RGB consoles to 15Khz VGA RGBHV monitor

UPDATE 27/Nov/2015

Made a new SCART-to-circuit cable, sync separator now works with both Amiga and Megadrive connected with their own, unmodified SCART cables.

N64 still having issues, but I guess they are due to RGB custom mod and/or cable... see details HERE

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UPDATE 25/Nov/2015

Sync separator circuit rebuilt from scratch with a brand new chip, working perfectly!
But video is FAR too dark... see details HERE

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ORIGINAL POST

Hi everyone,

I got my trusty Microvitec / Amiga Technologies 1538S back to operation, and I would like to use it with my retro consoles (Megadrive, Dreamcast, SNES, N64 and of course the Amiga as well).

It is a multisync/multiscan monitor 15KHz capable so that's a good starting point I guess. It is also fully functional, as I connected it to my Amiga A600 and it works like a charm.

Problem is, this monitor apparently only has RGBHV input, i.e. requires separate horizontal and vertical sync signals. The Amiga video port has everything natively available, but this doesn't apply to most of my other retro platforms unfortunately.

So, I've built a small electronic circuit with the TI LMH1980 sync separator: the idea was to get Hsync and Vsync and then merging them with R, G and B already available.

Spoiler

But this isn't working. :-\

Basically my test suite is like this:

A600 +-----> composite video -----> LMH1980 -----> Hsync/Vsync +----> monitor
_____+-----> RGB ----------------------------------------------+

Looking at the oscilloscope waveforms it *seems* the sync separator is properly outputting a 15Khz and 50hz hsync/vsync respectively, but my 1538S isn't "locking" to any discernible video signal. Those hsync and vsync signals are slightly (tens of ns if I'm not mistaken, but I might check that again if necessary) delayed compared to the native signals but that's the only thing I immediately notice... and I believe this might be expected due to the sync separator introducing it, right?
(please note I decided to start with the Amiga on purpose, so I could make comparisons with the signals that already come out from it and that are guaranteed to be working)

Given that I am NOT very good at electronics (I just have a little manual skill, nothing more... I'm a software guy after all) Is there something clearly wrong with my setup that I should be investigating?

Feel free to ask for any additional detail or even photos.

 

Is the monitor designed for analog 15KHz RGBHV or the digital 15KHz RGBHV used in CGA? If its meant for CGA, then 15KHz VGA won't work.

 

It's definitely analog RGB, monitor manual even claims digital RGB pins are not connected at all.

And yes, of course it's (analog) 15Khz compatible, as that's the default video mode of the Amiga I used to test it.

 

Are you sure you can't just feed composite sync to the horizontal sync pin?

 

Quite reasonably sure I'd say.
Tried that with the previous, LM1881-based* circuit test, but all I got was a messy picture, heavily distorted on the upper part of the screen. :-\

So I started investigating an a sync separator chip which had "real" hsync output, guessing my monitor actually required this kind of signal, and that led me to the LMH1980.

* = as many of you probably already know, LM1881 has no proper Hsync output, but it *does* have a csync that I used as hsync as per various schematics around the 'net, with the results above :-\

 

Think I used an el1883 some years ago

 

Looking at the datasheet it seems a LM1881 clone i.e. no "true" hsync output.

In the meantime, I'm afraid I somehow destroyed the LMH1980, so while I wait for the replacement chip to be shipped I'm going to try to assemble a new circuit with the EL4583. Let's see...

 

It has vertical and horizontal sync output, what did you see that suggests otherwise?

Like I said, I'm pretty sure that's the chip I used ages ago

 

Sorry, I stand corrected; as I said I'm not very much into electronics. :-\

Pin names were a bit misleading and I didn't understand "horizontal out" which I guess is actually hsync, is that correct?

And since we're here, do you happen to remember if those chips can separate H and V from a csync signal as well, or do they require a "full" composite video input?

 

That I'm unsure about, but most consoles that output RGB via scart use composite video for sync

 

Thanks, and again I apologize for the previous message, I understand it sounded like I was acting in a pretentious way, which is kinda ironic since I am definitely NOT that kind of person. I *really* should have worded differently. :-\

Back to the monitor "hack" there's another thing that I believe I need to know: most, if not all of the RGB-SCART connectors I own have capacitors and resistors installed... but the Amiga (for example) connects directly to this monitor.

Question is, are the RGB signals coming from the SCART directly compatible with the monitor (I'd need an adaptor of course from SCART to DB23 but that shouldn't be an issue), or do those capacitors/etc. have to be removed?

 

If there are caps/resistors in the cable - they are required. If your cable for a certain device doesnt contain them (and the cable is built correctly, not just someone making their own and not including them) then they are already in the circuit, but just on the PCB on the device itself.

I imagine most moved them into the cables to save money (if you dont use RGB, why pay to have the caps etc on the device, when you can move them into the cable for people who will pay the extra for it).

 

If I'm not mistaken the Amiga SCART-RGB cable has those capacitors/resistors as well.
BUT... this monitor has a direct-to-Amiga-DB23 cable/connector. It's built-in (can't even be unplugged from the monitor) so I have to assume there are no mistakes in wiring/etc. It works flawlessly with the Amiga by the way.

Would it be reasonable to assume caps/resistors are on the PCB of the monitor or, generally speaking, somewhere at the other end of the cable?

 

Sorry for the bump.
I've read on this page here that there are monitors that accept composite sync... on the vertical sync pin.

Is it worth trying this? That would solve me lots of problems, since an external electronic circuit to split H+V sync wouldn't be necessary anymore (at least when using a console with composite video out).

 

I say give that a shot. You can also try sending C-Sync to both pins if you haven't already done so.

 

Ok, I will look into it then.

Thanks!

 

BIG UPDATE

Ok, I've rebuilt the circuit from scratch using a brand new chip, and sync separation works!

Problem now is, picture is FAR too dark when connecting the Amiga via SCART: by disassembling the cable (picture is ok with this cable on a TV) I've noticed there are three resistors on R, G and B lines.

Are those resistors responsible for the DRAMATIC brightness decrease (I can barely see the picture) or do I have to look into something else?
If so, can those resistors be removed altogether or should the value just be lowered? I've also noticed some SCART cables for other gaming hardware have resistors *and* capacitors, will those cause issues as well with my monitor?

 

You need the capacitors. From what I've read online, the image will be darker if you don't add caps.

 

I don't get it. :-\
I mean, when connecting the Amiga directly (i.e. monitor cable plugged to Amiga video port) there are obviously no extra caps nor resistors, and picture is ok.

When putting the sync separator in the middle and the SCART cable, apparently the only extra stuff between the native signal and the monitor is, indeed, the resistors.

So, if I understand it correctly, you're suggesting I should add some caps, and that would lead brightness to an acceptable level?

 

You could try adding 220uf caps to the RGB lines. I've seen with the old method N64 RGB mods that you need to add caps to the RGB lines or the image is too dark. Maybe it is the same case here?