Sunday, August 23, 2009

Wire transmission

Scottish inventor Alexander Bain worked on chemical mechanical facsimile type devices and in 1846 was able to reproduce graphic signs in lab experiments. Frederick Bakewell made several improvements on Bain's design and demonstrated his device at the 1851 Great Exhibition in London. Bain and Bakewell's systems were inferior and could reproduce only poor quality images. They lacked synchronization between the transmitting mechanism and the receiving mechanism. In 1861, the first practical operational electro-mechanical commercially exploited telefax machine, the Pantelegraph, was invented by the Italian physicist Giovanni Caselli. He introduced the first commercial telefax service between Paris and Lyon some 11 years before the invention of workable telephones.[1][2]
In 1881, English inventor Shelford Bidwell constructed the scanning phototelegraph that was the first telefax machine to scan any two-dimensional original, not requiring manual plotting or drawing anymore. Around 1900, German physicist Arthur Korn invented the Bildtelegraph, widespread in continental Europe especially since a widely noticed transmission of a wanted-person photograph from Paris to London in 1908, used until the wider distribution of the radiofax. Its main competitors were the Bélinograf by Édouard Belin first, then since the 1930s the Hellschreiber, invented in 1929 by Rudolf Hell, a pioneer in mechanical image scanning and transmission

Typical characteristics

Group 3 fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 204×98 (normal) or 204×196 (fine) dots per square inch. The transfer rate is 14.4 kbit/s or higher for modems and some fax machines, but fax machines support speeds beginning with 2400 bit/s and typically operate at 9600 bit/s. The transferred image formats are called ITU-T (formerly CCITT) fax group 3 or 4.
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 (for 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 can also use the fact that two adjacent scanlines are usually quite similar, saving bandwidth by encoding only the differences.
Fax classes denote the way fax programs interact with fax hardware. Available classes include Class 1, Class 2, Class 2.0 and 2.1, and Intel CAS. Many modems support at least class 1 and often either Class 2 or Class 2.0. Which is preferable to use depends on factors such as hardware, software, modem firmware, and expected use.
Fax machines from the 1970s to the 1990s often used direct thermal printers as their printing technology, but since the mid-1990s there has been a transition towards thermal transfer printers, inkjet printers and laser printers.
One of the advantages of inkjet printing is that inkjets can affordably print in color; therefore, many of the inkjet-based fax machines claim to have color fax capability. There is a standard called ITU-T30e for faxing in color; unfortunately, it is not yet widely supported, so many of the color fax machines can only fax in color to machines from the same manufacturer.

Modified Read

Modified read (MR) encodes the first scanned line using MH. The next line is compared to the first, the differences determined, and then the differences are encoded and transmitted. This is effective as most lines differ little from their predecessor. This is not continued to the end of the fax transmission, but only for a limited number of lines until the process is reset and a new 'first line' encoded with MH is produced. This limited number of lines is to prevent errors propagating throughout the whole fax, as the standard does not provide for error-correction. MR is an optional facility, and some fax machines do not use MR in order to minimise the amount of computation required by the machine. The limited number of lines is two for 'Standard' resolution faxes, and four for 'Fine' resolution faxes.
The ITU-T T.6 recommendation adds a further compression type of Modified Modified READ (MMR), which simply allows for a greater number of lines to be coded by MR than in T.4. This is because T.6 makes the assumption that the transmission is over a circuit with a low number of line errors such as digital ISDN. In this case, there is no maximum number of lines for which the differences are encoded.

.Data transmission rate

Several different telephone line modulation techniques are used by fax machines. They are negotiated during the fax-modem handshake, and the fax devices will use the highest data rate that both fax devices support, usually a minimum of 14.4 kbit/s for Group 3 fax.
ITU Standard
Released Date
Data Rates (bit/s)
Modulation Method
4800, 2400
9600, 7200, 4800
14400, 12000, 9600, 7200
Note that 'Super Group 3' faxes use V.34bis modulation that allows a data rate of up to 33.6 kbit/s.


A "fax machine" usually consists of an image scanner, a modem, and a printer.
Although devices for transmitting printed documents electrically have existed, in various forms, since the 19th century (see "History" below), modern fax machines became feasible only in the mid-1970s as the sophistication increased and cost of the three underlying technologies dropped. Digital fax machines first became popular in Japan, where they had a clear advantage over competing technologies like the teleprinter, since at the time (before the development of easy-to-use input method editors) it was faster to handwrite kanji than to type the characters. Over time, faxing gradually became affordable, and by the mid-1980s, fax machines were very popular around the world.
Although many businesses still maintain some kind of fax capability, the technology has faced increasing competition from Internet-based systems. However, fax machines still retain some advantages, particularly in the transmission of sensitive material which, due to mandates like Sarbanes-Oxley and HIPAA, cannot be sent over the Internet unencrypted[citation needed]. In some countries, because electronic signatures on contracts are not recognized by law while faxed contracts with copies of signatures are, fax machines enjoy continuing support in business.
In many corporate environments, standalone fax machines have been replaced by "fax servers" and other computerized systems capable of receiving and storing incoming faxes electronically, and then routing them to users on paper or via an email (which may be secured). Such systems have the advantage of reducing costs by eliminating unnecessary printouts and reducing the number of inbound analog phone lines needed by an office.


