Reminds me of frequency-shift-keying modulation. (Used by teletypes since the 1930s or so.) Demodulators typically used filters to detect the two frequencies us

Reminds me of frequency-shift-keying modulation. (Used by teletypes since the 1930s or so.) Demodulators typically used filters to detect the two frequencies used (with an -expected- difference). Unusual frequency-shifts would only allow filtering one channel, at most (not even one, if the shift was very small).