In FSK (radioteletype or 'RTTY' is a well-documented case used commercially for decades) the most important factor in accurate recovery (especially for deterior

In FSK (radioteletype or 'RTTY' is a well-documented case used commercially for decades) the most important factor in accurate recovery (especially for deteriorated signals) is the quality of the filters and clocking. Cheap circuits avoid using them, and so the methods they fall back on give poorer results. High-quality circuits will have excellent results even when the signal level is below the noise level. Since your software has to work with the output of an A-D converter, that has to be as good as you need it to be. A couple of widely-used, vintage standards: Manchester encoding: https://www.allaboutcircuits.com/technical-articles/manchester-encoding-what-is-it-and-why-use-it/ https://www.allaboutcircuits.com/technical-articles/manchest... Kansas City Standard: http://www.dabeaz.com/py-kcs/index.html http://www.dabeaz.com/py-kcs/index.html On the other side of archiving, get the best tapes and recorders you can manage, and get a good, strong signal to the tape. (Time will weaken that signal.) Then keep them shielded from magnetic fields.