A fax machine is the combination of an optical image scanner, a modulation / demodulation interface device (otherwise known as a modem), and a printer.

A piece of paper, or nowadays any item which could sit underneath the lid of a flatbed scanning device, is fed into the bay of the scanner. It is then either dragged over the imaging eye of the scanner or, if the scanner is a flatbed variety, the imaging eye will move over the document.

The scanner will help the fax machine encode the document into a data file which represents the position of any writing or markings on the document and its associated contrast.

The fax machine will then attempt to connect to the remote machine by way of a telephone number. The number, when dialled, will hopefully ring and if there is a fax machine on the other end, it will answer with the now very familiar carrier tone.

The carrier tone is an audio signal telling the fax machine that there is a fax presence on the other end of the line, and to change the settings to allow the most efficient transmission of data.

Once this carrier tone “greeting” period has passed, and the parameters for the transmission are established, the sending machine will feed the data file into the modulator. The modulator converts the data file into sound, compressing it and transmitting it through the phone line to the receiving fax machine. The receiving machine will then de-modulate and decompress the data file.

The fax machine then passes the data file to the printer, interpreting it so that it can reproduce the positions of any writing or markings on the received document, along with the required contrast settings.

Once the transmission has been completed, the receiving machine sends a confirmation signal to the transmitting machine, and both devices hang up, finishing the session.

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