Your Fax Machine -- A Brief Tour

Like computers, printers, and photocopiers, fax machines have become essential elements of the modern office. A fax machine's task is impressive: it enables you to transmit a document anywhere in the world instantly. But the principles on which the fax machine works are actually quite simple.

Scanning the page

As a page is fed into your fax machine, a small fluorescent bulb lights up a row of pixels, which extends from one side of the page to the other. Most fax machines can scan an entire such row at a time, using a collection of 1,728 sensors. Each sensor detects whether its part of the page is black or white; this information is stored as 1,728 separate bits of data. Since a page typically consists of 1,145 rows of pixels, the sensors break down a one-page document into almost two million bits of information.

Sending the data

Different fax machines have different ways of compressing this data, so as to reduce the number of bits to be transmitted. These bits of information are then encoded in a form that can travel through the phone line: typically, the fax machine uses two different types of audible tones to represent the black pixels and the white pixels of the document. This information is sent from your fax machine to a faraway fax machine in a matter of seconds, through a normal telephone line.

Receiving the fax

When a fax machine receives an incoming fax, it simply has to decode the sounds that travel through the telephone line, decompress the data, and translate that data into a pattern of black and white pixels. It then prints that pattern onto a page, using one of several technologies.

Printing the fax

Twenty years ago, most fax machines printed on thermal paper. Thermal paper uses no ink, but is coated with a chemical that turns black where it is exposed to heat; these fax machines would heat the page in the spots where the black pixels needed to be. This paper, however, has the significant drawback of turning yellow over time and turning completely black if accidentally exposed to too much heat. Today, most fax machines use the exact same technology and paper as printers: you can buy inkjet or laser fax machines, each of which contain the same mechanisms as the corresponding printers. Some people receive faxes through their computers instead of through an actual fax machine: this process uses a fax modem to store the incoming fax data as a graphics file, and then print the file out using a normal inkjet or laser printer.