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Frequency-shift keying (FSK) http://en.wikipedia.org/wiki/Frequency-shift_keying
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On-off keying (OOK) http://en.wikipedia.org/wiki/On-off_keying
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Amplitude-shift keying (ASK) http://en.wikipedia.org/wiki/Amplitude-shift_keying
Zero crossings:
more advance:
- Sliding DFT (FFT)
- Sliding Görtzel Filter
- Phase Locked Loop