Adding a microcontroller, a couple of solenoids, relays, and IR sensors (for leader skip) will enable fully automatic playback, seek, and recording for a cassette data storage deck
Error correction would have to be added to the transfer protocol and I'm not sure if the C64 era ones would do. Back then, you had to push the buttons yourself too
If this can be moved up to a hacked VCR, we can probably bring back tape storage. 3D printing is just barely good enough for enclosures and lift doors
Keeping the audio vibe, conventional stereo 1/4 inch (6.35mm) jacks can be used with R-485 signaling since 3 connections provide the bare minimum:
RX, TX, GND
In fact a lot of connection types can be boiled down to just 3 leads if the speed lowered. The connection doesn't necessarily have to be RS-485, but I used it as an example because it's already out there and very simple to implement
Also, it can serve as the lower hardware layer with error correcting software protocols on top
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