Hi, currently I am experimenting with recording modem sounds on cassette tape and later replaying it and decoding it.

I was thinking about writing my own modem software. I have some general understanding how modems works but I lack that important bit - how does it detects there is a certain tone in incoming signal? (in case of AFSK-styled two tones modem e.g.)

Any explanation, article link and/or csound example is welcomed.

Sorry for somewhat offtopic post, but I am pretty sure there is a person (in this list) who knows exact solution of this problem.

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Severák