Light Gun Commando: LCD-compatible light gun support for original console hardware?

Tuesday, 13th December 2022

As the previous entries on this site might indicate, I do enjoy a good light gun game. Unfortunately, when it comes to playing them on home consoles you usually need to use a CRT television for the gun controllers to work. I have a CRT or two at my disposal but I'm well aware they won't last forever and they're not really the most convenient devices even when they are working at their best.

Bearing this in mind I decided to try to find a way to get light guns working on LCD TVs. My experiments back in early 2015 – called DC-LiGuE, the "Dreamcast Light Gun Emulator" – never amounted to much. I was able to build a circuit that I could feed coordinates into and translate that into light gun inputs without involving a CRT display, but I had a couple of major issues. The first was that I was unable to talk to the Dreamcast directly; its controller protocol is much too fast to decode reliably using the ATmega microcontrollers I favour in my projects, so my interface involved shining a bright white LED into the end of a regular Dreamcast light gun. The second issue was disappointment in the Wii remote I was hoping to use as the light gun input; I'd tried some homebrew programs that translated the data from its IR tracking camera into pointer events as well as the Mayflash DolphinBar (which shows up as a regular USB mouse on a PC) and the accuracy was generally pretty poor and not subsitute for a light gun.

More recently I was made aware of other IR-based light gun solutions such as GUN4IR from Boojakascha's YouTube video on the project. I thought I should give the Wii remote another chance! I'd also realised that Mayflash's DolphinBar has a fourth mode intended for use with the Dolphin emulator in which it presents the four paired Wii remotes as four USB HID devices. This makes experimenting with the Wii remote very easy, as you don't need to worry about any of the Bluetooth pairing side of things. I stuck some IR LEDs around my monitor in the GUN4IR configuration and knocked together a quick prototype in C# that analysed the tracked points and translated them into pointer coordinates. Having the four LEDs around the monitor (instead of just two in the Wii sensor bar) makes the aim tracking much easier and much more accurate.

Skipping ahead in the story a bit, the video below shows the C# prototype running in the monitor on the left. The diamond shape is drawn around the four tracked IR emitter points. If the Wii remote is aimed far enough away from the centre of the screen it may lose sight of one or more of these points, but the software does a reasonable job of reconstructing the diamond based on the remaining points it can see. The large bright white dot is the calculated pointer position.

In example the Wii remote is being used to play Time Crisis II on the PlayStation 2. This is a happening on a real PS2, not an emulator on the PC. To achieve this a circuit that mimics a Guncon light gun is connected between my PC and the PS2, and the PC sends button and pointer data to this circuit. Different circuits could be built that could mimic different light guns for different consoles, providing a somewhat universal light gun solution. One issue is that my PC isn't anywhere near my consoles, though, and not everyone is going to have a DolphinBar so replacing the PC side with a dedicated unit seemed like a good idea.

If I was going to use Wii remotes then I'd need something that could speak Bluetooth. The ESP32 microcontroller is cheap and provides a system-on-a-chip with Bluetooth (and a corresponding software stack) that seemed somewhat easy to get into. There was a simple Bluetooth HID host demo that served as a good starting point, and once I'd got the Wii remote paired with the ESP32 and transferring HID reports I translated the C# prototype code I'd written into C and got the ESP32 talking to the simulated Guncon instead of my PC.

The choice of the PlayStation Guncon was deliberate, as simulating this sort of light gun if you know the pointer coordinates you wish to send is very easy. The Guncon is quite unusual compared to other light guns of the era in that it handles the position calculation within the gun itself and then transfers the coordinates directly to the console over the standard controller protocol used to normally send button statuses or analogue joystick positions. Most other light guns just send a pulse on a dedicated "light gun" input pin when they see flashes of light from the CRT's raster scanning pattern and rely on the console to handle the position calculation, which the console can do as it can ask the video chip which part of the frame it was sending to the TV at the moment the light gun saw the light and from that determine where the gun was aimed. For the Guncon to be able to do this without direct access to the video chip it needs to have access to the generated video signal, which is why the Guncon has an RCA connector on it to pass the console's video signal through it. To be able to make most other light guns work, I'd need to generate the pulses seen when the Wii remote is pointing at the part of the screen that the console is currently outputting, and to do that I'd need to be syncronised with the console's video output.

