The best replacement for fsk is afsk.

The best alternatives to fsk are rated as following:
★★★★★  afsk
★★★★★  psk
★★★  ask
★★  dtmf
★★  fm
★★  gfsk
★★  ook
★★  frk
★★  fsk
★★  msk
★  bpsk
  cpfsk
  foil faced

Your alternative for fsk is not listed? Suggest!
Speaking about fsk, one probably shouldn't forget about mmtty.

Other search terms related to fsk and their ratings are:
★★★★★ mmtty
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fsk
afsk
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