Genesis Model 1 to Commodore 1084s-D1 RGB: "Smearing"?

[allyourblood]

Hey folks.

I finally got my hands on a 1084s-D1, and now I've made an RGB cable for my Genesis. I originally tried connecting everything straight across with no caps or resistors, "just to see". The result was a color-shifting image appearing on the screen for about 2 seconds, and then fading to black. I added 75ohm resistors to each of the RGB lines and that gave me perfect color but that same disappearing image. I then added a 75ohm resistor to the sync line as well and that gives me an almost perfect picture, but I'm left with some "smearing", or horizontal blurring going on.

I have read several posts mentioning 220uf caps on the RGB lines, so I tried that with no change, so I took them out. Finally, I tried adding the same type of cap to the sync line, but then I get a rolling/scrolling picture. Aside from the smearing I'm seeing, the image looks absolutely excellent: colors and sharpness are fantastic.

The attached photos were taken quickly and I admit they're a little lacking, but they do show the weird effect I'm talking about. Anyone have any ideas how to correct it?

(Also, I've been using the search function for the last 3 hours, trying different terms in an attempt to find someone describing the same situation, to no avail. I kept finding situations that were similar to my own, but trying out the few suggestions offered haven't changed things for me. I figured it was time to post.)

NFG Note: The attachments on this message are missing.  Sorry.

[albino_vulpix]

Give this a shot.

[allyourblood]

Thanks for the reply, albino_vulpix.

Before I do that, I should mention that I tried out a few other consoles with my cable this morning. In total, I have tried 3 Genesis consoles (1 "HD" model and 2 newer ones) as well as 2 Master systems, and all exhibit the same smearing effect. Taking this into account, should I still go forward with that fix you posted? I mean, is it possible that all 5 consoles will need this same adjustment?

A couple other things that might help rule out a few possibilities: I have tested this monitor with composite video and the image is normal; obviously it doesn't come close to the look of RGB, but the smearing/horizontal line issue isn't there. As far as I can tell, it also isn't there when I remove the resistor from the sync line, but then obviously I get that rolling, unstable picture.

EDIT: Screw it, I'm'a try it anyway. I'll post back in a bit...

[allyourblood]

Okay, I tried that. Cut the trace to pin 10 and added a 220uf cap and 75ohm resistor to pin 11, attached them to the c-sync pin on the DIN. Unfortunately, it just results in a rolling picture very similar to when I had a cap on my original configuration. I removed the cap from pin 11 and am still left with an identical flickering, rolling screen; it would appear adding the cap to that line isn't making any sort of visible difference. It would also seem that pin 11 isn't going to solve the issue...

So I took everything off and connected the sync pin (on the DIN) back onto pin 10 of the CXA chip. Here's an interesting tidbit: I left the resistor inside the console in place and added a second 75ohm resistor on the cable itself. I'm definitely getting less of that smearing issue. Does that offer up any clues as to what's going on?


UPDATE: I added yet a third 75ohm resistor to the sync line and I have a nearly perfect image.  There is now only the slightest hint of flicker/smearing on some areas of the screen; it's almost nonexistent (actually, now I've been letting it run for a few minutes and indeed, it looks to be a perfect image). I'm happy that the signal is tamed, but can anyone confirm that this is normal. I don't recall reading anyone having to add so much resistance to the sync line in order to get an accurate picture.

[allyourblood]

Took all the 75ohm resistors off the sync line, slapped on a 220ohm... looks good 90% of the time, certain games and certain screens seem to cause a slight amount of that horizontal line-stuff to occur. Even when it's happening, it's hardly noticeable and most of the time I don't see it at all. I still don't get exactly what I'm seeing here, and several more searches (both here and on several other sites) have turned up nothing. I find it strange that all of my Genesis systems behave identically, considering I haven't seen anyone else mention theirs being wired in this way.

 

[albino_vulpix]

Odd that you need to attenuate the sync line like that. Does the Genesis' composite video work properly on the monitor?

[allyourblood]

Odd that you need to attenuate the sync line like that. Does the Genesis' composite video work properly on the monitor?

Yes, perfectly. I initially tested the monitor by feeding it a PS2 and Saturn over composite and everything looks great. At this point, I'm guessing it's the monitor that's to blame. I have another 1084s to test with, but I"m gonna have to start a new cable for that one because it uses a 6-pin/8-pin DIN.

[viletim]

allyourblood,

It's not your monitor to blame, this is a design flaw of the Megadrive/Genesis. The composite sync output from the console cannot reliably drive a typical monitor's composite sync input. This problem only occurs with monitors which require a TTL composite sync source such as many models of Commodore 1084.  There are two ways to fix it:
1) Build a digital buffer circuit for the composite sync signal. For example you could use a CD4050 with all the gates tied in parallel for a good strong output which will drive any monitor.
2) Build a sync stripper circuit which will generate a digital TTL composite sync signal from the composite video output. The LM1881 is the most popular.

Anything less than this will not be reliable, placing just the right amount of resistors and capacitors in series with the line might work for now but your screen will roll at even the smallest solar flar.

It's unfortunate that only some European models of 1084 can sync to a composite video signal without modification...

[allyourblood]

Wow Tim, thanks for that. That sounds like what's happening, although for now I get a really steady and accurate picture  almost all of the time (Although I found some games to cause a small amount of flicker, most others look perfect).

Well, at least now I have an excuse to build an LM1881! Thanks for shedding some light on this for me.