minidisc
I am a huge fan of the MiniDisc format and use it even in 2024, even with its ATRAC-based flaws. It can actually be understood fully end-to-end by a dedicated person, unlike many systems that exist today, making it a good base for retro-tech hacking that still works with modern hardware.
- minidisc.wiki — The modern-day MiniDisc resource. Start here if you are at all interested in the format.
- minidisc.org — The "old" MiniDisc resource from the early 00s. The Minidisc System Overview is a good start.
- M Karcher's linux-minidisc project — Gets very deep into the technical details of data transfer for NetMD or Hi-MD devices.