Video output

The Jammafier does not touch the video provided by Jamma game.  It is routed directly to the female VGA connector, using standard pinout:

Pin Signal
1 Jamma RED
2 Jamma GREEN
3 Jamma BLUE
4 n/c
5 GND
6 GND
7 GND
8 GND
9 Optional 5V output*
10 GND
11 n/c
12 n/c
13 Jamma CSYNC
14 n/c
15 n/c

*) The 5V output can be enabled by closing the solder-loop on the back of the board.  This is a convenience feature only, and is not needed for normal operation.

IF your monitor supports 15kHz, csync and 0-5VPP RGB with 1k Ohm impedance - you can feed this directly to your monitor.  Unfortunately, monitors in most JVS cabinets do not support this, and the signal has to be either up-scaled or conditioned.

 

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 has 75Ohm termination resistors.  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 re-amplifying the signal to 75Ohm with levels within 0-1VPP, it also does this without loading the game out of spec.