The circuit for VGA to scart connection.

Я радиолюбитель

The circuit for VGA to scart connection.

Here is the circuit for VGA to scart connection. It is basically a circuit which takes VGA signals and converts it to RGB + composite sync signal which can be fed to TV via SCART connector. VGA card picture components RED, GREEN and BLUE are already at the correct voltage level (0.7Vpp) and has correct impedance (75 ohm) for direct connection to correspondign inputs in the TV. What needs to be done is to combine separate horizonal and vertical sync signal from VGA card to one composite sync signal which is feed to TV video in pin in SCART connector. This sync signal conversion is done by the electronics in the circuit. The circuit has also sends correct level signal to the TV RGB input enabling control pin in the SCART connector (pin 16).

This picture is available in GIF and Postscript versions.
How the circuit works

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 from composite sync singals. The circuit is simply based on one TTL chip with four XOR ports, two resistors and two capacitors. TTL chip was 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.

The circuit needs well regulates +5V (+/-5%) power supply and takes about 120 mA current.

Building the circuits

VGA to TV converter is quite easy to build if you have some experience in building electronic circuits. The electronics of the circuits can be easily built to a small piece of veroboard and no special circuit board is needed. I used this approach in my prototype.

Remember to add powerfeed to the chip U1. It has not been marked to the schematic. U1 has ground at pin 7 and +5V power input at pin 14.

The circuit need well-stabilized power +5V power source (actual voltage can be in 4.75V to 5.25V range). 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. If your computer graphics card is VESA DDC compliant, it might have +5V available at VGA connector pin 9 (the standard have option that VGA card can have +5V at pin 9, but it does not necessarily have to have this). You can test easily if your computer has this +5V output using multimeter. If your VGA card does not have power ooutput, then check my How to get power from PC to your circuits article for more ideas.

I have built most of adapters I have made to a small circuit board which is designed by me and Aki Suihkonen. You can see as small picture of the PCB in the picture below (copper traces viewed from component side):

If you want to make this simple one side circuit board then download either the copper sive view or component side view whichever suits your use better. The circuit board picture here is a scanned version (scanned at 300x300 dots/inc resolution) of the actual circuit board (version 1.0) in GIF format (size of picture is 1227x672 pixels). You need to use suitable image viewing/exiting program to print it in correct resolution so that you get a picture with size of 4.09x2.24 inches (you can't get anything useful by printing from your web browser).

Component side view of the PCB is very useful if you hare making the circuit using photographic method (turn the slide you just printed around and use it as the PC master film for exposure) or if you use heat transfer method (just print the file to a special meterial which allows transfer of toner to the copper on the PCB using presure and heat).

I have also provided you are zipped Postscript versio of the component side view of the circuit board. You can print this picture to correct size using any Postscript printer or by using a Postscript viewer/reader program like Gsview. After you ve printerd this picture to a slide you turn it around (make the in to face the copper on circuit board) and start to make the board using whatever method you find suitable.

If you have HP laserjet printer (or compatible) you can download the HP Laserjet PCL format file of the component side view of PCB copper. The PCL file can be directly copied to your HP laserjet laser printer using DOS copy command: COPY VGA_PCB.PCL LPT1 if your printer is connected to port LPT1. This version of the picture gives the highset resolution, because it is directly generated from the PCB I were using (EasyPC). All other versions are somewhat scan/conversions of this version.

The circuit board is designed for fitting the electronics around 74LS86 chip and also a small +5V voltage regulation circuit. If you are building the circuit to this PCB, check also the component layout picture. Look at the picture below to see how the ready made circuit looks like.

VGA connector, SCART connector and other connectors must be fitted outside this circuit board. Remeber that the SCART connector pinouts in the circuit diagrams are according the pinout used in TV end. If you should your circuit so that it has male SCART connector which goes directly to TV everything works as planned. If you want that your circuit has female connector and you use some ready-made SCART wire you must remeber tow things: you have to use a wire which has all pins connected and the SCART interconnection cables switch pins 19 and 20 inside the cable (those cables swap all audio and video incoming/outcoming wires).

