VGA視頻接口轉(zhuǎn)PAL制信號(hào)的電路圖

./VGA視頻接口轉(zhuǎn)PAL制信號(hào)的電路圖

我國(guó)許多飛機(jī)使用過(guò)類似彩色電視的PAL制顯示器,在顯示圖形分辨率要求不是很苛刻的情況下,這些設(shè)備完全可以作為機(jī)載計(jì)算機(jī)的顯示終端。但一般計(jì)算機(jī)的顯示接口并不直接支持PAL制,需要在VGA接口和PAL制之間進(jìn)行適當(dāng)?shù)霓D(zhuǎn)換。美國(guó)模擬器件公司的AD725就具有此項(xiàng)功能,圖1是具體的轉(zhuǎn)換電路。AD725支持PAL制和NTSC制式,PAL即逐行倒相正交平衡調(diào)幅制,是我國(guó)電視采用的標(biāo)準(zhǔn);NTSC是全國(guó)電視系統(tǒng)委員會(huì)制式,是美用的標(biāo)準(zhǔn)。VGA接口主要包括2個(gè)同步信號(hào)和3個(gè)色彩信號(hào),即水平同步<行同步>信號(hào)HSync,垂直同步<幀同步>信號(hào)VSync,以及R<紅>、G<綠>、B<藍(lán)>三色強(qiáng)度信號(hào)。其中,HSync和VSync是TTL邏輯電平,RGB是模擬信號(hào)。RGB輸出經(jīng)過(guò)交流耦合連接到AD725相應(yīng)的輸入,在靠近AD725附近要對(duì)模擬信號(hào)端接75的電阻,其電壓最大峰-峰值為714mV。為增強(qiáng)抑制噪音,需要較大的輸入阻容,每個(gè)輸入端都接有0.1F的串聯(lián)電容。HSync和VSync邏輯電平輸入在AD725部共同產(chǎn)生綜合同步。如果直接使用綜合同步信號(hào),則綜合同步信號(hào)接HSync輸入端,而VSync端要接高電平H<+2V>。PAL/NTSC模式轉(zhuǎn)換由管腳STND決定,當(dāng)輸入STND=H時(shí),芯片工作在NTSC模式；當(dāng)STND=L時(shí),芯片工作在PAL模式。圖中工作模式由H1、H2調(diào)線設(shè)置,兩者應(yīng)同時(shí)為H或L。當(dāng)工作在PAL制時(shí),G1的晶振頻率應(yīng)選17.734475MHz；工作在NTSC模式時(shí),G1應(yīng)為14.318180MHz。圖1中的視頻輸出提供兩種方法：一種是所有的TV都有R/F輸入,對(duì)應(yīng)信號(hào)為管腳10的綜合視頻<COMP>輸出；另一種采用亮度和色度組成圖像控制,圖中管腳11<LIMA>是亮度輸出,管腳9<CRMA>是色度輸出。VGA信號(hào)轉(zhuǎn)換為彩色電視機(jī)PAL和NTSC制式信號(hào)本方摘自.chinaecnet.

本文介紹的轉(zhuǎn)換器用于計(jì)算機(jī)VGA信號(hào)轉(zhuǎn)換為彩色電視機(jī)PAL和NTSC制式信號(hào),電路的主要技術(shù)參數(shù)如下。

電源：穩(wěn)壓的＋5V電源,電流小于300mA。

視頻輸入：RGB＋HSYNC＋VSYNC信號(hào),取自VGA卡,刷新率與NTSC標(biāo)準(zhǔn)兼容。

視頻輸出：混合視頻和S－視頻〔Y/C。

支持的視頻標(biāo)準(zhǔn)：PALB、G、H和NTSCM。

電路要求VGA卡能發(fā)送與PAL或NTSC標(biāo)準(zhǔn)視頻時(shí)序兼容的RGB格式視頻信號(hào)。

工作原理

電路原理如附圖所示。

該電路以模擬集成電路AD722為基礎(chǔ),AD722是可將RGB信號(hào)轉(zhuǎn)換為NTSC/PAL信號(hào)的編碼器,產(chǎn)生與PAL或NTSC標(biāo)準(zhǔn)一致的、對(duì)應(yīng)的亮度〔基帶振幅和彩色信號(hào)〔調(diào)幅和調(diào)相,還可將亮度和彩色信號(hào)混合生成混合視頻輸出,所有輸出可直接驅(qū)動(dòng)標(biāo)準(zhǔn)75Ω端口的視頻電纜,無(wú)需額外的放大器,該器件只需一組＋5V電源,而且無(wú)需外部延遲線或?yàn)V波器。

