You can walk into nearly any office in the United States today, big or small, hi-tech or lo-tech, and you will find a fax machine. Connected to a normal phone line, a fax machine allows you to transmit pieces of paper to someone else instantly! Even with FedEx and e-mail, it is nearly impossible to do business without one of these machines today.

Now, you'll learn how a fax machine works its magic!

The Basic Idea
Fax machines have been around in one form or another for more than a century -- Alexander Bain patented the first fax design in 1843 (
click here to learn more). If you look back at some of the early designs, you can get a very good idea of how they work today.

Most of the early designs involved a rotating drum. To send a fax, you would attach the piece of paper to the drum, with the print facing outward. The rest of the machine worked something like this:

  • There was a small photo sensor with a lens and a light.
  • The photo sensor was attached to an arm and faced the sheet of paper.
  • The arm could move downward over the sheet of paper from one end to the other as the sheet rotated on the drum.
In other words, it worked something like a lathe.

The photo sensor was able to focus in and look at a very small spot on the piece of paper -- perhaps an area of 0.01 inches squared (0.25 millimeters squared). That little patch of paper would be either black or white. The drum would rotate so that the photo sensor could examine one line of the sheet of paper and then move down a line. It did this either step-wise or in a very long spiral.

To transmit the information through a phone line, early fax machines used a very simple technique: If the spot of paper that the photo cell was looking at were white, the fax machine would send one tone; if it were black, it would send a different tone (see How Modems Work for details). For example, it might have sent an 800-Hertz tone for white and a 1,300-Hertz tone for black.

At the receiving end, there would be a similar rotating-drum mechanism, and some sort of pen to mark on the paper. When the receiving fax machine heard a 1,300-Hertz tone it would apply the pen to the paper, and when it heard an 800-Hertz tone it would take the pen off the paper.

Modern Fax Machines
A modern fax machine does not have the rotating drums and is a lot faster, but it uses the same basic mechanics to get the job done:

A typical fax machine that you find in an office is officially known as a CCITT (ITU-T) Group 3 Facsimile machine. The Group 3 designation tells you four things about the fax machine:
  • It will be able to communicate with any other Group 3 machine.
  • It has a horizontal resolution of 203 pixels per inch (8 pixels/mm).
  • It has three different vertical resolutions:
    • Standard: 98 lines per inch (3.85 lines/mm)
    • Fine: 196 lines per inch (7.7 lines/mm)
    • Super fine (not officially a Group 3 standard, but fairly common): 391 lines per inch (15.4 lines/mm)
  • It can transmit at a maximum data rate of 14,400 bits per second (bps), and will usually fall back to 12,000 bps, 9,600 bps, 7,200 bps, 4,800 bps or 2,400 bps if there is a lot of noise on the line.
The fax machine typically has a CCD or photo-diode sensing array. It contains 1,728 sensors (203 pixels per inch), so it can scan an entire line of the document at one time. The paper is lit by a small fluorescent tube so that the sensor has a clear view.

Understanding the scanning process: The scanner in a fax machine looks at one line of the sheet of paper. The scan line is shown here in red. It sees a group of black and white spots, shown blown up in the red rectangle at the bottom of the figure. It encodes the pattern of spots and sends them through the phone line.

The image sensor looks for black or white. Therefore, a single line of the document can be represented in 1,728 bits. In standard mode, there are 1,145 lines to the document. The total document size is:

1,728 pixels per line * 1,145 lines = approximately 2,000,000 bits of information.
To reduce the number of bits that have to be transmitted, Group 3 fax machines use three different compression techniques:
  • Modified Huffman (MH)
  • Modified Read (MR)
  • Modified Modified Read (MMR)
See this page for a discussion of these compression types. The basic idea in these schemes is to look for "runs" of same-color bits. For example, if a line on the page is all white, the modem can transmit a dozen or so bits rather than the full 1,728 bits scanned for the line. This sort of compression can cut transmission time by a factor of at least two, and for many documents much more. A document containing a significant amount of white space can transmit in just a few seconds.

Receiving the Fax
The bits for the scanned document travel through the phone line and arrive at a receiving fax machine. The bits are decoded, uncompressed and reassembled into the scanned lines of the original document. There are five common ways to print the fax, depending on the type of machine that receives it:

For more information on fax machines and related topics, check out the links on the next page!

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