VGA to Sync on Green

[HellasTechn]

Hello my dear friends.

I have a pc and a monitor that has three BNC inputs (RGB). this monitor uses sync on green.

I know very little about video signals...

I have tryed to make a circuit i found on the internet to convert the signals but it does not work. this is the circuit i used



Any idea why ???

***More info in my next post***

 

[davenn]

can you please draw the circuit in MS paint or something
just so we all can see what you are doing... the above ascii representation is a little difficult to follow

Dave

 

[HellasTechn]

can you please draw the circuit in MS paint or something
just so we all can see what you are doing... the above ascii representation is a little difficult to follow

Dave

I am sorry. When i copy-pasted it didnt look like that.

I will post picture.

I have read online that there are different kinds of sync on green monitors that use different pulses. is that correct?
Mabe i am useing the wrong circuit ? all i know is that the monitor is a NOVATION display intercafes and that the old pc that used to be hooked up with used a Gemini P1 video accelerator (this had a set of dip switches and a chart to choose between classic PC VGA (separate H and V sync) and Sync on Green

 

[HellasTechn]

One thing that comes to my mind is that maybe a normal pc output use negative Vsync. Could that be the case ?

One other thing that poped in my mind is that maybe the PC was set to output 75Hz and i think the monitor can handle up to 60Hz. (I have to test that)

But still is the circuit correct ?

 

[HellasTechn]

Okay. now i have found this circuit on this site

http://www.epanorama.net/circuits/vga2rgbs.html

That provides RGB with separate composite sync (H+V sync).

I think that if i modify the circuit a little it will work.

This is the circuit i am talking about with my mod

 

[Rleo6965]

You need also mix all RGB signals into 1 composite signals. Even if you able to properly sync signals. You will be missing dot display Red and Blue.

Example: You have red , green , blue circles from your RGB signals. Only green circle will only pass on interface circuit and displayed on CRT.

 

[HellasTechn]

1) You need also mix all RGB signals into 1 composite signals.


2)Even if you able to properly sync signals. You will be missing dot display Red and Blue.

3) Example: You have red , green , blue circles from your RGB signals. Only green circle will only pass on interface circuit and displayed on CRT.

Are you sure about (1) ?

1) I think this is not true because my monitor uses RGB with sync on green and does not use composit video.

2) I think that is also not true because the sync pulses are for both three colours. the reason we get it trgough green is that the first pc monitors where monochrome and their colour was green.
The RGB sync on green is the same as the RGB with separate sync signals (VGA output or 5 BNC output) The only difference is that the sync pulses are combined and sent in green colour line ( they dont interfere with each other because they occure at different times).

3) It is actually a flat panel LCD screen.

 

[HellasTechn]

1) I think this is not true because my monitor uses RGB with sync on green and does not use composit video.

2) I think that is also not true because the sync pulses are for both three colours. the reason we get it trgough green is that the first pc monitors where monochrome and their colour was green.
The RGB sync on green is the same as the RGB with separate sync signals (VGA output or 5 BNC output) The only difference is that the sync pulses are combined and sent in green colour line ( they dont interfere with each other because they occure at different times).

3) It is actually a flat panel LCD screen.

Take a look Here

http://www.raphnet.net/electronique/sync-on-green/sync-on-green_en.php

 

[HellasTechn]

Anyone ?

Other opinions ?

Comments about the circuit ?

 

[HellasTechn]

Okay The previus schematic was wrong in a few spots.

This is the correct one !!!

It actually worked !!!

Many Thanks to the guy that designed this circuit !!!!

 

[HellasTechn]

For the above circuit i need 6V power supply (though it says it works with 5 volts). i have done this two ways.

First i took a 9v power supply and added a potentiometer to bring voltage down to 6v (that worked. but with the load on voltage drops to 3.3v why does that happen ?).

Second i used an old pc power supply that provides 5v but it couldnt work. for some reason it was giveing me a green screen and a dimm picture on the background. (no idea why also)

My thought was to use the potentiometer on the 12v output of the pc power supply and reduce it to 6v but the circuit couldnt work at all. (with the load on i had no voltage drop)

all theese are a little strange to me.

Now i will either keep the 9v power supply i have with the potentiometer or i will look for a 6v regulator and use the 12v output of the pc power supply.

Any ideas ?

