RGB composite sync Amplifier from 0.7vpp to 4vpp

Thread Starter

xnihilo

Joined Nov 20, 2008
33
Hello,

I need to amplify VGA card sync output.
The VGA card has 0.7vpp horizontal and vertical sync connectors.

I need to connect my VGA card to an arcade monitor which expects Composite sync with level between 1 to 5VPP negative polarity.

I need to combine the two sync signals (vertical and horizontal) and then amplify from 0.7Vpp to something like 1~4VPP.
The sync signals have negative polarity.

Combining the signal should not ve so difficult but I have no idea of how to amplify the signal.

Could anyone help me out with that challenge?
Thank you very much for any hint.
 

rjenkins

Joined Nov 6, 2005
1,013
If the monitor takes a standard composite signal, it should handle 1V p-p.

Doesn't you video card have a TV (composite) output connector? Many do, either as a phono (RCA) socket or a mini-DIN with an adapter to a phono socket.

With that, it should be a direct connection.
 

retched

Joined Dec 5, 2009
5,207
Right, the monitor is an arcade monitor.

What rjenkins was saying was your VIDEO CARD most likely has a composite(TV) out. That way you need no conversion.

And for the price of Composite-ready video cards, ~$20, you should go that route.

Unless you REALLY want to build the adapter. But you could really screw things up. "Releasing the magic smoke" from your arcade screen could be a definite possibility.

Are you hard-set on building this?
 

Thread Starter

xnihilo

Joined Nov 20, 2008
33
You still don't understand.
It is an upright arcade cabinet monitor. It has NO composite video input.
It has 5 wires: R,G,B,sync and GND. That's it.

Moreover composite video is a PAL signal, whereas RGB produces clearer image.

Anyway, I need to amplify my sync signal...
 

Thread Starter

xnihilo

Joined Nov 20, 2008
33
I'm looking for Synch signal amplification.
At ultimarc they have a CGA amplifier which will cost me about 50 USD. I wish to make a simple sync amplifier...
 

Thread Starter

xnihilo

Joined Nov 20, 2008
33
Thank you. I already have visited this page.
Which of the two circuits should I use? The one without the BC547? It seems the VGA sync is a TTL level signal 0 or 5V... so the 74LS86 chip should be enough, is that correct?
 

Thread Starter

xnihilo

Joined Nov 20, 2008
33
According to what I just read, the VGA signals, except for R,G and B are TTL levels, so I don't need to amplify it, just combine V and H sync.

I guess I got it right.
 

SgtWookie

Joined Jul 17, 2007
22,230
It looks to me like you could simply add in the other stage if necessary.

Pretty cheap solution.

Make certain to use 0.1uF bypass caps across the Vcc/GND terminals of the IC's used.
 

SgtWookie

Joined Jul 17, 2007
22,230
Lower output impedance. There's a limit to how much TTL IC's can sink or source. Transistors are much more capable. Ideally, your converter should have an impedance of 75 Ohms to match the impedance of the cable and the monitor. If there is an impedance mismatch, you might get "ghosting" caused by ringing on the cable due to the mismatch.
 

Thread Starter

xnihilo

Joined Nov 20, 2008
33
SgtWookie,

Thank you for answering all my question, I appreciate it.
I'm sorry if my questions look dumb, I don't know much about electronics so that's why I'm seeking for some help.

I'm not building a big project, I'm just trying to connect a PC to an arcade monitor to run arcade games with an emulator and trying to have a good picture quality.
Can I ask you what you would advise me, regarding the two schematics on the first link you gave me.
I successfuly connected the VGA card to a TV as the levels were the same.

As the VGA sync signals are TTL level, I don't need any amplification, just combining the two sync signals (V and H sync to create a composite signal). I might have used the Composite video output to get the sync signal but the level is incompatible with the arcade monitor.
I guess combining the VGA TTL sync signals should be fine but then, I still don't understand why I should user more circuitry than just the 74LS86.

Please could you clarify this for me?

The circuit is quite simple, because RGB signal ouput from VGA card is already standard 0.7Vpp to 75 ohm load.
???

TTL chip ws logical choise because VGA sync signals are TTL level signals.
Ok

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).
Uh? I read that VGA sync level is TL (0-5v)...

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
This sounds like chineese to me

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.
Is that the case of my arcade monitor?
What is "buffering"?

Hope I can get some answers...

Thank you anyway.

Regards

PS: with the circuit I'm currently using (based on diodes for sync combining and some amplifier I see ghost lines running down the screen at something like 1Hz... I think I'm amplifying what is already a TTL output from the VGA card and I might have some interference somewhere, the rendered picture looks lame and I would like to have a nice stable picture...

On the first of the two circuits the pin 13 is connected to GND while in the second schematic it is connected to +5V... Why?

Whats exactly the use of the two BC547? It amplifies? It acts like a kind of darlington pair???

What if I use the Composite Video output with the LM1881N IC to get a composite signal? I don't know if this chip will output a TTL level output sync signal...

I also read:
Depending on your console, you may also need coupling caps on the video lines to remove DC offset. 220uF is usually the standard, but you can get by with (sometimes much) lower when driving a high impedance input like an arcade monitor.
And this:
A LM1881 (crossed by Intersil as the EL1881 which is often cheaper) will extract just the sync information from the CVBS line and output TTL level composite sync
But I thought the output level was about 0.7VPP while the lines above state that it will be TTL level...

And finaly this:
> > - input a CVBS signal (0.5-2 Vp-p composite video signal). The LM1881N
> > first extracts the C-sync portion of the CVBS signal, and outputs it as
a
> > TTL level C-sync signal.
So it is what I should use, provided my VGA card simultaneously outputs on the VGA and on the Composite output connectors...


at: http://www.mail-archive.com/msx@stack.nl/msg13217.html

I'm using circuits from:

http://members.optusnet.com.au/eviltim/vga2arc/vga2arc.htm#acvid

And it not bad but I have the problem with the ghost interference...
 
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