OSSC Pro and the NAC Splitfire Jamma Capture Compatibility Thread-a-go-go.

[marqs]

Let’s first try to rule out the particular OSSC Pro unit is not faulty. I just a built a sync buffer with SN74HC14N which should be relatively close to Splitfire (can’t find large enough picture of it to see which chip(s) are used for sync – maybe SN74HCT04?) but could not replicate the issue.

[themisterfalcon]

Hi @marqs and others, thanks for your help in trying to figure this out and please let me know if I can provide any additional details or high-resolution photos. As mentioned, I am happy to send a unit to you directly for testing. I  can send it as a gift etc. as well to avoid customs stuff since this is being donated for research.

 

[marqs]

If you can provide details of the IC used as the sync buffer and any other possible components on sync path of Splitfire, I should be able to build exactly the same circuit and test it quickly.

[marqs]

Tested a few things with the replica circuit and noticed that adding a 500ohm series resistor after the buffer significantly narrowed down the TTL hsync range where Pro could reliably lock. I don’t know if Splitfire has a resistor like that on board, but in that case one workaround could be replacing it with 0ohm resistor (or maybe 100ohm to make it more safe with cables connecting sync line directly to SCART) to make it more compatible with Pro. I’ll be also trying different ADC settings to see if it can be made less sensitive to sync characteristics even though hysteresis and glitch filtering should be already enabled.

[marqs]

Depending on the sync circuit of Splitfire, even more simple fix/workaround may be possible which does not affect connection to other devices. I’m waiting details / pictures of the Splitfire before writing instructions here.

[djpsykom1]

OK good to know.  I’m hoping Birdland has one on the way.  If not I can ping them.

[marqs]

I noticed that Splitfire manual has the relevant details I was looking for, confirming 74HCT04 inverter (presumably 2-pass to preserve sync polarity) is used with 470ohm series resistor. I’m assuming you used this configuration instead of utilizing THS7374 as sync buffer. If that’s correct assumption, then fixing compatibility with OSSC Pro involves just adding a jumper wire between VGA pins 4 and 13 on Splitfire, and using AV3_RGBS input on Pro. A fully wired VGA cable is also requred for this configuration to work.

I’m still trying to to figure out the root cause why some VGA sources like Splitfire or X68k do not work reliably as-is. I have replaced HV sync level conversion circuitry on one OSSC Pro board with SN74LVC2G17 buffer which seems to fully fix stability issues, but it’s not very easy to add afterwards (and theoretically is inferior to existing circuit).

[djpsykom1]

Ok good to know.  I have a few spare splitfires.
I’ll see if I can make the jumper happen.  If you still need one, I can ship one as well.

Thank you for continuing to look into this.

[themisterfalcon]

Hey Marqs, apologies as I didn’t follow up on the thread. I will DM you now so we can get a unit over to you for testing and such.

 

[djpsykom1]

Hello,  has this been sorted out?   Will this potentially be resolved in a future version of the ossc pro?  Is there anything I can do for my current unit that will resolve the incompatibility?

[djpsykom1]

Sorry double posted

• This reply was modified 10 months, 1 week ago by djpsykom1.

[marqs]

I didn’t get hear back after responding the DM, but I have modified a couple boards as a test with SN74LVC2G17 added on HV sync path. That resolved the stability issue on my test setup made to mimic Splitfire, and also X68k on another tester’s setup. I can post the instructions for anyone willing to do this on his/her board, and after some further testing this can be included in future HW revisions.

I’d like to believe there is some way to fix this by just configuring the ADC chip, but so far this has been a dead end. Perhaps there are some undocumented registers that would need to be tinkered with, but this kind of info has maybe been passed to big customers only. The only way then to get the right settings might be finding an old TV/projector with the same chip and dumping the I2C traffic.

[BuckoA51]

The fix is discussed in detail here:- OSSC Pro AV3 HV sync fix for Splitfire, X68k etc. – VideoGamePerfection.com

• This reply was modified 9 months, 2 weeks ago by BuckoA51.

[Arghya77]

This thread is a treasure trove for arcade enthusiasts diving into OSSC Pro and NAC Splitfire Jamma capture setups! The discussions on compatibility and signal quality are incredibly helpful. Whether you’re fine-tuning your arcade display or ensuring perfect capture for streaming, this is the spot. Big props to the contributors for sharing insights and solutions—proof that retro gaming communities are alive and thriving. Keep the knowledge flowing!

[BuckoA51]

Thanks! That’s why we like to keep a forum running, and not a discord where the information gets buried after a few weeks.

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