little backgrounder
early modems used what is called FSK or frequency shift keying
by default a 1 is some frequency say 400 hz
a 0 might be 440 hertz
so you only need to detect if the sound is above or below 420 hz
this makes it very easy electrically to decode
the same type of thing is used on the tv and radio in the usa for the emergency alert system
thats the noise you hear on the tv ( or radio) speaker when it goes off
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