I've seen the other way around, gameport to usb but I don't think what you're after exists

I think this will be a highly specific and complicated device. You would need to decode and interpret the joystick‘s protocol. There have been people reverse engineering this. So you won’t need to start from scratch. Then you need to translate the axis values into analogue electrical signals. Old Joysticks for PCs used potentiometers which would charge capacitors on the game card. The time it took for this would be proportional to the axis position. So your converter would need a DAC or some PWM to simulate this.

Regarding MIDI: did I miss something? I thought MIDI cables didn’t change in the last 40 years…?

I'd be happy with just digital / d-pad support to two PlayStation controllers or even 9-pin Atari/Amiga/Megadrive pads. For now, a gameport splitter and two 90s control pads are still working.

BushLin wrote:

I'd be happy with just digital / d-pad support to two PlayStation controllers or even 9-pin Atari/Amiga/Megadrive pads. For now, a gameport splitter and two 90s control pads are still working.

I actually have an adapter from 9-pin Atari to 15-pin gameport. It works like a dream for arcade-style games, with the slight limitation that the best Atari-style joysticks only have one button, where most PC games need at least two.

Hamby wrote:

Or a USB MIDI device (like a musical keyboard) to the gameport of a vintage soundcard?

USB "MIDI" is not simply MIDI signals sent through a USB port. Inside the keyboard is a USB (from/to) MIDI interface that converts the MIDI signals to something that can be sent over USB.

If you want MIDI to your gameport, use the MIDI IN/OUT ports on the keyboard and a MIDI to gameport cable.

USB HID has a standard interface for MIDI devices just as DOS systems usually had the MPU-401 interface. If the USB controller is built into the synthesizer device, a USB interface could conceivably deliver MIDI data without ever utilizing a MIDI current loop or classical serial encoding.