[Charlie] was killing some time hacking on some cheap FPGA dev boards he bought from eBay. Initially, he intended to use them to create HDMI ports for a different project before new inspiration hit him. Instead, he added an HDMI port to Neo Geo MVS games. The Neo Geo MVS was a 90’s arcade machine that played gems like the Metal Slug and Samurai Showdown series. [Charlie] has a special knack for mods, being featured on Hackaday before for implementing Zork on hardware and making a mini supergun PCB. What’s especially nice about his newest mod is that the HDMI outputs both audio and video.
[Charlie] obtained the best possible video and audio signal by tapping the digital inputs to the Neo Geo’s DACs (digital-to-analog converter). The FPGA was then used to convert the signals to HDMI, maintaining a digital signal path from video generation to display. While this sounds simple enough, there was a lot that had to be done. The JAMMA video standard’s lower resolution was incompatible with the various resolutions offered by the HDMI protocol. [Charlie] solved this problem by implementing scan doubling using the RAM on the Cyclone II dev board. He then had to downsample the audio to 32kHz (from 55.6kHz) in order to meet the HDMI specs. Getting the sound over HDMI required adding data islands to the signal, a feat [Charlie] admits was a frustrating one.
When he tested the HDMI with his monitor, it was out of spec but still worked. His TV, on the other hand, refused to play it at all. This was due to the Neo Geo outputting 59.1 fps – not the standard 60 fps. Using the FPGA, [Charlie] overclocked the NeoGeo by approximately 1% and used the 27Mhz pixel clock to change the FPGA output to a 720 x 480p signal.
For those that love the scan lines of yore, they can be enabled with the push of a button. [Charlie] notes that there are some slight differences in the shadow effects of some graphics, but he has done his best to minimize them. He also admits that the FPGA code contributes only 100 microseconds of delay compared to analog output, which is fast enough for even the most hardcore gamers.
Check out the video after the break to see how the Neo Geo looks in HDMI along with a side-by-side comparison to a CRT TV.