在AD725部共同产生综合同步。如果直接使用综合同步信号,则综合同步信号接HSync输入端,而VSync端要接高电平H<+2V>。PAL/NTSC模式转换由管脚STND决定,当输入STND=H时,芯片工作在NTSC模式；当STND=L时,芯片工作在PAL模式。图中工作模式由H1、H2调线设置,两者应同时为H或L。当工作在PAL制时,G1的晶振频率应选17.734475MHz；工作在NTSC模式时,G1应为14.318180MHz。图1中的视频输出提供两种方法：一种是所有的TV都有R/F输入,对应信号为管脚10的综合视频<COMP>输出；另一种采用亮度和色度组成图像控制,图中管脚11<LIMA>是亮度输出,管脚9<CRMA>是色度输出。VGA信号转换为彩色电视机PAL和NTSC制式信号本方摘自.chinaecnet.

本文介绍的转换器用于计算机VGA信号转换为彩色电视机PAL和NTSC制式信号,电路的主要技术参数如下。

电源：稳压的＋5V电源,电流小于300mA。

视频输入：RGB＋HSYNC＋VSYNC信号,取自VGA卡,刷新率与NTSC标准兼容。

视频输出：混合视频和S－视频〔Y/C。

支持的视频标准：PALB、G、H和NTSCM。

电路要求VGA卡能发送与PAL或NTSC标准视频时序兼容的RGB格式视频信号。

工作原理

电路原理如附图所示。

该电路以模拟集成电路AD722为基础,AD722是可将RGB信号转换为NTSC/PAL信号的编码器,产生与PAL或NTSC标准一致的、对应的亮度〔基带振幅和彩色信号〔调幅和调相,还可将亮度和彩色信号混合生成混合视频输出,所有输出可直接驱动标准75Ω端口的视频电缆,无需额外的放大器,该器件只需一组＋5V电源,而且无需外部延迟线或滤波器。

该转换器仅需很少的外部电子元件,但需外接时钟晶体振荡器,该电路可为PAL提供4.43MHz、为NTSC提供3.58MHz的色载波频率,还提供用于电源去耦和合适的视频线路端口〔输入和输出所需元件。

电路最复杂的部分是同步信号处理。因为从VGA卡送出的同步信号是任意极性,当信号进入转换电路U1〔四2输入异或门,用于极性转换时,围绕着VGA的U1及电路将保证同步信号总是处于正确的极性,该电路总是处于工作状态；但是,有些VGA卡对于某些图形模式,很难产生宽度非常准确的HSYNC信号,而HSYNC的宽度必须与合适的彩色视频信号接收保持一致。在电路中,围绕着U2〔555定时器的单稳多谐振荡器为LSYNC信号产生合适的脉冲宽度,该电路的脉冲宽度由R4调整。电路的使用

使用该电路时,必须插入从VGA转换到TV的驱动卡,其功能是使VGA卡产生正确格式和刷新率的图形信号。该电路的驱动卡与从VGA转换到SCART的电路驱动卡相同。然后,选择与视频输出标准相匹配的输出标准。接着按附表设置开关SW1和SW2,以匹配所使用的视频输出标准。

利用R4可以调整HSYNC信号的宽度,NTSC信号要求4.59ms,而PAL要求4.6ms。如果HSYNC信号脉冲宽度不对,电视机在接收彩色信号时就会出现问题,甚至根本不能与视频信号保持同步。调试HSYNC信号最好使用示波器、矢量显示器或频率计数器〔具有周期测量功能等。如果没有这些仪器,则可调试R4,使电视机的彩色信号能良好地工作,通常要对PAL和NTSC标准〔二者之间的时序非常接近做同样的设置工作。

最后,微调彩色载频,C6用于微调NTSC载频,而C7用于微调PAL载频,调整载频的目的是获得最佳的彩色形成,应使用矢量显示器或视频分析仪,否则,只能得到接近于电视机的设置。

另外还需准备以下材料。

75Ω的BNC插头〔可安装PCB。

4针的微型插头〔可安装PCB。

金属盒。制作

制作时,小心地将细线与AD722集成电路的每只引脚相接,然后把它固定在印制板上〔并非容易的任务,但做起来挺有趣。制作印制板时,可以以合适的分辨率,将电路图输出到激光打印机打印〔6.83×3.87英寸,以自己制造电路板。改进措施

由于AD722已经淘汰,AD724是可用来替换AD722的最接近的器件,幸运的是,AD724与AD722非常相符,并具有相同的引脚功能,唯一需要修改的是,RGB的输入第⑥、⑦、⑧脚各需增长1只0.1μF瓷或塑料电容器,该电容器接在75Ω端口电阻和AD724RGB输入之间使用AD724的RGB→ＮＴＳＣ／ＰＡＬ制信号转换电路类别：电子综合

