system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What were we actually hearing during the classic dial-up sound?
gpt
The modem is communicating over the analogue telephone line, normally used to transmit spoken language.

Computers only speak binary though. What you are hearing is actually pure binary in the form of an analogue square wave, where each "up" is a 1 and each "down" is a 0 (or the other way around, can never remember).

Since modems use standardized "phrases" to identify themselves, shake hands and other steps needed to establish the connection, the actual sound made is generally the same or at least constructed of the same chunks of sound each time you connect.

Typically these "soundbites" start with a slow and easily readable greeting that even superslow modems on the other end can handle. This would be the "beee-booo-bi-beee" part in the beginning. Then, after they have agreed on the maximum speed both parts can handle, it switches to that speed and that's when you instead hear the white noice kind of sound "pschheeoooiiaauuaa...".

Finally, when it's all set up and ready for communication, the modem switches of it's speaker so you don't have to keep listening to the noice. It is however still sending said noice over the line.