Sega Mega Drive 1 PAL S-Video 50Hz color fine, 60Hz B/W

[marquisor]

hello!

as the RGB mod for 1084S-D1 still not working, i tried another one on my own.

as seen in this picture: 


i built two cinch cables (similar to the c64 ones) for connecting to the 1084S-D1 luma/chroma ports.

luma: pin 16 "Y OUT" + 220 µF + 75 Ohm tocinch  "signal" and pin 24 "GND2" to ground
chroma: pin 15 "C OUT" + 220 µF + 75 Ohm to "signal" and no ground (doesn't matter for my issue if GND2 is connected or not, as luma GND seems sufficient)

50Hz mode working fine, pristine display. but if switched to 60Hz it all gets black/white. what's the fault??
(with RGB mod 60Hz works flawlessly, except sync issues in another thread from me)

[Midori]

If you switch the console to 60 Hz, you change the clock in the system which is used for the PAL encoding. RGB is pure and has no encoding, therefore you still get colour :-)

You can probably fix this though by going to your cxa1145 encoder in your Mega Drive, disconnect the system clock and installing a dedicated crystal providing PAL clock for it. That is the only real solution however. You must add a crystal for it.

[marquisor]

thx for quick answer:

alright, so i need an "asynchronous" clock with 4,43361875 MHz. is that right? how to apply that?
and are you sure, that it simply DOES display less lines in 60Hz mode?

the clock is calculated by

PAL: 15625 Hz * 283,75 Periods + 25 Hz = 4,43361875 MHz ;
NTSC: 4,5 MHz / 286 * 227,5 Periods = 3,57954545 MHz
(source wikipedia)

though i'm not sure about PAL60, yet!

[Midori]

I'm no expert on crystals and the clock they provide really, so I won't pretend that either :-) But I do know that they are used in the system and sort of what for. And switching the Hz operation of the MD/GEN changes the clock divider which produces this problem. As to applying the stuff Phreak here wrote in this thread how he did it:

http://nfggames.com/forum2/index.php?topic=894.0

Like the datasheet that you linked in the first post says :-) Although, I'd disconnect pin 5 and 6 from the board. Not sure why he didn't... Perhaps I'm missing something...

And yes, it sure does display fewer lines when operating in 60 Hz, otherwise the TV wouldn't be able to display the signal :-) At all. You'd get a rolling picture or something even less useful.

[adamj1664]

you can do 2 things here

the easy one is buying a rgb cable and use that, look on ebay one seller is selling the cheap, i bought one.

the tricky one. look up mmmonkeys guide on it, just find the link on the home page, there is a guide on what to do there.

[l_oliveira]

Here's why it gets B&W:

NTSC =  53.639Mhz   3.57595x15
PAL    =  53.203Mhz   4,43361x12

When you switch to 60hz the VDP changes divider from 12 to 15 and the clock it gets is 3.546Mhz.

So yes, only solution is asynchronous clock.

Here's what you do:

1- Cut the copper tracks which connect to  pins 6 (Chroma clock) and 7 (NTSC/PAL) of the CXA1145.
2- Connect pin 7 of the CXA1145 to GND (this forces it to stay on PAL mode. the track that comes from the VDP changes to +5V when in 60hz)
3- Connect the PAL 4mhz crystal to pins 5 and 6 of the CXA1145 chip.
4- Test and if it does not work verify the connections.

[phreak97]

the mod i did wasnt my own creation, someone helped me with it, so i cant verify its accuracy, only that it does work. I did that years ago before i had much understanding of clock signals etc. There may be a better way to do it.

I didnt think you could just connect in a crystal like that, dont you need a couple of capacitors to get it oscillating?

[dadou]

for md1 here's the solution: http://www.zipplet.co.uk/index.php/content/electronics_mdpal60 

[marquisor]

@dadou:

very nice link, i will order parts and see how i can get it done! i love s-video, good compromise between sharpness and blurryness on sega megadrive and its games

thx!

[MangledLeg]

FWIW, I wrote a tute a few years back following help from Viletim! and others on this forum, and recently re-added it to my blog. You can view it (complete with crappy pics ) here.

[dadou]

do you know how to do this mod for mega drive model 2?

[albino_vulpix]

The only reason this does not work with the Mega Drive II is because the video IC doesn't have a crystal oscillator driver built in. Making your own, if you know what you're doing, should be a trivial matter. It seems no one who has this knowledge has gotten around to doing it :p

[dadou]

i tried to do a crystal oscillator but i didn't suceed  i'm sure that someone can do it but i didn't find this guy yet ^^