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本文介绍的信号转换电路可将计算机显卡VGA端口输出的RGB信号转换为NTSC/PAL制式信号,适用于普通电视接收和显示不带TV-OUT的显卡输出的图像信号。此电路是目前低成本、效果较好的信号转换电路之一,其电路原理图如图1所示。此电路仅需很少的外部分立电子元件,核心元件为一片AnalogDevices公司出品的模拟集成电路AD724oAD724是可将RGB信号转换为NTSC/PAL信号的编码器,产生与PAL或NTSC标准一致的、对应的亮度信号和彩色信号。还可将亮度信号和彩色信号混合生成混合视频输出,所有输出均可直接驱动标准75Ω端口的视频电缆,无需再加额外的驱动放大器,并且不需要外部延迟线或滤波器,简化了电路,方便需要的爱好者自制。该集成电路采用稳压的单+5V电源供电,工作电流小于300mA,较低的功耗使得其能长时间稳定地工作。AD724的封装示意及各引脚定义见图2及表2。该转换器所需晶振的理论精确频率在PAL输出模式时为4.433620MHz,NTSC输出模式时为3.579545MHz,但实际应用中可分别选用频率为4.43MHz和3.58MHz的晶振,通过微调和它们并联的电容可以微调彩色载频。C8用于微调PAL载频、C9用于微调NTSC载频以获得最佳的彩色形成。按表1设置S1和S2,可以使电路输出在NTSC信号和PAL信号之间切换,以选取所需要使用的视频信号格式。该转换器中的Y1,Y2,C8,C9需加屏蔽罩,所用的容量为O.1μF的电容均要使用瓷片电容,其余元件无特殊要求,按图1中所标选取使用即可。因为AD724为CMOS电路,制作过程中应注意避免人体静电和烙铁的感应电对其造成损坏。且其为16脚的贴片封装,引脚较细较密,业余制作时要小心用细线焊接将其引脚引出,整个转换电路均可安装在通用试验板上。制作成的电路板要安装到一个金属盒避免外界电磁干扰。如果该转换器再加上射频调制器和高频功放,则可将电脑显示的容以开路的方式发射出去,便于在一定围传播信号,免去了布线的麻烦,扩展了其用途,在某些场合还是具有一定使用价值的。有需要者可自行参考相关电路,但要注意的是现在大多数成品射频调试器的输人视频信号都要PAL制式信号,以免在制作调试过程中多走弯路。VGAtoRGB+compositesync-converter

