Proper 50Hz/60Hz mods for Master System/Mega Drive/Sega Saturn? (Crystal Switch)

[Link83]

Hi all,
So I have just recently acquired an XRGB-Mini for scaling my retro systems to a HDTV. It works great The problem is some PAL Sega systems suffer from slight 'stuttering' problems when switched into 60Hz using the standard 50Hz/60Hz mods as described here:-
http://junkerhq.net/xrgb/index.php/Sega_Genesis

This is due to the crystal oscillators in PAL and NTSC systems using different frequencies, which results in a very slight speed difference (Apparently less than 1% out) this is usually unnoticeable to most users (Especially on CRT TVs etc) but its enough to cause some stutter when using the XRGB-Mini (and possibly on some other scalers/HDTV's as well) Now although I could just swap the systems PAL crystal for an NTSC one, this doesn't really solve the problem since then 50Hz games suffer from stutter instead of the 60Hz versions.

I am hoping for a mod which would allow the use of two crystals - one for each region (I realise that for true NTSC/PAL output the video encoder would also need to be switched between PAL/NTSC, but lets ignore this for now since I will be using RGB to connect all my systems any way)

The standard crystal frequency for color encoding NTSC is 3.579545 MHz, and for PAL its 4.43361875 MHz, so all the systems use crystals whose frequency is a multiple of these two base frequencies:-

-MASTER SYSTEM- (Crystal Oscillator 'OSC1')
53.693175 MHz - NTSC (x15)
53.203425 MHz - PAL (x12)

-MEGA DRIVE- (Crystal Oscillator 'OSC1')
53.693175 MHz - NTSC (x15)
53.203425 MHz - PAL (x12)

-SEGA SATURN- (Crystal Resonator 'X1')
14.31818 MHz - NTSC (x4)
17.734475 MHz - PAL (x4)

AFAIK the Dreamcast should be unaffected by these issues, since all motherboards are identical worldwide(?) and the region is set in the flash memory.

The Saturn crystal resonators for each region are relatively easy to find, the Master System/Mega Drive crystal oscillators are more difficult, but by searching on Taobao I think I may have found a few suppliers for the 53.693175 MHz oscillators needed (I cant find the PAL equivelant, but this doesnt matter since it can be taken directly from the PAL system being modded)

-THE MOD-
The 'simple' method to switch between crystals would be to simply connect wires to each crystal and use a switch to select which one is used, but this is not recommended as you would need to run long wires/traces which is not a good idea when your dealing with clock signals.

In Argentina some Mega Drive III's came with 'official' 50/60Hz mods which also switched between the crystals, as you can see from this post by chinitosoccer and this picture:-

Just recreating that circuit would be great, but if we could improve upon it that would be even better (It appears to require three wires)

What I am hoping for is a circuit board design/schematic which I could use to design a small PCB that could basically be 'dropped in' to the system where the original crystal was (Like the Argentinian Mega Drive pictured above) and would use one high/low pin like the standard 50/60Hz mod to switch between the two crystals (+5V=60Hz, GND=50Hz) this would make it very easy to retrofit this mod to consoles already 50/60Hz modded.

I have seen the SNES '1CHIP' 50Hz/60Hz mod which uses a similar idea:-
http://sd2snes.de/files/1chip-5060.pdf
However i'm not sure if this 74HCU04 design could be used directly with the MS/MD/SS. Also I am guessing two different designs would be needed, since the Master System and Mega Drive use a crystal oscillator, whereas the Sega Saturn uses a crystal resonator.

Thanks for reading, and for any help/advice you can give me

[TmEE]

You only need the 74AC00 / 74F00 chip arrangement of the SNES 1CHIP mod for this clock swap. The freq is really high however and you need to pay attention to maximum swithing freqs of particular logic gates.

[zyrobs]

I'm looking for something like this as well so I can have my Saturn with both crystals installed. I've found that using 50hz mode with ntsc crystal will tilt the picture to the left a lot, even with RGB cables... in fact, my Megadrive is also tilted to the left like that.

[Drakon]

Wow, selectable master clock oscillators.

[Link83]

You only need the 74AC00 / 74F00 chip arrangement of the SNES 1CHIP mod for this clock swap. The freq is really high however and you need to pay attention to maximum swithing freqs of particular logic gates.

Thanks, so just to confirm, this layout should work for MS/MD/SS for switching between crystals?

Although i'm not sure if anything needs to be changed when using a crystal oscillator for the MS/MD, as opposed to a crystal resonator (plus capacitors/resistors) for the SS?

The Sega Saturn also already has the capacitors to ground, 1M resistor and 100R resistor to create an oscillator circuit, so is it still nessary to add these components on the above circuit?


Lastly, its worth noting that the Sega Saturn uses different form factors for the X1 Crystal, which means we cant design a direct a drop-in PCB as i'd originally hoped:-

[MaxWar]

I do not see an oscillator in My NTSC sms. It only has a 2 pin 10.7386mhz crystal. *It is an older version

[Drakon]

I do not see an oscillator in My NTSC sms. It only has a 2 pin 10.7386mhz crystal. *It is an older version

Yeah that 2 pin crystal is wired into some caps and that's the oscillation circuit. The main chip divides it down to 3.58 mhz and that drives everything on the pcb.

[RichB93]

I'd be interested in buying a crystal from you should you get any NTSC ones as I'd probably just do a straight swap with my current PAL one.

[lumclaw]

I'd need a PAL crystal, and we all know how rare PAL consoles are here.