300 bps; frequency shift keying
https://vigrey.com/blog/emulating-bell-103-modem https://dsp.stackexchange.com/a/67287/60742 https://github.com/ViGrey/modem-tap/blob/master/src/modem-tap-go/modem-tap-go.go
https://dsp.stackexchange.com/questions/29946/demodulating-fsk-audio-in-python
The client used a mark tone of 1270Hz and a space tone of 1070Hz, while the server used a mark tone of 2225Hz and a space tone of 2025Hz.
sample: 300 Baud Acoustic Modem Emulation & calling a BBS - YouTube
https://bournetocode.com/projects/GCSE_Computing_Fundamentals/pages/img/ascii_table_lge.png
ITU V.21
https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-V.21-198811-I!!PDF-E&type=items
client 1080, server 1750