RGB SCART->HD15/VGA problems

[it290]

Hi all, first post.

I've been trying to get the best output out of my consoles and finally decided to take the plunge on some RGB SCART cables and other stuff. However, I'm having some problems.

Right now, the only two consoles I've gotten cables for are my US SNES and Saturn. I'm using a PAL Gamecube cable for the SNES. I have these runnning into an SCART->VGA adapter (manufactured by Gefen), which then runs into a VGA->Component transcoder and into my TV. The Gefen adapter has a GS4981 chip inside, which is supposed to perform the same function as the LM1881 and give me separate sync. Also, I know the transcoder is OK as I can run the output from my Extron Super Emotia (RGBS, 4BNC to HD15) without problems, as well as the output from my Amiga 3000. The TV is 15khz only.

So here's the problem - with the SNES I get only a distorted and out of sync image, although this stabilizes depending on the output (although the image remains distorted). Colors are correct. With the Saturn I don't get any image at all. I also tried hooking both consoles to a Commodore 1942 monitor, which is a 15/31khz device, and I get an even more distorted and rolling image with both consoles.

So, sounds like a sync problem, right? The SCART->VGA adapter should in theory provide the right voltage and separate the signals, so perhaps it's just not functioning properly? Any ideas on what I could try?

[NeWmAn]

Is the SCART->VGA adapter powered by the Scart?
 

[it290]

That's something I need to find out definitively. Right now I can only say it looks like it is, since there is a wire inside on the proper pin that connects to the chip. I believe both consoles should be putting out power over the cable, but I have yet to measure it and make sure.

[viletim!]

What do these "adaptors" actually do? You understand that VGA and TV video timings are not compattible don't you?

BTW, There's no reliable way to draw usable power from a SCART lead. If you've got a device that tries to do this (dodgy!) then you might need to do some rewiring?

[it290]

viletim!,

I'm not attempting to do any kind of upscanning, rather all the devices in the chain are dealing with 15khz. All the adapter is supposed to do is take the RGB scart input, split the sync, and spit out 15khz RGBHV on the HD15 end. However, it looks like it's not doing that correctly.

But yes, as of right now I'm thinking that trying to power the chip using an external PS might be the best bet... although I thought the blanking/mode selection pin could be used for that purpose?

[viletim!]

Pull apart both your Gfen adaptor and your SCART cable (the SCART end should come apart easily) then, plug everything in then check, with a multimeter, that the chip inside your Gfen adaptor is getting it's required power supply voltage.

Some SCART cables have the consoles's +5v line connected directly to pin 16 of the SCART, some (like the official PAL SNES lead) have the pin 16 connected to +12v, some don't connect anything to pin 16.  

[it290]

Hmm, well, I got it working, kind of. Putting +5v on pin 8 of the GS4981 (from an external PS) and tying the H+V sync together on the output seems to do the trick. This gives me a beautiful picture out of the SNES, although it is shifted about half an inch to the left. The Saturn and Genesis (actually 32X) now give me a stable picture, but with incorrect colors. I'm going to check and make sure my SCART cables are wired correctly, but other than that, any suggestions (particularly on the SNES' offset picture)?

[it290]

Ok, another update... I have everything pretty much working now. Putting some 75ohm resistors on the RGB lines on the Genesis and Saturn seemed to do the trick, and I adjusted for the offset in the TV's service menu.

The Genesis is now 100% perfect, but the Saturn is a bit dark (which I can live with), and also has some distortion problems... occasionally if a horizontal line is filled with a bright color, that part of the image will tweak and distort to the right. This is very noticeable on fighting games, for example, where the energy bars will be tweaked. It also seems to be happening with the SNES, although more rarely (and in that case the brightness is correct). Any ideas?
 

[viletim]

Have a look at my SCART Cable Diagrams page if you are building your own. Turning down your TV's contrast will fix your distortion problems.

[it290]

Oddly enough having the Saturn RGB lines running straight through doesn't work for me, it needs the resistors on there or else the colors are tweaked both with the transcoder and the 1942. Would putting capacitors on the RGB lines brighten up the colors?

Also, kind of weird, but the Genesis cable is actually only working with my Genesis 1 (using the 32X adapter cable), coming out of the 32X or Genesis 2 it is still incorrect. Not a big deal as I don't play many 32X games anyway, and prefer the Gen 1 because it has no TMSS and the sound is a bit richer.

[it290]

Well.. after screwing with it for a while I still cannot get this distortion to go away on the Saturn and SNES. It's not related to the brightness or contrast on the TV. It does appear to be related to the brightness of the RGB signals, however, as a bright game like Super Turrican will do it often, while a darker one like Super Castlevania IV will not.

A couple of things I have figured out:

1. Putting resistors on the RGB lines has no effect on the distortion.
2. Doesn't appear to be a grounding problem, as far as I can discern.
3. I believe it's related to the strength of the composite video signal coming out of the system, as putting resistors on that line causes the distortion to become worse. I tried putting a 220uf cap on that, no effect.

Attached is an image of what it looks like, it's like the sync goes out of whack in that particular spot. Any suggestions? The only other thing I can think of is that maybe I need to add something in before tying together the H+V sync out of the GS4981..

[NeWmAn]

My Saturn RGB cable has electrolitic capacitors on the RGB lines (10v 100uF), with the Negative leg connected to the scart plug and the positive leg connected to the color signal wire.
Check how your Saturn cable is wired: there's composite sync on pin #1 and composite video (which includes sync) on pin #8. Maybe switching from one to the other can cure your problems.

maybe I need to add something in before tying together the H+V sync out of the GS4981..

Why do you join H+V from the GS4981??? Sync signals are supposed to go to a monitor that wants them separated, so why joining them???
If  for some reason you do really need the comp sync why don't you use pin 1?


Viletim: there's a slight error on your Saturn cable diagram description, the line:

...9 composite video, 9 luminance, 10...
Should be:
...8 composite video, 9 luminance, 10...

[it290]

Using the composite sync did the trick, alright. I suppose I should have just done that in the first place, only I didn't want to go to the bother of opening up the SNES and hooking it up (it's an SNESjr and I hadn't connected that line previously), not to mention the trouble of opening up the Saturn end of the Saturn cable.. which I did, getting rid of great gobs of epoxy crap, only to realize that the sync line was already running to the blanking pin on the SCART end.. oh well, live and learn I suppose.

As for tying together the H and V sync, I don't know why it's needed, but it is. Both the transcoder I'm using and my 1942 monitor will only sync if it's hooked up that way. Perhaps it's something to do with the peculiarities of the GS4981 chip and/or the signal I'm feeding into it, as the transcoder deals OK with just having the H sync from other devices, and IIRC the 1942 gets both H and V when hooked up to an Amiga. I could use pin 1 from the chip, but all that stuff is on a little PCB and jammed into the SCART->VGA box and it's easier just to work on the cables.

[viletim!]

it290,

Connecting H and V sync together does not make composite sync. Unless you know exaclty what your doing you should never connect two digital outputs together. Why don't you just throw this adaptor of yours in the bin, where it evidently belongs, and connect the RGB + composite sync from you game consoles to your other equipment (which seems to grow with each post you make)...

[it290]

True enough, I probably should have just built an adapter with the LM1881 circuit in the first place, but everything seems dandy now, so I probably won't mess with it unless other problems crop up. As for my growing list of equipment, that's true as well... I just picked up a Sony RGB TV at the thrift store which I plan to use for tate mode, so the next thing to do is get that hooked up....