I did not discover this, but I wanted to make sure this information is in the general Retro Duo information thread:
Want to hook up RGB? Use this PDF link
for the data sheet of the video encoder of the Retro Duo. Basically it says that you can get RGB from pins 21, 22, and 23 from the chip labeled "KA2198BD".
Grahf and I have messaged each other a couple of times about the subject, and I thought I would post his excellent response to a question I had for him:
Life of Brian wrote:
I was wanting to message you about getting RGB off of the Retro Duo. You mentioned in zzatack's thread (I think that's his name) about not even needing to use caps for the mod if using a PSOne screen? I was wondering if you could elaborate a little bit more about it.
Check out the reply I made in that thread. Do a search for KA2198BD in google, and you'll find it's datasheet.
Basically the purest form of video inside consoles is RGB. The video encoder ICs take this raw signal, and normalize it. They then output composite, s-video, or RGB, depending on the console maker. Basically in the FC Twin/Retro Duo the potential for RGB or S-video is there, even though it never makes it out the back. These same ICs (CXA1645 or clones) are used in many newer Genesis models, most Playstations, Saturns, etc. There are older varieties that could only do RGB -> composite or crap s-video (early Genesis and NeoGeo). Some systems, such as the real SNES don't rely on any external video encoders, and could output everything on its own.
Anyhow, the cap+resistors are only needed on the output lines of these chips to further "trim" the signal to be suitable for a TV. On the PSone LCD, it's not needed. On the CXA at least, don't know if the clone is with the range it can handle.
Sometime next week I'll be testing whether or not caps and resistors are needed with the Retro Duo's video encoder chip when used with a PSOne/Zenith screen. If I don't, then hip hip hooray! An easy way to get RGB from the Retro Duo!
One last thing - do I need to hook up composite sync? Is that even relevant to RGB?
sweet. ive spent a third of my life on benheck!