RGB Genesis to Arcade Monitor... Help Wanted

[Sensato]

I've finally seen the Sega Genesis on S-Video and I am highly satisfied with the signal improvement, however I now want more, I want it better. Can't sleep at night. My eyes want to see what RGB is all about!!!

I've got an Arcade Raster Monitor (Sanyo 20 EZV) which I think I could use to display proper RGB. I've done a quick test, pulling the Red, Green and Blue, as well as Sync directly form the AV Port. Didn't work, nothing was displayed.  I'm thinking that maybe I didn't inject the signal at the right place on the monitor circuit. Might be that or the fact that the signal has to be pulled form the video encoder chip through a 75 Ohms and 220uF (assuming I'm using a Gen3). In which case the Sync has to be taken from... the AV Port or the video chip too?

Basicaly, I'm just not sure how to hook up my Gen to an Arcade Monitor and a little help would be welcomed. Thanks in advance!

[viletim!]

a few things to think about:

-- The Genesis model 2 has an unbuffered sync output - you shouldn't drive your monitor with it. You'll have to either buffer the signal with a logic chip or strip the sync signal off the composite video with a LM1881 IC.
-- Have you tried other consoles on your monitor? Many arcade monitors can't properly display the video from a console without an amplifier (though you suold still see a dim picture)
-- Did you connect everything properly (including ground!)?


BTW You won't need a capacitor and resistor in series with the signal when connecting it to an arcade monitor.

[Sensato]

Thanks for the tips viletim!

-- The Genesis model 2 has an unbuffered sync output - you shouldn't drive your monitor with it. You'll have to either buffer the signal with a logic chip or strip the sync signal off the composite video with a LM1881 IC.

That applies to every system using the CXA1145 for video chip I assume (Genesis 1 and 2, SMS...)? What about the Genesis 3?

I'm also not sure what buffering the Sync does to the signal. Inverting it's polarity maybe? While I'm there, does the Genesis 3 output Negative or Positive Sync? I'm asking because another arcade monitor manual I have states that this other one uses Negative Sync. I'd run tests on this monitor too but the Cab it comes from has to be fixed, I'm not even sure the monitor works properly.

Have you tried other consoles on your monitor? Many arcade monitors can't properly display the video from a console without an amplifier (though you suold still see a dim picture)

Sadly, I don't have any other RGB-ready console at hand right now. In fact, I've never seen a game system displaying RGB. I believe everything is properly connected to the monitor. By turning the Master Screen pot up a bit, I could get this ''negative'' image :

http://img.photobucket.com/albums/v299/jpb...ests/GenRGB.jpg

[viletim!]

You many be right... I checked my original model Mega Drive and the signal is also unbuffered. By unbuffered I mean that there's no buffer circuit between the signal's internal connections and external connections.

Inside the Mega Drive !CSYNC comes out from one of the ASICs and connects directly to the CXA and the video output port. If the signal were to be short circuited at the output port then the signal will no longer reach the CXA. I suspect your monitor has a lowish value resistor to ground on the sync line (to reduces noise). The output from the ASIC is not very strong (at least in the case of the MD2), and when connected to the monitor it may be pulled down too much for the CXA to to recognise it.

Code Select Expand

.   INSIDE SMD             INSIDE
.                          MONITOR
.-----+    |cxa  |
.     |    |1145 |
.big  |    +-----+
.sega |      |                
.asic |------+---------//---+---etc    
.     | !CSYNC              |    
.-----+                     R
.                           R ~1K
.                           R
.                           |
.                       GND---

If the CXA doesn't receive it's !CSYNC then it's internal clamp circuit will not function and it won't output any usable video (remember that the RGB must pass through the CXA).

At least that's the theory.... to test this I suggest simply disconnecting the sync signal. If you see out of sync video on the screen as a resault then you'll know this is the problem. If it is, there may be a simple soultion. If there is a genuine CXA1145 (inside the MD2 you'll often find a clone chip made by fujitsu or samsung) it will have a buffered sync output at pin 10 (sync input is pin 9). Try using this signal instead.

[viletim!]

BTW, Everything outputs negative sync.

[NFG]

Practically nothing uses positive sync.  Some older Williams arcade PCBs are the only things that spring to mind...

[Sensato]

You many be right... I checked my original model Mega Drive and the signal is also unbuffered. By unbuffered I mean that there's no buffer circuit between the signal's internal connections and external connections.

Sorry if I sound totally ignorant but may I ask what excatly does buffering a signal do?

Without SYNC, the monitor I'm using will not display anything, like it's cutting the video signal. I should get an oscilloscope in the next days I'll be in a better position to understand what's going on exactly.

If nothing works out in the end, I might end up with the usual Scart + RGB to Component converter option but I'd really like to turn one of my cabs into an RGB Genesis, using the arcade stick as a controller.

[viletim!]

Sorry if I sound totally ignorant but may I ask what excatly does buffering a signal do?

In this case it would isolate the internal signal from the external signal. If it were buffered you could short-circuit the external signal ground (or abuse it in some other way) and the internal signal will remain unharmed.

[RGB32E]

While this might not be useful for your Sanyo arcade monitor, filtering the composite sync output with a 220uf cap fixed the sync problem for me (on a Sony PVM-2030).