 

[KrisBlueNZ]

That circuit uses an LSTTL IC that should be powered from 5V +/- 0.5V. Use a 7805 three-terminal regulator (see http://www.fairchildsemi.com/ds/LM/LM7805.pdf) to drop a supply of 8V or higher down to 5V.

 

[HellasTechn]

In theory it sould be ok with 5 volts.

The power supply i use also provides 5 volts.

When i use the 5 volts to feed the circuit it doesnt work correctly. I can not get the sync pulses with green colour and all i get at the monitor is a green screen.

But the circuit works fine when i use the 9 volts power supply nd bring it down to 6 volts useing a potentiometer.

 

[KrisBlueNZ]

That's a pretty clever circuit but it's a bit naughty. It connects the output of U1D (pin 11) directly onto the green signal output from the VGA card. This could be the reason why it isn't working for you at +5V supply voltage.

I've tried to do some research on VGA signals. It appears that the actual R, G and B signals range from 0V (which is black) to +0.7V (full brightness for that colour). Sync pulses are normally "blacker than black" and this implies that the signal must go negative, i.e. below 0V, during the sync pulses. That circuit will not do this.

That circuit uses a clever method of combining the horizontal and vertical sync, while also automatically detecting the sync polarity. The first three exclusive-OR gates do this. The signal on pin 8 of the 74LS86 is an active-high sync signal, and it's converted into an active-low sync by the fourth gate.

It seems to me that this active-low sync signal needs to be mixed into the green video signal using a method that produces a negative voltage. This can be done using a capacitor and two diodes. If you want to try it, here are the steps.

Disconnect pin 11 from the green video signal.
Get a capacitor of around 10 µF and connect its positive terminal to pin 11.
Connect a 1N914 diode with its anode to the negative terminal of the capacitor and its cathode to the 0V rail.
Connect a second 1N914 diode with its cathode to the negative terminal of the capacitor and its anode to the green video signal.

This may work. If it doesn't, you could try inserting a resistor of around 47 ohms in series with the green video signal connection to pin 2 of the VGA connector.

If these things don't work, we will really need to see some waveforms. If you have an oscilloscope, disconnect pin 11 and look at the waveform on the green output from the VGA card, and also look at the signal on pin 11. Use DC coupling, and take photographs. On the photographs, show where the 0V DC voltage is, and state the number of volts per division, so we can see the maximum and minimum voltages of the video signal. Then post the pictures here. We may be able to figure out what's going on, and recommend a change to that design.

 

[HellasTechn]

I will do that first chance i get. in the meanwhile +6volts supply to the circuit works fine.

Thank you for your suggestions ! they are always welcomed.

 

[HellasTechn]

I have finally tried the changes you recommended but it didnt work.

As a matter of fact when i attached the monitor on the circuit i got a black screen (not the blue screen thet normally has if no signal) and then it wouldnt work at all.
It got back to normal after a few times tunred it on and off.

I tried this twice times and twice the same thnig happened.

I used an Oscope and got the waveforms. it looked ok useing yor recommendations.

Luckily i have an Oscope that can store waveforms in floppy disk (not that lucky to have UBS)
so i will post some time tomorrow or the day after.

To tell you the truth i am a little reluctant to try that new circuit again fearing i might permanently short circuit the monitors input.

Nevertheless i will post waveforms of old and new circuit output.

 

[KrisBlueNZ]

Yes, some oscilloscope traces would be very helpful. I would like to see the green video, horizontal sync and vertical sync waveforms from the VGA card.

You don't need to use the monitor at all for that test. I would also like to see the waveform on the composite output of the circuit without the monitor connected, and with the monitor connected (but not powered up) if that makes any difference.

 

[HellasTechn]

i will give you pictures of all.
I will also look if i can find any info about that monitor. doubtful though.

 

[HellasTechn]

Theese are the pictures from the vga adapter
from right to left you see:

1.R
2.G
3.B
4.H SYNC
5.V SYNC

Without anything attached to the output of the VGA card

 

[HellasTechn]

with the circuit on and fed with 6V (through the potentiometer voltage divider)

1. pin 11 of 74LS86 wihtout anything attached on it

2. pin 11 with the green from the VGA card attached on it

3. pin 11 with with the green from the VGA card and the RGB monitor attached on it