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，是美国采用的标准。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0.1 F的串联电容。HSync和VSync逻辑电平输入在AD725内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

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：PAL B、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陶瓷或塑料电容器，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 阅读：2277

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Analog Devices公司出品的模拟集成电路AD724 o AD724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内避免外界电磁干扰。

如果该转换器再加上射频调制器和高频功放，则可将电脑显示的内容以开路的方式发射出去，便于在一定范围内传播信号，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求是PAL制式信号，以免在制作调试过程中多走弯路。

VGA to RGB + composite sync -converter

Designed by Tomi Engdahl

This article consists of one circuits which I have designed for connecting VGA card to other

display devices than just a VGA monitor. This circuits makes it possible to connect your VGA

card to fixed frequency monitors and video projectors. For some applications and system

combinations some additional software is needed to get the system working.

I keep all the rights to this circuit. You can freely build this circuit for your own use but you are

not allowed to use those circuit designs commercially without written permission from the

designer.

VGA to RGB + Composite Sync converter

First circuit is for connecting VGA card to video projector or a monitor which accept VGA card

frequencies and has RGB + Composite sync input. This circuit has been succesfully used with

Electrohome Projection Systems ECP 4100 data and video projector.

This circuit is designed for converting normal VGA signals standard RGB signals and

composite sync signal. The circuit is quite simple, because RGB signal ouput from VGA card

is already standard 0.7Vpp to 75 ohm load.

For sync signals there is a circuit which combines horizonal and vertical sync signals to form

composite sync singals. The circuit is simply based on one TTL chip with four XOR ports, two

resistors and two capacitors. TTL chip ws logical choise because VGA sync signals are TTL

level signals.

The sync signal combiner has a system to adjust to different sync polarities so that it always

makes correct composite sync signals. VGA card uses different sync signal polarities to tell

the monitor which resolution is used. This circuit adjusts to sync signal polarity changes in

less than 200 milliseconds, which is faster than setting time of a normal VGA monitor in the

display mode change.

Analogue composite sync signal converter

First circuit is for connecting VGA card to video projector or a monitor which accept VGA card

frequencies and has RGB + Composite sync input (1 Vpp signal level, 75 ohm input

impedance).

This picture is available in GIF and Postscript versions.

Note: The circuit diagram does not show the wire giving power to IC1. The +5V power is

connected to pin 14 and ground is connected to pin 7.

This circuit has been succesfully used with Electrohome Projection Systems ECP 4100 video

projector in many VGA and SuperVGA modes. The circuit have been also used succesfully

with one old Barco video projector using my VGA to TV drivers to get the VGA card to

generate suitable signal frequencies which that old video projector can handle.

Component list

C1 22 uF 10V electrolytic

C2 22 uF 10V electrolytic

R1 2.2 kohm

R2 2.2 kohm

R3 1.8 kohm

R4 1.8 kohm

R5 2.7 kohm

R6..R8 47 ohm

U1 74LS86 or 74HCT86

T1 BC 547

T2 BC 547

TTL level sync signal

Many computer monitors have been designed to accept TTL level sync signals. If you happen

to own a monitor which uses TTL level sync signals the circuit above does work with with it,

because the sync signal level from that circuit is not enough for the monitor. I have designed

another simpler circuit for monitors which need TTL levels.

Note: The circuit diagram does not show the wire giving power to IC1. The +5V power is

connected to pin 14 and ground is connected to pin 7.

This circuit is basically the same as the first circuit. The only only difference is that the

transistor driver stage have been left out from the circuit, because not so much driving

capacity is needed and TTL levels signals from the IC are what is wanted. Because the 74LS86

IC directly drives the output without any further buffering, it is only suitable for driving high

impedance (1 kohm or more) sync inputs.

Component list

C1 22 uF 10V electrolytic

C2 22 uF 10V electrolytic

R1 2.2 kohm

R2 2.2 kohm

U1 74LS86 or 74HCT86

Simple sync combining BOX

Sometimes you want to use a ready-made BNC cable made for connecting VGA card to high

quality monitor. Those BNC cables have 5 BNC outputs with following functions:

Signal Function Color in cable

---------------------------------------

R Red Red

G Green Green

B Blue Blue

H/HV Hsync(+Vsync) Black

V Vsync White or Gray

Tha cables are usually built (should be built!) from high quality 75 ohm coaxial cable and 75

ohm BNC conenctors to guarantee good image quality. You can see a typical schematic of

commercial VGA to BNC cable below:

This kind of cable can be used with monitors with 5 BNC inputs. But if you unfortinately have

a monitor with RGB and composite sync inputs only, that cable might not be of any use,

unless your graphics card can generate composite sync signals (this is not a standard option).

If you cna get composite sync directly from graphics card, then it is enough to just connect

the H/HV connector to the composite sync pinput of the monitor.

If you are on unfortunate person who deos not have this kind of graphics card or your drivers

don't allow using that option, then your only choice is to build a circuit which combines the

separate HSYNC and VSYNC signals to composite sync. You can see this kind of circuit below.

Component list

C1 22 uF 10V electrolytic

C2 22 uF 10V electrolytic

R1 2.2 kohm

R2 2.2 kohm

U1 74LS86 or 74HCT86

Connecting VGA to Sync-on-Green monitor

Some monitors use sync-on-green syncronization instead of separate sync. In that case you

can't used my circuits directly. You have to modify those circuits to add Sync-on-Green

capability. Generating sync signals to screen picture component is quite easy. I have not had

any need for such a circuit, so I haven't designed such option. Check this article to get the

idea what to add to my circuits. For more complete explanation how to deal with sync on

green monitors, check the Sync on Green FAQ.

