I ll be trying this circuit very soon.
How would perfect one would look ?

 

How would perfect one would look ?

When I said it is not perfect, I was saying the way V-sync is made in that circuit is ok but not right.
The X-OR circuit inverts H-sync curing the sync time period. Actually H-sync needs to be moved over a little in time but that is too hard to do for your simple test. If your picture looks bad in at the top of the picture we can change the circuit. If it looks ok then stay with this simple version.

There is a PLL in TVs and monitors to make the monitor run at the H-sync frequency. By simply inverting the sync for 3 lines the PLL gets confused because it thinks the picture has moved over in time a little. (an Edge Lock Loop (PLL) only looks at one edge of H-sync and an inversion moves that edge to the other side of sync) Don't worry about it for now.

 

Composite video signal should look like this:



I wrote a blog that introduces Video Basics.
https://forum.allaboutcircuits.com/ubs/diy-pong-part-1-of-4-getting-started.822/

 

Warning: Trivial Nit-Picking . . .

In the lower drawing, doesn't the first equalizing pulse of even fields occur in the middle of the line? That would be in the middle of the third step of the staircase waveform, where the drawing shows it after the first step. Granted the sync pulse widths are exaggerated for detail, but still ...

ak

 

Today tested new schematic. Sync works. Takes few tryes but finaly syncs and then runs smoothly. Picture is stable with no tremor.
But now problem is very dim picture. Nearly unreadable. Cant say if bold text is separable yet.
Do I need to increase resistor R4 on the transistor emiter to solve that ?

P.S. Found VGA monitor - will be testing direct connection too.
And thank you for a good read MrChips

 

If you are referring to the circuit I posted then try reducing the value of R2.
If that does not help then you need to amplify the video signal before it reaches R2.

 

Can you hand draw a picture? What is happening on the Emitter?
The 74HC86 & R1 is probably driving too hard and Video & R2 is not hard enough.
--------------
Now that I look at it why R3? If video is about 1 to 2V on R4 then there is too much voltage across R3. I would short R3 out or change to 50 ohms or 0 ohms.

 

Decreased R2 to 0.5 ohm.
Picture is much better now. Bold characters are separable, but regular ones ones looks strange. Seems to me like horizontal lines looks ok , but vertical lines are much dimmer. Hard to read as a result.
Can there be something to be done to improve ?



Moderator edit: rotated image

 

I believe it is either related to the limited bandwidth or the amplitude of the video signal.

What transistor are you using?
Have you tried different values of resistors for R1 - R4?

Are there brightness and contrast adjustments on your screen?

 

I only tryed R2. There is brightness and contrast on output board. This picture is best I managed to get out of current setup.
Possibly there is adjustments on monitor itself. I can see "menu" and other buttons like + and - . Do you thing it is worth trying to disasemble to get to physical buttons and trying ? Buttons are not easily accesibly but I will do the job if it is worth

 

I only tryed R2. There is brightness and contrast on output board. This picture is best I managed to get out of current setup.
Possibly there is adjustments on monitor itself. I can see "menu" and other buttons like + and - . Do you thing it is worth trying to disasemble to get to physical buttons and trying ? Buttons are not easily accesibly but I will do the job if it is worth

No. Don't disassemble anything. It would appear that MENU etc are "soft" buttons. You need to find a user manual or try to figure out how to access the MENU options.

 

Look at this picture. The vertical lines in the "OUT" are dim and the horizontal lines are bright. Look at the top of the "T".
There are dim 1 pixel dots, bright 2 pixels and very bright 5 pixel areas.
This shows your bandwidth is low. This is coming from several places. Maybe inside the monitor. Maybe from your computer. Maybe because your coax is not terminated right.

The value of R3 is so wrong that the transistor is not functioning. It is just a diode at this point.
Please make R3=0. Now play with R2 if you want. Probably some where between 1k and 200 ohms. Not 0.5!

 

Do as Ron says or try replacing R3 with a variable resistor and adjust for the best display.

 

1. Replaced R3 to 0 and picture became unstable. Cant sync at all if R3 < 500 ohm. So left at 500
2. Replaced R4 to 70 ohm and picture became more stable. More than 70 has no effect.
3. R2 should be not more 20 ohm for picture to be bright enough.

At his stage picture improvement is obvious but still not perfect. I can manipulate output board pots to achieve nicer picture or separable bold characters ,but then sync is imediatelly lost.

Now situation is like that . Output pots are at max.

I can achieve separable bold characters, but regular ones become almost unreadable plus sync problem.

Tere are pots on output. There is one more wire on connector. I dont know what it is for.

 

The difference between plain and bold text has to do with the number of pixels in the horizontal scan.
If your video switching circuit is not fast enough, i.e. poor high frequency response, then single-pixel width will be dim.

You never said what transistor you are using.
Here are factors that could affect your video image:

1) high frequency limitation
2) video amplitude
3) voltage levels of black and white, i.e. DC offset
4) brightness and contrast settings

Do as you did. Try adjusting the values of the resistors R1-R4 for best overall results.

 

Transistor is N2222 .

 

Transistor is N2222 .

There are different pinouts for 2N2222 transistors. Make sure that you have it wired correctly.

 

And why they make life harder than it needs to be.
That I definitelly need to check firts of all.

 

And why they make life harder than it needs to be.
That I definitelly need to check firts of all.

It's to check that people are still using their brains. It is like people who follow GPS instructions blindly and end up driving into a swamp.

 

Very true