該轉(zhuǎn)換器僅需很少的外部電子元件,但需外接時(shí)鐘晶體振蕩器,該電路可為PAL提供4.43MHz、為NTSC提供3.58MHz的色載波頻率,還提供用于電源去耦和合適的視頻線路端口〔輸入和輸出所需元件。

電路最復(fù)雜的部分是同步信號(hào)處理。因?yàn)閺腣GA卡送出的同步信號(hào)是任意極性,當(dāng)信號(hào)進(jìn)入轉(zhuǎn)換電路U1〔四2輸入異或門,用于極性轉(zhuǎn)換時(shí),圍繞著VGA的U1及電路將保證同步信號(hào)總是處于正確的極性,該電路總是處于工作狀態(tài)；但是,有些VGA卡對(duì)于某些圖形模式,很難產(chǎn)生寬度非常準(zhǔn)確的HSYNC信號(hào),而HSYNC的寬度必須與合適的彩色視頻信號(hào)接收保持一致。在電路中,圍繞著U2〔555定時(shí)器的單穩(wěn)多諧振蕩器為L(zhǎng)SYNC信號(hào)產(chǎn)生合適的脈沖寬度,該電路的脈沖寬度由R4調(diào)整。電路的使用

使用該電路時(shí),必須插入從VGA轉(zhuǎn)換到TV的驅(qū)動(dòng)卡,其功能是使VGA卡產(chǎn)生正確格式和刷新率的圖形信號(hào)。該電路的驅(qū)動(dòng)卡與從VGA轉(zhuǎn)換到SCART的電路驅(qū)動(dòng)卡相同。然后,選擇與視頻輸出標(biāo)準(zhǔn)相匹配的輸出標(biāo)準(zhǔn)。接著按附表設(shè)置開(kāi)關(guān)SW1和SW2,以匹配所使用的視頻輸出標(biāo)準(zhǔn)。

利用R4可以調(diào)整HSYNC信號(hào)的寬度,NTSC信號(hào)要求4.59ms,而PAL要求4.6ms。如果HSYNC信號(hào)脈沖寬度不對(duì),電視機(jī)在接收彩色信號(hào)時(shí)就會(huì)出現(xiàn)問(wèn)題,甚至根本不能與視頻信號(hào)保持同步。調(diào)試HSYNC信號(hào)最好使用示波器、矢量顯示器或頻率計(jì)數(shù)器〔具有周期測(cè)量功能等。如果沒(méi)有這些儀器,則可調(diào)試R4,使電視機(jī)的彩色信號(hào)能良好地工作,通常要對(duì)PAL和NTSC標(biāo)準(zhǔn)〔二者之間的時(shí)序非常接近做同樣的設(shè)置工作。

最后,微調(diào)彩色載頻,C6用于微調(diào)NTSC載頻,而C7用于微調(diào)PAL載頻,調(diào)整載頻的目的是獲得最佳的彩色形成,應(yīng)使用矢量顯示器或視頻分析儀,否則,只能得到接近于電視機(jī)的設(shè)置。

另外還需準(zhǔn)備以下材料。

75Ω的BNC插頭〔可安裝PCB。

4針的微型插頭〔可安裝PCB。

金屬盒。制作

制作時(shí),小心地將細(xì)線與AD722集成電路的每只引腳相接,然后把它固定在印制板上〔并非容易的任務(wù),但做起來(lái)挺有趣。制作印制板時(shí),可以以合適的分辨率,將電路圖輸出到激光打印機(jī)打印〔6.83×3.87英寸,以自己制造電路板。改進(jìn)措施

