Advice modifying a circuit

[Steve]

Hi all,

I have a few SGI computers which all output Sync-on-Green. This causes problems when I want to use VGA-to-HDMI converters (cheap ones off Amazon) as they do the job, but the entire screen is 'tinted' green. I do realise there are larger types of scalers I could get, but they're not suitable for what I'm doing.

I found this: https://www.instructables.com/Arcade-PS ... move-Gree/

Since the PS2 is SoG, I could potentially use this circuit to strip out the green tint. But this is designed for taking the PS2 AV out and converting it to VGA. I want to take VGA > VGA (with the green tint removed)

I am just wondering the best way to modify this circuit to simply take VGA input instead of PS2 AV, as it's not taking in any horizontal or vertical sync on the input - is that a problem, if I change it to VGA should I wire it up differently to pass through those signals? etc.

Thank you,

[dml]

Hi Steve,

I am going to have the same problem soon - I have the correct 13W3 connector but no monitors that can support sync-on-green properly. So I'll need to figure out some kind of sync separator. I thought I might have to do that manually with a small pcb + transistors etc.

I'm not very familiar with the PS2 video signal setup (I feel bad - I was writing code on those for some years!) but I'll have a look and try to make sense of it.

[mrbombermillzy]

I had this problem a while ago and trawled the net for ages for a low cost/simple solution.

I did come across a very simple circuit. I will try to find it for you. Not sure if I still have it, but I will have a good look.

[dml]

The SGI side should have both H&V sync encoded in green - so just R/G/B in - which matches the PS2 circuit there. So I would expect any other sync sygnals on the VGA input side should just be ignored?

There might be changes relating so signal ground but maybe the rest of it will just work.

[edit]

Not sure about the potentiometer part - seems a bit sketchy. Maybe its good enough but a reasonably fast opamp here would be better.

[dml]

I think another simple variant on that potentiometer hack would be to use a very low forward voltage diode (schottky <=0.2v or something 'special' and active, like a FET) and to do exactly the same thing on each R,G,B line so there is no extra messing with the green signal to align them. They will all have the same offset.

However using diodes at all will probably result in dropout so I think an active rectification circuit on the green line is better.

[edit]

Seems like mrbombermillzy might be able to dig up a simpler solution for us - but if the problem is just rectifying the sync signal out of the green, there are still some better options than a diode. An opamp can do it but isn't going to be fast enough to keep up with pixel clock speeds in the MHz - but an active/synchronous rectifier with a FET can do it. This will give you a diode with almost zero dropout, at the cost of more parts and some external power. And a bunch of fiddling around with values of course.

e.g.

jtQTo7hF.png (10.55 KiB) Viewed 222 times

[dml]

one more hack worth trying (maybe) is just to shunt the inverted peak to GND with a reversed schottky diode. so instead of it being in series with green, its in parallel with ground, flipped.

[rubber_jonnie]

I have a couple of Octanes and bought a 13W3 to VGA adapter, but I've never tried over HDMI. I think I should and I'll report back. I have several 'cheapo' adapters so I'll see what's what.

One thing I do remember from buying the 13W3 to VGA adapter was that at the time I also had an Indy (Wish I still did) and the 13W3 adapters for that didn't work with the Octanes and vice versa.

[Steve]

One thing I do remember from buying the 13W3 to VGA adapter was that at the time I also had an Indy (Wish I still did) and the 13W3 adapters for that didn't work with the Octanes and vice versa.

I think the 13w3 out of an Octane and indy are supposed to be the same. The issue might be to do with the adapter, I have heard that some Sun adapters aren't always working properly with SGI.

Luckily for me I'm dealing with an o2 which is simply VGA. This is why I need VGA-VGA SoG removal.