Video output
On revisions prior to 2.0, the Jammafier does not touch the video at all.
Revision 2.0 has two modes, JAMMA which is untouched like earlier versions and Tri-Sync mode (default). In Tri-Sync mode RGB is attenuated and re-amplified, while sync either split into H/V, or processed into regenerated CSYNC.
Regenerated CSYNC will fix issues some games have with Sanwa PFX monitors, while Toshiba PF monitors seem to prefer H/V sync.
| Pin | Jamma setting | Tri-Sync setting |
| 1 | JAMMA Red | 75-ohm Red |
| 2 | JAMMA Green | 75-ohm Green |
| 3 | JAMMA Blue | 75-ohm Blue |
| 4 | n/c | n/c |
| 5 | GND | GND |
| 6 | GND | GND |
| 7 | GND | GND |
| 8 | GND | GND |
| 9 | Rev 2.0 n/c Earlier optional 5V | n/c |
| 10 | GND | GND |
| 11 | n/c | n/c |
| 12 | n/c | n/c |
| 13 | JAMMA CSYNC | HSYNC or CSYNC |
| 14 | n/c | VSYNC or n/c |
| 15 | n/c | n/c |
Some notes on Jamma video for the technically inclined
Adding 220 Ohm series resistors on RGB lines
A few people have reported success in bringing the video levels down to 0-1VPP levels by adding 220 Ohm resistors in series with the RGB signals, which is required by some tri-sync CRT monitors.
If this signal is fed into a monitor with 75 Ohm termination resistors, it effectively results in a voltage divider that divides the signal by approximately 4 – IF the output impedance of the game is very low (strong).
There are 2 issues with this approach:
- The game is loaded with 295 Ohm, which is out of spec
- It does not take the output impedance of the game into account.
The resulting video levels will be highly dependent on the games output impedance, which unfortunately varies a lot.
Namco issues
The Namco adapter uses active electronics to change the impedance of the signal – and in the process introduces a noticeable distortion/softening of the image.
TODO: Check RGB output levels when loaded 75Ohm.
Riverservice RS issues
The RS use 75Ohm termination on RGB. This is not great for a couple of reasons:
- Jamma games do not have an impedance of 75Ohm, meaning the load put on them is too high and may cause failure
- If this signal is fed to a scaler/monitor that also terminates the video with 75Ohm – the resulting load will be 37.5Ohm, about 30 times higher than what the games are designed for (1kOhm).
Very few games actually have 1kOhm output impedance, somewhere in the hundreds is more normal – regardless, the 75Ohm termination serve no good purpose (that I know of), and put a load on the RGB outputs that is way out of spec.
Konami
The Konami does a very good job of actively attenuating and re-amplifying the signal to 75Ohm with levels within 0-1VPP, it also does this without loading the game out of spec.
Last Updated on 2022-08-20 by admin