For best signal quality all of the wiring should be made of 75 ohm coaxial cable (thin antenna wire) for best image quality just like in any video circuit. Because TV is not very high resolution display, the wire type does not matter much if the wire is only few meters long. I have succesfully used shielded twisted pair computer cable for the connections (one pair takes one signal and one ground line). Use well shielded cable, because poorly shielded video cable can generate quite an amount of radio interference.

Modification ideas

Adding monitor ID signals

If you are planning to use the TV as the only monitor for the computer, you might want to add the monitor type identification function to this circuit. This can be easily added to circuit by connecting VGA connector pin 11 to pin 5 with piece of wire. In this way the VGA card gets color monitor ID from the VGA to TV adapter and you can boot your computer to color monitor mode when VGA to TV adapter is connected.

Solving screen mode problems in some 16:9 TVs

The voltage which the circuit sends to Function select (AV control) pin of SCART connector (pin 8) is +5V voltage which is well enough for sending typical TVs to AV mode. Unfortunately some TVs might have problems on this because in new TVs this voltage is defined to activate 16:9 screen size mode. If you experience problems with this select signal replace it with 9.5-12V voltage from some other source.

Adding sounds from computer to TV option

Other nice modification is to add computer sound to TV speakers option. This can be done connecting soundcard right line level output to SCART connector pin 2, left line level output to SCART connector pin 6 and connect sound card audio ground to SCART connector pin 4. In this way the sound is nicely routed from soundcard to TV spakers.

It is a good idea to use well shielded cable for audio to prevent audio lines from picking high frequency noise from video signals going to TV.

Using female SCART connector as circuit output

Sometimes you might want to build the circuit using female SCART connector which will fit nicely to plastic case and you can connect the circuit to TV using standard SCART inteconnection cable. If you are planning to use female connector in the circuit and you use standard scart vire to connect it to TV you must remeber the following things:

  • use a SCART cable which has all pins connected (all cables don't have RGB wires connected
  • standard RGB cable crosses all audio and video lines so that what is input in one end is connected to the output in another end
The necessary modification if you want to use female SCART connector is to connect the composite sync signal output to pin 19 instead of pin 20. (If you plan to use the sound option then you connect left channel to pin 3 and right channel to pin 1).

What if you encounter problems

If you have problems in starting using the circuit you just built, read the VGA to TV circuit troubleshooting guide. If this does not give any help and you have double checked everything then you can e-mail me a question with GOOD description of the problem, your system setup and what you have already tried to solve the problem. Without good description it is quite impossible to help you in troubleshooting the circuit via e-mail.

Warning and disclaimer

If you try this circuit and those drivers and do something wrong, there is danger that you damage your TV, graphics card and monitor. So think what you do and double check everything. And remember that you try this at your own risk: I am not responsible if something harmful happens. The material in the document have been checked and is beleieved to be correct, but there is always possibility of errors. And remeber that there are some differences in different graphics cards and TVs, so it is possible that the circuit might not work in your system for some reason. The system has been tested succesfully with 6 different graphics card in 5 different computers using 6 different TVs/monitors. VGA to TV converter component list Main circuit U1 74LS86 (74HC86 or 74HCT86 can also be used) C1 22 microfarads 16V electrolytic capacitor C2 use 47 uF 16V electrolytic for more reliable operation (22 uF listed schematic can cause problems in some cases) R1,R2 2.2 kohm, 1/4 W R3,R4,R5 2.2 kohm, 1/4 W R6,R7,R9 47 ohm, 1/2 W R8 120 ohm, 1/2 W T1,T2 BC547B (2N2222 should also work but note the different pinout) P1 15 pin SUB-D connector (DE-15) Output connector:21 pin EURO/SCART connector Wiring:Red, Green, Blue and Composite Sync lines should be wired using 75 ohm coaxial cable for best picture quality, but can be replaced with normal shielded wire. Power supply components7805 regulator chip 100 uF electrolytic 25V 10 uF electrolytic 16V 100 nF polyester or ceramic condensator Wall adapter which outputs 8-18V DC and 150 mA or more current Connector for connecting wall adaptor to circuit

Tomi Engdahl < Этот e-mail адрес защищен от спам-ботов, для его просмотра у Вас должен быть включен Javascript >
Источник материала.

Рекомендуемый контент

Copyright © 2010-2017 Контакты: При использовании материалов веб-сайта Домашнее Радио, гиперссылка на источник обязательна.