Fortunately the ESP32 has a motor PWM controller that can be synchronised to external sources and so by adding an LM1881 sync separator circuit to extract the composite (horizontal) and vertical sync pulses to give it something to synchronise itself to I was able to generate pulses that looked like they might have come from a light gun based on the point on the screen the Wii remote was aimed at. I first tested this with the Master System, pretending to be a Light Phaser, before feeding the signals into a circuit that pretends to be a Mega Drive Justifier.

The point I'm aiming for with all of this is to have a central unit sitting under my TV that has the Wii remotes paired to it and the video signal passing through it. It will then have a socket on the front that can be connected to the console-specific light gun simulating cables which will then be plugged into their corresponding consoles. One system that can hopefully cover all the possible combination of light guns on original hardware. When I think of being prepared for any eventuality with a large collection of guns I naturally think of the classic Arnold Schwarzenegger film Commando, and so I've decided to call this project the Light Gun Commando.

Unlike John Matrix, however, I prefer to go into battle with a friend and so it's very important to me that not only does this system cover as many light gun types as possible, but also more than one player. I can't think of any console light gun games that support more than two simultaneous players so at the moment I'm concentrating on two player support but will probably leave space in protocol specifications for more. The following video shows a demonstration of two simultaneous guns in action:

In the video we're back to the PlayStation, but now simulating a Hyper Blaster instead of a Guncon. This is using the same circuit as the Guncon adaptor, but the firmware now supports different gun modes. Before the main host device sends any data to a console-specific gun adaptor it asks it to describe its capabilities. The adaptor replies with a block of formatted data describing each gun it can simulate; this includes a list of all buttons it has (including a descriptive name and a generic button type such as "trigger button", "start button" or "back button") and any axes it has (such as the pointer X and Y for the Guncon). These gun descriptions can then be repeated for multiple players if an adaptor can simulate more than one gun at a time, and then these player/gun descriptions can be grouped into different modes (e.g. "Two Guncons", "Two Hyper Blasters"). Pressing a button on the host device can then cycle between the different gun modes. The host device can use these mode, player and gun descriptions to construct status reports based on the current state of the Wii remotes, and by using generic button types (e.g. "trigger button" or "start button") the Wii buttons can be mapped automatically to suitable simulated gun buttons.

This automatic mapping is not just a matter of convenience, but is also intended to keep the host device and simulated guns separated. The simulated guns only need to worry about the guns they are simulating and not about button mapping from a Wii remote specifically. This is to allow the host device to be replaced by other hosts but still be able to use the same console-specific adaptors; the idea is that a GUN4IR or Sinden light gun host could be put together in the same way that I'm putting together a Wii remote host, and as long as the communication protocol between them is sensibly designed this should be a pretty straightforward endeavour.

To this end I'm currently trying to hack together as many different simulated light guns as I can to ensure that the communication protocol works well for them all. I've already mentioned the Sega Master System's Light Phaser, Mega Drive's Justifier and PlayStation's Guncon and Hyper Blaster. I did also get the Saturn's Virtua Gun working:

This all started back in 2015 with an attempt to mimic the Dreamcast's Light Gun, though, so if it's nearly eight years later and still no closer to that goal it's surely a bit disappointing? The main problem I had with the Dreamcast is that its controller bus uses a wire protocol that's not directly compatible with any existing standard that might be built into a microcontroller (the PlayStation uses a minor variation of SPI, for example) and is too fast to decode reliably in software on the microcontrollers I typically use. I have seen projects that perform clever tricks to work around this, such as dumping the bus state to RAM in a tight loop before decoding it (slowly) afterwards but these never seemed particularly robust and I didn't want to risk dropping data coming in from the host via the serial port if I was busy spending all available CPU time on speaking to the Dreamcast.

My solution was to look into the dsPIC33, specifically a model capable of running at 140MHz (up to 70 MIPS) which has sufficient grunt to decode the controller protocol in software with cycles to spare, enough RAM to store large data frames and DMA capabilities to be able to keep receiving data from the host device with zero CPU overhead if we're otherwise busy talking to the Dreamcast – all in in a hobbyist-friendly breadboardable DIP28 package!

As the video above hopefully demonstrates, the dsPIC33 seems to have done the trick and I can join the dogs of the AMS and hopefully not suffer like G did.

There's quite a lot work to go but I'm already feeling slightly less worried that if my CRT TVs all conk out I'll be stranded with no way to play my light gun games.

Electronics Light Gun Commando Light guns Sega Sega Dreamcast Sega Mega Drive Sega Mega-CD Sega Saturn Sony PlayStation

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