Digital communication thru analog 3.5mm plug with modem

[cleansugar]

https://hackaday.io/project/186309-simple-universal-modem
In early days, modem was used to both DAC and ADC.
If Rockbox device has only one analog plug, it can be used as digital communication with ADC or DAC converter.
And we are likely to have external ADC converter to decode analog signal from inside device.
Keyboards, mice, other external expansion devices could be attached via this manner.
For example, some guaging sensors like thermometer, luxmeter communicate via Android phone 4 pin jack.
Two Rockbox devices can communicate each other like old calculators or Gameboys.
There is PSP keyboard hack which converts to serial signal.
https://forums.ps2dev.org/viewtopic.php?t=11001