Building the circuit

The circuit is quite simple to built if you have basic skills in building electronic circuits. The

electronics of the circuits can be easily built to a small piece of veroboard so no special circuit

board is necessarily needed (I used this approach in my prototype). I have designed a circuit

board for my VGA to TV converter and this same PCB design (GIF picture scanned at 300x300

dots/inc resolution, also available in zipped postscript file format) and component layout can

be used with this project also. Note that there are some differences in connection of the IC U1

pins in PCB compared to the schematic, but the circuit built to this PCB fuctions in the same

way.

The circuit need well-stabilized power +5V power source (+/- 5%). The circuit takes less than

150 mA current, so you don't need a large power supply. If you don't have anythign suitable

avalable, you can always use a small general purpose wall transformer and a small +5V

voltage regulation circuit. Another option is to take the +5V power from PC using methods

explained in my How to get power from PC to your circuits document.

The wiring should be made carefully and 75 ohm coaxial cabling for picture signals should be

used everywhere, especially if you are going to use this circuit with a high resolution monitor.

Signals reclections caused by bad wiring can really mess up your picture quality. I would

recommend using metal box, shielded cables and shielded connectors for circuit to keep noise

and radio interference minimum.

VGA to RGBS converter component list

U1 74LS86

C1,C2 22 microfarads, 16V

R1,R2 2.2 kohm, 1/4 W

R3,R4 1.8 kohm, 1/4 W

R5 2.7 kohm, 1/4 W

R6,R7,R9 47 ohm, 1/2 W

R8 120 ohm, 1/2 W

T1,T2 BC547B

P1 15 pin SUB-D connector

Output connector:

75 ohm BNC connectors (you need 4 BNC connectors)

Wiring:

Red, Green, Blue and Composite Sync lines should be

wired using 75 ohm coaxial cable.

If you are building my TTL level output model, you can leave out R3-R8 and T1-T2 out of this

list.

Getting your equipment to accept signal from PC

Many modern compter monitors are multisync monitors which accedpt easily wide selection

of different refresh rates, scan frequencies and resolutions. This same applies to modern data

projectors. But many older monitors (monitors not made for PC) and video projectors can't

accept the signals your VGA card puts out.

If your video projection system or monitor don't accept VGA scanning frequences you have to

get a suitable driver for adjusting your VGA frequencies. For normal VGA modes the horizonal

sync frequency is 31250 Hz and vertical sync frequency is 60 or 72 Hz. For information about

SuperVGA modes you have to consult you display card adapter technical data.

Another use for the same circuit is to connect to connect VGA card to a large old color monitor

from an old workstation. If you manage to set your VGA adapter to generate suitable

scanning frequecies for the monitor you have, you can use it with your VGA card. For more

information how to do this, check the VGA to workstation monitor FAQ.

How to find fixed frequency monitor technical data

Befor you can start eny useful experimenting have to find out the horizonal frequency and the

refresh rates your monitor needs. If you try to use the monitor with wrong frequency signals

the monitor does not work properly and some monitors can be damaged if you try to use

them at wrong frequency.

Your monitor user's guide technical info section is the first place to check. If that does not

help try the manufacturer's website. Monitor databases like The World Wide Web Monitor

Database and Apple Macintosh Monitor Database are very useful information sources for

getting this information. If you can't find any technical info on the exact monitor you have you

might check the information about some very similar monitor model (for example some older

revision of that model) and use it as some kind starting point. If you know the computer

model where the monitor was connected you can try to look at the technical specs or some

FAQ of that computer (if you get to know what kind of signal that computer has sent to

monitor you know that the monitor can at least show it).

How to configure Windows

Your video card drivers quite propably has options to set the the screen refresh rate. Check

you graphics card manual and driver documents how to adjust those settings (this depends

on graphics cards). Try to find a refresh rate and resolution which matches you monitor

specifications. Typically the selections in typical drivers are quite limited and many driver's

don't have any further options for user customization.

Matrox has added excellent user configurability oprtions for their Millenium and Mystique

graphics card Windows 95 drivers. For more information take a look at my info page how to

confire Matrox cards to TV frequencies because thet page has also lots of information how to

change tha settings and this information is useful if you have to make configuration for some

fixed frequency monitor you have.

Configuting Linux XFree86 for fixed frequency monitors

Linux XFree86 has excellent user configurability by editing XF86Config file which has all

necessary video card parameters in it and that file can be edited with any ASCII editor.

Changing those parameters in gonfiguation file properlu needs some experinece. For more

information on configuring XFree86 take a look at Use your old Workstation Monitors with

Linux/XFree86 and The Hitchhiker's Guide to X386/XFree86 Video Timing.

How to use old video projectors and TV studio monitors

If you happen to own an old video projector which is capable of displaying only

PAL/NTSC/SECAM video picture but has RGB inputs, you might find my signal conversion

circuits still useful. You can make your VGA card to generate signal which is suitable for this

type of monitos or video projectors using the drivers designed for my VGA to TV circuit. Those

drivers are available for standard VGA modes in DOS. Some support is also available for some

cards for SuperVGA modes in DOS, Windows and Linux Xfree86.

Other related projects

I have also designed a VGA to TV adapter using this same basic circuit. With some additional

software and small modifications this circuit can be used for connecting VGA card to TV

equipped with SCART connector.