FFax (short for facsimile, from Latin fac simile, "make similar", i.e. "make a copy") is a telecommunications technology used to transfer copies (facsimiles) of documents, especially using affordable devices operating over the telephone network. The word telefax, short for telefacsimile, for "make a copy at a distance", is also used as a synonym. Although fax is not an acronym, it is often written as “FAX”. The device is also known as a telecopier in certain industries. When sending documents to people at large distances, faxes have a distinct advantage over postal mail in that the delivery is nearly instantaneous, yet its disadvantages in quality have relegated it to a position beneath email as the prevailing form of electronic document transfer.

Saturday, August 22, 2009

What is a FAX?

The transmission of photographs, drawings, maps, and written or printed words by electric signals. Light waves reflected from an image are converted into electric signals, transmitted by wire or radio to a distant receiver, and reconstituted on paper or film into a copy of the original.
Facsimile is used by news services to send news and photos to newspapers and television stations, by banks, airlines, and railroads to transmit the content of documents, and by many other businesses as an aid in data handling and record keeping.
Facsimile systems involve optical scanning, signal encoding, modulation, signal transmission, demodulation, decoding, and copy making.
Scanning is done in a manner similar to that used in television. An original, a photo for example, is illuminated and systematically examined in small adjacent areas called pixels (picture elements). Light reflected from each pixel is converted into electric current by an electronic device, a photocell, photodiode, or charge-coupled device (CCD).
A single such device may be used to cover one pixel after another in a row, row after row from top to bottom until the entire image has been translated into electric impulses. This is rectilinear scanning. Scanning may also be done a row at a time by a battery of devices; this is array scanning.
In multispot scanning, a vertical array of photodevices moves across the image, examining the pixels column by column. As the array passes down the copy, it produces a set of current pulses from each photodevice. The separate currents, however produced, are then transmitted successively over a single circuit to the distant receiver.
To secure fine detail in the reproduced image it is necessary to use very small pixels. In one standard, Group 3 of the International Telegraph and Telephone Consultative Committee ( CCITT ), each pixel is a rectangle 0.12 by 0.13 mm ( 1 inch=25.4 mm ). On this standard, subject copy measuring 8 by 11 inches ( 20 x 28 cm ) is divided into 3.6 million pixels.
This compares with about 200,000 pixels for televised images. The pixels used in high-resolution facsimile systems have dimensions one-fifth those of the CCITT standard mentioned above, whereas in low-definition systems the dimensions may be twice as great.
The image may be illuminated as in rectilinear scanning, or a relatively large area of the image may be illuminated, the photodevice viewing the image through a lens aperture that restricts its field to a single pixel at a time.
In a commonly used facsimile scanning system ( invented by Frederick Bakewell in 1848 and based on Alexander Bain's work of 1842 ) the subject copy is wrapped around a drum. A finely focused spot of light falls on the copy and the light reflected from that pixel is picked up by the photodevice. The drum is rotated so that the light spot traces a line across the copy, examining each pixel in turn.
As the drum rotates, the light source is moved slowly on a carriage parallel to the drum axis, tracing out a spiral of adjacent lines until the entire area of the copy has been scanned. At least once in each rotation of the drum a signal transmitted to the recorder keeps the scanner and the recorder in step.
In drum scanning, the copy may also be illuminated broadly and examined by a photodevice fitted with a lens aperture.
Copy cannot always be conveniently wrapped around a drum. In such cases, flat copy may be scanned by a spot of light directed across its surface by a moving mirror. Mirror scanning may also be used when the copy is wrapped on a drum, or while it is being pulled from a roller. Laser light produces a very fine beam that travels across the copy, row by row, as the copy moves vertically.
In one arrangement the mirror is rocked back and forth, moving the beam across the copy. In another, a rotating polygonal mirror is used. This mirror typically has 18 flat mirror surfaces on its periphery, each capable of scanning a row of pixels.
Very fast scanning can be achieved by rapid rotation of the mirror and corresponding vertical motion of the copy. The beam is reflected from each pixel into a photodevice that converts successive light values into corresponding currents. Electronic scanning of flat copy may also be done by arrays of photodiodes or charge-coupled devices.
For scanning rates higher than about 6 rows per second laser beams with polygonal mirrors and arrays of photodevices are favored.