eli5: why do fax machines sound like satan himself?


My job involves calling a lot of people, some of whom are listed under outdated phone numbers. Some of the phone numbers are apparently connected to fax machines now.

Why do they sound like that? It’s alarming. They make these bizarre computerized screeching noises. And they seem to keep going indefinitely until I hang up.

I understand it has to do with the radio transmission (??) but why does the caller need to hear this?

In: 5


Oh man that makes me feel old. You are basically listening to the fax machine requesting information from the caller. If it was connected to another fax machine that machine would respond with screeches of it’s own. It’s the sound of data.

No radio signals.

Faxes are still sent over phone lines, not radio. You are hearing the digital transmission converted to sound by your phones speaker. It sounds harsh because it’s meant to be a strong enough signal for the receiving end to interpret clearly and its sending a rapid series of electrical changes to relay the information. This signal isn’t ever meant to be interpreted by a speaker, so how it sounds has never been a concern.

If you are old enough to have used dial-up internet, its the same type of sound that was emitted by dial-up modems when connecting to the internet. Complete guess on my part, but I think the dial-up modems only emitted the sound so that people would know they were doing something and not get frustrated as nothing happened while the modem connected.

When you call a dedicated fax line the fax on the other end is expecting a fax on your end. The sounds you hear are the audio-ized handshake which is that computer reaching out with signals to your end to try to negotiate a connection and determine the parameters of the connection, ie speed. Because it’s not a digital computer network connection (ie ethernet) it has to do it through sound. Because there’s no fax on your end the machine on the other end continually tries to get a response.
You’re only torturing yourself when you don’t hang up as soon as you hear that sound. Why you would continue to listen or so it seems is beyond me.

The screeching you’re hearing is the fax on the receiving end trying to contact the expected fax machine at the calling end. The fax signals go through the same voice-grade-channel that your voice does when speaking to someone on the other end, which is why you can hear the data. The modems (short for MODulator-DEModulator) know what they’re listening for. They take the entiretly of the received audio signal and break it down, looking at the various modulated frequency tones (which is all your voice is, by the way) and demodulate them into the appropriate data stream.