Originally posted by twhitehead
But where will the fax signal be coming from?
Just like any other originator of signals, on a radio station its the transmitter site, where you see all those huge AM towers. On a computer, it generates signals that can easily be picked up by a short wave radio, in fact they are very annoying to us hams because they cause all kinds of interference even though they are supposed to be confining their signals to hard wires or optical cables, they are electrical before they hit the fiber and so some of it leaks out into the world, right into my ham and shortwave receivers.
A fax machine is no different, it has a circuit board that generates those kind of signals also but at a much lower frequency so you would be hard pressed to actually hear those signals in a shortwave radio.
I will have to try that experiment to see if I can hear them over a radio! I did that with some digital watches and you wouldn't think they would give a signal capable of being picked up by a radio but they do!
Only within a few inches of the radio that has a built in antenna but it is there nonetheless! So that is on my experiment plate, see what I hear on a shortwave or AM radio when the fax is being sent.
But that signal then goes to the wires of the phone company and is sent down the line to another fax machine that just takes the signal in reverse and applies it to the printer section.
It is like a slow scan tv signal, the old fashioned analog tv's. A beam of electrons would sweep across the screen back and forth and another signal inside the tube would force that beam to move up and down at the same time so the action of the two signals causes the electron beam to sweep across from the top, then down a tiny bit and another sweep and so forth to the bottom of the screen then the cycle starts over at the top of the picture tube.
It's like that with a fax, kind of like how you would take a piece of paper and hold it over some sculpture or other and run a pencil over the paper which gives an impression of what is underneath.
Instead of a pencil, its a narrow beam of light, maybe even a laser in the newer ones but it does a back and forth scan and a light sensor picks up the variations in brightness, meanwhile the paper moves down a bit and the next scan starts and so forth till the end of the page.
So what is transmitted as the fax signal is just a stream of information about the brightness of that light beam, a single scan doesn't have much information in it till a bunch of scans has gone by and revealed what letters are to be sent.
That is an analog signal, not digital. It is easy to digitize such signals however.
You feed the analog signal into an analog to digital converter and a no light signal may be represented by all zeros, 00000000, say in this case an 8 bit stream, and the max signal as 1111111 which the other end would integrate all the ones and zero's and put them in the proper order to recreate the letters of the fax.
That's how it COULD be done but is not, fax is just an analog signal to an analog receiver at the other end.