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.
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