This will get you started: Tony Fisher wrote V.32 soft modem stack for SGI Indy back in 1996. York University couldnt handle hosting whole 8 megabytes of his s

This will get you started: Tony Fisher wrote V.32 soft modem stack for SGI Indy back in 1996. York University couldnt handle hosting whole 8 megabytes of his stuff https://www.cs.york.ac.uk/news/fisher/home-page https://www.cs.york.ac.uk/news/fisher/home-page but at least they had the decency to upload it to github instead of just deleting https://github.com/university-of-york/cs-www-users-fisher/tree/master/software/modem https://github.com/university-of-york/cs-www-users-fisher/tr... Randy Rossi rewrote this project and got working V.21/23 https://github.com/randyrossi/fisher-modem https://github.com/randyrossi/fisher-modem Fabrice Bellard, because of course he did :), implemented his own version all the way to V.34 with parts of V.90 https://bellard.org/linmodem/ https://bellard.org/linmodem/ Currently actively maintained by Osmocom project https://osmocom.org/projects/linmodem https://osmocom.org/projects/linmodem "In Late 2021 we started a project to support AMR_Modem_Riser cards via a custom break-out board (#5294) and an associated FPGA softcore to interface the AC97 master interface with USB (audio)." V.21 modem implemented in Gnu Radio https://web.archive.org/web/20170627151353/http://aaronscher.com/GNU_Radio_Companion_Collection/Audio_Modem_loop_back_test_FSK.html https://web.archive.org/web/20170627151353/http://aaronscher... amodem - good source of ready made modulation implementations https://github.com/romanz/amodem https://github.com/romanz/amodem minimodem- simple radio link FSK modem https://github.com/kamalmostafa/minimodem https://github.com/kamalmostafa/minimodem audsl - leased line project, not compatible with standard modems https://www.araneus.fi/audsl/ https://www.araneus.fi/audsl/ but might give some ideas.