HDMI is a commonly used video connector on modern PCs and monitors, but vintage PCs from the 1980s have difficulty connecting to modern displays without additional adapters. Graphics Gremlin (GG) is an open-source project that addresses this problem by using scan doubling and increasing the vertical refresh rate to output to a VGA port. However, there are some limitations with the original design, such as the inability to use the Composite video and VGA port simultaneously and the need for a VGA-to-HDMI adapter. To overcome these issues, the author modified the GG design to connect natively to an HDMI monitor while still servicing the internal Composite-based CRT. The modified design includes an added HDMI port, a DVI transmitter, and various changes to the HDL code. The author also encountered some issues during testing, such as shimmering, unsupported frequencies, and a constant high-pitch sound. Overall, this project was a long journey for the author and builds upon the original work of Eric Schlaepfer.
https://yeokhengmeng.com/2023/09/hdmi-isa-graphics-card-for-vintage-pcs/