由于AD722已經(jīng)淘汰,AD724是可用來(lái)替換AD722的最接近的器件,幸運(yùn)的是,AD724與AD722非常相符,并具有相同的引腳功能,唯一需要修改的是,RGB的輸入第⑥、⑦、⑧腳各需增長(zhǎng)1只0.1μF瓷或塑料電容器,該電容器接在75Ω端口電阻和AD724RGB輸入之間使用AD724的RGB→ＮＴＳＣ／ＰＡＬ制信號(hào)轉(zhuǎn)換電路類別：電子綜合

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本文介紹的信號(hào)轉(zhuǎn)換電路可將計(jì)算機(jī)顯卡VGA端口輸出的RGB信號(hào)轉(zhuǎn)換為NTSC/PAL制式信號(hào),適用于普通電視接收和顯示不帶TV-OUT的顯卡輸出的圖像信號(hào)。此電路是目前低成本、效果較好的信號(hào)轉(zhuǎn)換電路之一,其電路原理圖如圖1所示。此電路僅需很少的外部分立電子元件,核心元件為一片AnalogDevices公司出品的模擬集成電路AD724oAD724是可將RGB信號(hào)轉(zhuǎn)換為NTSC/PAL信號(hào)的編碼器,產(chǎn)生與PAL或NTSC標(biāo)準(zhǔn)一致的、對(duì)應(yīng)的亮度信號(hào)和彩色信號(hào)。還可將亮度信號(hào)和彩色信號(hào)混合生成混合視頻輸出,所有輸出均可直接驅(qū)動(dòng)標(biāo)準(zhǔn)75Ω端口的視頻電纜,無(wú)需再加額外的驅(qū)動(dòng)放大器,并且不需要外部延遲線或?yàn)V波器,簡(jiǎn)化了電路,方便需要的愛(ài)好者自制。該集成電路采用穩(wěn)壓的單+5V電源供電,工作電流小于300mA,較低的功耗使得其能長(zhǎng)時(shí)間穩(wěn)定地工作。AD724的封裝示意及各引腳定義見(jiàn)圖2及表2。該轉(zhuǎn)換器所需晶振的理論精確頻率在PAL輸出模式時(shí)為4.433620MHz,NTSC輸出模式時(shí)為3.579545MHz,但實(shí)際應(yīng)用中可分別選用頻率為4.43MHz和3.58MHz的晶振,通過(guò)微調(diào)和它們并聯(lián)的電容可以微調(diào)彩色載頻。C8用于微調(diào)PAL載頻、C9用于微調(diào)NTSC載頻以獲得最佳的彩色形成。按表1設(shè)置S1和S2,可以使電路輸出在NTSC信號(hào)和PAL信號(hào)之間切換,以選取所需要使用的視頻信號(hào)格式。該轉(zhuǎn)換器中的Y1,Y2,C8,C9需加屏蔽罩,所用的容量為O.1μF的電容均要使用瓷片電容,其余元件無(wú)特殊要求,按圖1中所標(biāo)選取使用即可。因?yàn)锳D724為CMOS電路,制作過(guò)程中應(yīng)注意避免人體靜電和烙鐵的感應(yīng)電對(duì)其造成損壞。且其為16腳的貼片封裝,引腳較細(xì)較密,業(yè)余制作時(shí)要小心用細(xì)線焊接將其引腳引出,整個(gè)轉(zhuǎn)換電路均可安裝在通用試驗(yàn)板上。制作成的電路板要安裝到一個(gè)金屬盒避免外界電磁干擾。如果該轉(zhuǎn)換器再加上射頻調(diào)制器和高頻功放,則可將電腦顯示的容以開(kāi)路的方式發(fā)射出去,便于在一定圍傳播信號(hào),免去了布線的麻煩,擴(kuò)展了其用途,在某些場(chǎng)合還是具有一定使用價(jià)值的。有需要者可自行參考相關(guān)電路,但要注意的是現(xiàn)在大多數(shù)成品射頻調(diào)試器的輸人視頻信號(hào)都要PAL制式信號(hào),以免在制作調(diào)試過(guò)程中多走彎路。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