eli5: The “handshake” between dial-up modems and why it made those sounds

Question

eli5: The “handshake” between dial-up modems and why it made those sounds

In: 294

Answer 1

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

Answer 2

Here’s a handy page + chart that talks about the handshake sequence in detail: [https://www.windytan.com/2012/11/the-sound-of-dialup-pictured.html](https://www.windytan.com/2012/11/the-sound-of-dialup-pictured.html)