For decoding old FSK tapes, look into sliding Goertzel filters. They're easy to implement filters that extract the amplitude of a frequency bin within a sliding

For decoding old FSK tapes, look into sliding Goertzel filters. They're easy to implement filters that extract the amplitude of a frequency bin within a sliding window, and are often referenced in literature about DTMF decoding. Compare the outputs of a pair of these to produce a digital output. You can also use a sparse sliding DFT, but interpolating between frequency bins is more of a hassle, whereas the Goertzel filter handles that for you.