Bizzare RGB issue. (Sega Genesis)

[SL_]

The port on th X'eye was a Genesis II RGB capable port  but...

I'm enountering a really weird visual defect when attempting to connect it to my external component transcoder via RGB-Scart. There's no white. Anything that would be white is simply displaced as black on the screen outlines, details, sega start up text, but all the other coloring is there.

Does anyone have the slighest idea about what's going on?

All of my other RGB capable consoles work absolutely perfect with the JSTech encoder and all the connections are secured...

[NFG]

Sounds like you're overloading your transcoder.  It can happen in an electrical circuit that the signal is too bright and it copes by 'looping' back to zero, essentially it's dealing with failure by killing the inputs entirely.  I don't know what to suggest as a fix, perhaps some 220uf capacitors on the RGB lines?  

[atom]

Wouldn't he want to try a pot to reduce the signal? I'm no expert at Ohm's law but wouldnt the resistance = weaker signal? I have never really understood why people seem to try everything to avoid resistors and pots. Take DirectPad Pro interfaces, they use diodes to weaken the voltage. I am not saying its wrong I'm just saying I don't understand so if someone could explain it to me thatd be great.

[Aidan]

This isn't really the right place to explain, but I'm going to anyhow.

With the directpad pro interface, the diodes are used for specific reason. The pads expect to see about 3.3V or so. You could put a resistor in place to drop the voltage, but for two things. Firstly, the diodes only allow the current to pass from the pins on the parallel port to the pad. No current can pass backwards from the pad to the parallel port. Windows is known to wiggle pins on the parallel port when it's starting up.

Secondly, the pads are not constant current. When they're operating, they take more power than when they're idle. The heaviest draw is taken when the pads are being polled and are operating in analog mode.

If you put a resistor in place that was the right size to drop the voltage when the pad was idle, there wouldn't be enough power to run the pad when it's running flat out in analog mode. Result is the pad is unstable and restarts itself.

On the other hand, if you put a resistor in place that was the right size to drop the voltage when the pad was running flat out, the voltage would be too high when the pad was idling.

Diodes have a property known as a forward voltage drop. Think of it like a weir on a stream. The voltage is the amount of water. If there's not enough voltage (water), then it doesn't come up high enough on the wier to spill over. The height of the wier is the forward voltage drop of the diode. As you can see, when the diode does conduct electricity, it loses a portion of the voltage. For the average small signal silicon diode, it's not unusual for the voltage to be around 1.2V.

As the parallel port outputs 5V, the diodes drop 1.2V of that voltage whatever the current is. That leaves the pads with about 3.8V, which is much better than 5V. (There's other issues with I/O voltage compatibility, but I'm not going to go into that now!)

[atom]

Awesome explanation! Thanks.

[Darklegion]

Have there not been issues in the past with genesis rgb output being too bright? This sort of  thing could happen if the transcoder is also an amplifier since it is probably using transistors to up the signal strength and any small change on the input side could mean a much bigger change on the output side.I think the genesis always required 75ohm resistors on each rgb line,but I don't think I still have the docs for it.Also dc restoration could be an issue here(it involves blacks and whites),in which case it may be because your rgb lines are not ac coupled and your transcoder probably expects that,and if it is not there you may lose brightness information on the output from the transcoder.Wiring in some ac coupling caps(220uf is recommended by most as this is what was used in the past but anything from 10uf would probably do it...especially if you are lacking space