Fax

From Wikinfo

Fax (short for facsimile or telefacsimile) is a telecommunications technology used to transfer copies of documents, especially using affordable devices operating over the telephone network. Such faxes became affordable and very popular in the 1980s. They transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 200 dots per inch. The transfer rate is 14.4 kilobits per second (kbit/s) or higher. The transferred image formats are called ITU-T (formerly CCITT) fax group 3 or 4.

The technique the fax is based on was invented in 1929 by Rudolf Hell.

The most basic fax mode transfers black and white only. The original page is scanned in a resolution of 1728 pixels/line and 1145 lines/page (A4). The resulting raw data is compressed using a modified Huffman code optimized for written text, achieving average compression factors of around 20. Typically a page needs 10 s for transmission, instead of about 3 minutes for the same uncompressed raw data of 1728*1145 bits at a speed of 9600 bit/s. The compression method uses a Huffman codebook for run lengths of black and white runs in a single scanned line, and it also uses the fact that two adjacent scanlines are usually quite similar, saving bandwidth by encoding only the differences.

There are different fax classes, including Class 1, Class 2 and Intel CAS.

Several different telephone line modulation techniques are used by fax machines. They are negotiated during the fax-modem handshake. Today, only the fastest 14000 bit/s modulation is normally used, although most machines are capable of supporting the lower transmission speeds.

• 9600 bit/s and 14400 bit/s modulation methods utilize trellis modulation
• 2400 bit/s and 4800 bit/s modulation methods utilize phase-shift keying (PSK) modulation
• 7200 bit/s and 9600 bit/s modulation methods utilize quadrature amplitude modulation (QAM)

Contents 1 Autonomous computer fax 2 Alternative 3 Historic 4 See also 4.1 References

Autonomous computer fax

Modern computer fax can be attached to a printer or multifunctional machine, using two standard USB ports, to autonomously send, receive and print faxes (without needing a computer).

Alternative

A modern alternative for sending a fax is sending an email with one or more image files as attachments. This allows color and is more versatile with respect to resolution.

Historic

The following passage is from Chapter V, Section 7 of the Royal Commission on National Development in the Arts, Letters and Sciences, 1949-1951:

"The vigorous brief of the Canadian Daily Newspapers Association was devoted entirely to a discussion of the consequences to the present newspaper business if the new device of facsimile broadcasting should become, as seems possible, an effective and popular rival to newspapers as we know them. We can claim only an imperfect knowledge of this new medium of communication. In brief, as we understand it, this process can deliver directly into the home a printed newspaper as readily and by essentially the same means as radio and television are now received. No printing machinery or delivery services are needed, and any radio station could go into the newspaper business for a small fraction of the investment required to establish a normal newspaper. The Canadian Daily Newspapers Association states that this development will attract newcomers to the newspaper field, and that the facsimile reader will be able in his home to dial any one of several newspapers just as now he tunes his receiving set to radio programmes."

Scottish inventor Alexander Bain is often credited with the first fax patent in the 1840's. He used his knowledge of electric clock pendulums to produce a back-and-forth line-by-line scanning mechanism.

See also

• facsimile
• telautograph
• fax machine

References

• Adapted from the Wikipedia article, "Fax" http://en.wikipedia.org/wiki/Fax, used under the GNU Free Documentation License