World Cup: Analog Controls and Audio Volume

Work on Tehkan World Cup electrical schematic diagrams is finished. It has brought up a number of new discoveries on this hardware:

• correct video timing (including frame rate)

• 3kHz antialias audio filter for PCM

• 16-step volume control for PCM audio

• priority control for character and sprite layers

Comparing to last week's update, the new version of the core implements:

• analog controls via analog stick on game pad

• antialias audio filter and variable audio gain, matching PCB

• video layer priority control, matching PCB

• DIP switches are correctly read by the CPU

Analog controls totally change the game experience. Not because you would like to walk slowly towards the goal, but because you can now do smooth diagonals so you can move around opponents in a natural way. Trackball controls are not supported yet. I have not been able to achieve a nice experience with them yet.

Volume Control

One of the most interesting parts of this board is the analog volume control for the PCM sound effects. The ADPCM used for sound samples on this game is the OKI-M5205. This chip did the ADPCM decoding and the digital-to-analog conversion, but it did not have any sort of volume control. Tehkan's hardware engineer added this feature by connecting four external resistors via controllable switches. The sound CPU could decide which resistors would be used to connect from the OKI chip to the sound amplifier, effectively reducing the sound from as much as 20.8dB to as little as 6dB.

This feature is actually used in the game in order to control the drums and other sound effects. This was an unknown feature. The MAME documentation had identified that there was data written by the CPU, but they could not guess what it was doing. I will report this in detail to MAME's team for implementation. But, as of today, you are the first people to hear this effect again.

Grid Iron 

There is another game currently playable on this core: Grid Iron. This is the American football game for which the hardware was originally done. The cabinet had a 7-segment display for each player, hidden from the view of the other player. This was used to select the team formation. We currently do not have a nice way of reproducing this feature. On top of that, I am not so convinced that this game will be played at all taking into account that it was a commercial flop. So for now, I am not implementing the formation displays. Indeed, I think modifying the software (via a ROM hack) to display the formation on screen would be better for the general modern player. Anyway, the game is available and playable on today's update.

Other Updates

• JTWWFSS: fixed bad graphics on right side of the screen on Analogue Pocket

• JTS18: fixed coin assignment on Moonwalker

Upcoming Work for End of 2024

I am finishing the work on The FairyLand Story beta core, which is the predecessor of Bubble Bobble. When we started working on the schematics, we were not expecting any findings. I still wanted to do them for preservation. But, extraction of the PCB schematics turned out to be full of surprises. The emulation of this game is surprisingly not accurate in many, many ways. More on this for the beta release.

There is one more beta core I want to get out. This time, a 90's game of my choice for a change! I am targeting the evening of the 5th of January 2025 for this. It is a big system and a hard schedule, but the motivation is high!