How a Fax Machine Works

Simple yet complicated devices

A lot of people know if they push a button on a fax machine, they can send a document to somebody who is thousands of miles away - and that's all they need to know. However, if you'd like to further your knowledge on the machines, and learn about the actual process they use to compress and transmit data across telephone wires, we must dig a little deeper.

Fax machines are used to send printed documents that can contain text and images. These days you can send documents in several different ways. Although the most common method is to send a fax is through telephone lines, you can also send them via e-mails, cell-phones, hand-held organizers, radio, satellite and cable. In this section we'll explain the functions of a basic fax machine that transmits by telephone lines.

A fax machine is made up of an optical scanner for digitizing images on the paper, a printer for handling incoming faxes and a telephone line for making the connection.

It works by digitizing an image and then dividing it into a grid of dots. Depending on whether it is black or white, each dot is on or off. Electronically, a dot is represented by a bit that has a value of zero for off and one for on. This enables the machine to translate text and images into a series of zeros and ones, which is known as a bit map. To reduce the number of bits, modern fax machines use three different compression techniques. The receiving fax machine then reads the incoming data, translating the zeros and ones back into dots, and duplicates and prints the original document.

There are different types of scanning methods implemented by fax machines. Rectilinear scanning is done when the original document is illuminated and examined in pixels. The light that is reflected from each pixel is transferred into an electric current by an electric device such as a photocell, photodiode or charge-coupled device (CCD). The electric device can scan the pixels in rows from top to bottom until the whole document has been changed into electric impulses. To make sure a fax transmission is good quality, you need to use very small pixels.

In many scanning systems, the copy is wrapped around a drum or cylinder. A light then strikes the copy and the light reflected from that pixel is read by the photo device. The cylinder rotates so the light spot can trace a line across the copy, examining each pixel. When the cylinder turns, the light source moves slowly on a carriage that is parallel to the cylinder axis. This traces adjacent lines until the entire document is scanned. In drum scanning, the document can also be illuminated broadly and examined by a photo device that is fitted with a lens aperture.

However the copy can't always be wrapped around a cylinder. In these instances, flat copy can be scanned by a beam of light that is directed across the document by a moving mirror. Mirror scanning can also be used when the copy is wrapped on a drum, or when it is pulled from a roller. Flat copy can also be scanned by arrays of photodiodes or charge-coupled devices. The receiving fax machine decodes, decompresses and reassembles the bits of the original document, then prints it. There are different types of fax machines available and they come with various features, but they all work on these same general principles.

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