DesignedbyTomiEngdahlThisarticleconsistsofonecircuitswhichIhavedesignedforconnectingVGAcardtootherdisplaydevicesthanjustaVGAmonitor.ThiscircuitsmakesitpossibletoconnectyourVGAcardtofixedfrequencymonitorsandvideoprojectors.Forsomeapplicationsandsystemcombinationssomeadditionalsoftwareisneededtogetthesystemworking.Ikeepalltherightstothiscircuit.Youcanfreelybuildthiscircuitforyourownusebutyouarenotallowedtousethosecircuitdesignscommerciallywithoutwrittenpermissionfromthedesigner.VGAtoRGB+CompositeSyncconverterFirstcircuitisforconnectingVGAcardtovideoprojectororamonitorwhichacceptVGAcardfrequenciesandhasRGB+Compositesyncinput.ThiscircuithasbeensuccesfullyusedwithElectrohomeProjectionSystemsECP4100dataandvideoprojector.ThiscircuitisdesignedforconvertingnormalVGAsignalsstandardRGBsignalsandcompositesyncsignal.Thecircuitisquitesimple,becauseRGBsignalouputfromVGAcardisalreadystandard0.7Vppto75ohmload.Forsyncsignalsthereisacircuitwhichcombineshorizonalandverticalsyncsignalstoformcompositesyncsingals.ThecircuitissimplybasedononeTTLchipwithfourXORports,tworesistorsandtwocapacitors.TTLchipwslogicalchoisebecauseVGAsyncsignalsareTTLlevelsignals.Thesyncsignalcombinerhasasystemtoadjusttodifferentsyncpolaritiessothatitalwaysmakescorrectcompositesyncsignals.VGAcardusesdifferentsyncsignalpolaritiestotellthemonitorwhichresolutionisused.Thiscircuitadjuststosyncsignalpolaritychangesinlessthan200milliseconds,whichisfasterthansettingtimeofanormalVGAmonitorinthedisplaymodechange.AnaloguecompositesyncsignalconverterFirstcircuitisforconnectingVGAcardtovideoprojectororamonitorwhichacceptVGAcardfrequenciesandhasRGB+Compositesyncinput<1Vppsignallevel,75ohminputimpedance>.ThispictureisavailableinGIFandPostscriptversions.Note:ThecircuitdiagramdoesnotshowthewiregivingpowertoIC1.The+5Vpowerisconnectedtopin14andgroundisconnectedtopin7.ThiscircuithasbeensuccesfullyusedwithElectrohomeProjectionSystemsECP4100videoprojectorinmanyVGAandSuperVGAmodes.ThecircuithavebeenalsousedsuccesfullywithoneoldBarcovideoprojectorusingmyVGAtoTVdriverstogettheVGAcardtogeneratesuitablesignalfrequencieswhichthatoldvideoprojectorcanhandle.ComponentlistC122uF10VelectrolyticC222uF10VelectrolyticR12.2kohmR22.2kohmR31.8kohmR41.8kohmR52.7kohmR6..R847ohmU174LS86or74HCT86T1BC547T2BC547TTLlevelsyncsignalManycomputermonitorshavebeendesignedtoacceptTTLlevelsyncsignals.IfyouhappentoownamonitorwhichusesTTLlevelsyncsignalsthecircuitabovedoesworkwithwithit,becausethesyncsignallevelfromthatcircuitisnotenoughforthemonitor.IhavedesignedanothersimplercircuitformonitorswhichneedTTLlevels.Note:ThecircuitdiagramdoesnotshowthewiregivingpowertoIC1.The+5Vpowerisconnectedtopin14andgroundisconnectedtopin7.Thiscircuitisbasicallythesameasthefirstcircuit.Theonlyonlydifferenceisthatthetransistordriverstagehavebeenleftoutfromthecircuit,becausenotsomuchdrivingcapacityisneededandTTLlevelssignalsfromtheICarewhatiswanted.Becausethe74LS86ICdirectlydrivestheoutputwithoutanyfurtherbuffering,itisonlysuitablefordrivinghighimpedance<1kohmormore>syncinputs.ComponentlistC122uF10VelectrolyticC222uF10VelectrolyticR12.2kohmR22.2kohmU174LS86or74HCT86SimplesynccombiningBOXSometimesyouwanttouseaready-madeBNCcablemadeforconnectingVGAcardtohighqualitymonitor.ThoseBNCcableshave5BNCoutputswithfollowingfunctions:SignalFunctionColorincableRRedRedGGreenGreenBBlueBlueH/HVHsync<+Vsync>BlackVVsyncWhiteorGrayThacablesareusuallybuilt<shouldbebuilt!>fromhighquality75ohmcoaxialcableand75ohmBNCconenctorstoguaranteegoodimagequality.YoucanseeatypicalschematicofcommercialVGAtoBNCcablebelow:Thiskindofcablecanbeusedwithmonitorswith5BNCinputs.ButifyouunfortinatelyhaveamonitorwithRGBandcompositesyncinputsonly,thatcablemightnotbeofanyuse,unlessyourgraphicscardcangeneratecompositesyncsignals<thisisnotastandardoption>.Ifyouagetcompositesyncdirectlyfromgraphicscard,thenitisenoughtojustconnecttheH/HVconnectortothecompositesyncpinputofthemonitor.Ifyouareonunfortunatepersonwhodeosnothavethiskindofgraphicscardoryourdriversdon'tallowusingthatoption,thenyouronlychoiceistobuildacircuitwhichcombinestheseparateHSYNCandVSYNCsignalstocompositesync.Youcanseethiskindofcircuitbelow.ComponentlistC122uF10VelectrolyticC222uF10VelectrolyticR12.2kohmR22.2kohmU174LS86or74HCT86ConnectingVGAtoSync-on-GreenmonitorSomemonitorsusesync-on-greensyncronizationinsteadofseparatesync.Inthatcaseyoucan'tusedmycircuitsdirectly.YouhavetomodifythosecircuitstoaddSync-on-Greencapability.Generatingsyncsignalstoscreenpicturecomponentisquiteeasy.Ihavenothadanyneedforsuchacircuit,soIhaven'tdesignedsuchoption.Checkthisarticletogettheideawhattoaddtomycircuits.Formorecompleteexplanationhowtodealwithsyncongreenmonitors,checktheSynconGreenFAQ.BuildingthecircuitThecircuitisquitesimpletobuiltifyouhavebasicskillsinbuildingelectroniccircuits.Theelectronicsofthecircuitscanbeeasilybuilttoasmallpieceofveroboardsonospecialcircuitboardisnecessarilyneeded<Iusedthisapproachinmyprototype>.IhavedesignedacircuitboardformyVGAtoTVconverterandthissamePCBdesign<GIFpicturescannedat300x300dots/incresolution,alsoavailableinzippedpostscriptfileformat>andcomponentlayoutcanbeusedwiththisprojectalso.NotethattherearesomedifferencesinconnectionoftheICU1pinsinPCBcomparedtotheschematic,butthecircuitbuilttothisPCBfuctionsinthesameway.Thecircuitneedwell-stabilizedpower+5Vpowersource<+/-5%>.Thecircuittakeslessthan150mAcurrent,soyoudon'tneedalargepowersupply.Ifyoudon'thaveanythignsuitableavalable,youcanalwaysuseasmallgeneralpurposewalltransformerandasmall+5Vvoltageregulationcircuit.Anotheroptionistotakethe+5VpowerfromPCusingmethodsexplainedinmyHowtogetpowerfromPCtoyourcircuitsdocument.Thewiringshouldbemadecarefullyand75ohmcoaxialcablingforpicturesignalsshouldbeusedeverywhere,especiallyifyouaregoingtousethiscircuitwithahighresolutionmonitor.Signalsreclectionscausedbybadwiringcanreallymessupyourpicturequality.Iwouldrecommendusingmetalbox,shieldedcablesandshieldedconnectorsforcircuittokeepnoiseandradiointerferenceminimum.VGAtoRGBSconvertercomponentlistU174LS86C1,C222microfarads,16VR1,R22.2kohm,1/4WR3,R41.8kohm,1/4WR52.7kohm,1/4WR6,R7,R947ohm,1/2WR8120ohm,1/2WT1,T2BC547BP115pinSUB-DconnectorOutputconnector:75ohmBNCconnectors<youneed4BNCconnectors>Wiring:Red,Green,BlueandCompositeSynclinesshouldbewiredusing75ohmcoaxialcable.IfyouarebuildingmyTTLleveloutputmodel,youcanleaveoutR3-R8andT1-T2outofthislist.GettingyourequipmenttoacceptsignalfromPCManymoderncomptermonitorsaremultisyncmonitorswhichaccedpteasilywideselectionofdifferentrefreshrates,scanfrequenciesandresolutions.Thissameappliestomoderndataprojectors.Butmanyoldermonitors<monitorsnotmadeforPC>andvideoprojectorscan'tacceptthesignalsyourVGAcardputsout.Ifyourvideoprojectionsystemormonitordon'tacceptVGAscanningfrequencesyouhavetogetasuitabledriverforadjustingyourVGAfrequencies.FornormalVGAmodesthehorizonalsyncfrequencyis31250Hzandverticalsyncfrequencyis60or72Hz.ForinformationaboutSuperVGAmodesyouhavetoconsultyoudisplaycardadaptertechnicaldata.AnotheruseforthesamecircuitistoconnecttoconnectVGAcardtoalargeoldcolormonitorfromanoldworkstation.IfyoumanagetosetyourVGAadaptertogeneratesuitablescanningfrequeciesforthemonitoryouhave,youcanuseitwithyourVGAcard.Formoreinformationhowtodothis,checktheVGAtoworkstationmonitorFAQ.HowtofindfixedfrequencymonitortechnicaldataBeforyoucanstartenyusefulexperimentinghavetofindoutthehorizonalfrequencyandtherefreshratesyourmonitorneeds.Ifyoutrytousethemonitorwithwrongfrequencysignalsthemonitordoesnotworkproperlyandsomemonitorscanbedamagedifyoutrytousethematwrongfrequency.Yourmonitoruser'sguidetechnicalinfosectionisthefirstplacetocheck.Ifthatdoesnothelptrythemanufacturer'swebsite.MonitordatabaseslikeTheWorldWideWebMonitorDatabaseandAppleMacintoshMonitorDatabaseareveryusefulinformationsourcesforgettingthisinformation.Ifyoucan'tfindanytechnicalinfoontheexactmonitoryouhaveyoumightchecktheinformationaboutsomeverysimilarmonitormodel<forexamplesomeolderrevisionofthatmodel>anduseitassomekindstartingpoint.IfyouknowthecomputermodelwherethemonitorwasconnectedyoucantrytolookatthetechnicalspecsorsomeFAQofthatcomputer<ifyougettoknowwhatkindofsignalthatcomputerhassenttomonitoryouknowthatthemonitorcanatleastshowit>.HowtoconfigureWindowsYourvideocarddriversquitepropablyhasoptionstosetthethescreenrefreshrate.Checkyougraphicscardmanualanddriverdocumentshowtoadjustthosesettings<thisdependsongraphicscards>.Trytofindarefreshrateandresolutionwhichmatchesyoumonitorspecifications.Typicallytheselectionsintypicaldriversarequitelimitedandmanydriver'sdon'thaveanyfurtheroptionsforusercustomization.MatroxhasaddedexcellentuserconfigurabilityoprtionsfortheirMilleniumandMystiquegraphicscardWindows95drivers.Formor