Fax (short for facsimile - from Latin "fac simile", "make similar", i.e. "make a copy" - or telefacsimile ) is a telecommunications technology used to transfer copies of documents, especially using affordable devices operating over the telephone network. The words telecopy and telefax are also used as synonyms .
A fax machine is essentially an image scanner , a modem , and a computer printer combined into a highly specialized package. The scanner converts the content of a physical document into a digital image, the modem sends the image data over a phone line, and the printer at the other end makes a duplicate of the original document.
Obviously, a very high-quality fax machine with some additional electronics can connect to a computer, and can be used to scan documents into a computer, to print documents from the computer, and to make photocopies. Such high-end devices are called multifunction printers and cost more than fax machines.
Modern fax technology became feasible only in the mid- 1970s as the sophistication and cost of the three underlying technologies improved to a reasonable level. Fax machines first became popular in Japan , where they had an clear advantage over competing technologies like the teleprinter ; 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.
However, although most businesses still maintain some kind of fax capability, the technology appears increasingly dated in the world of the Internet.
Fax machines transfer one or a few printed or handwritten pages per minute in black-and-white (bitonal) at a resolution of 100x200 or 200x200 dots per inch. The transfer rate is 14.4 kilo bits per second (kbit/s) or higher(but fax machines support speeds beginning with 2400 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 ( 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 1728Ã—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.
The codes used for sending facsimiles are: Modified Huffman (MH), Modified READ (MR) (known also as CCITT Group 3 fax encoding or CCITT T.4 ) and Modified Modified READ (MMR)(known also as CCITT Group 4 fax encoding or CCITT T.6 ).
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 , and the fax devices will use the highest data rate that both fax devices support, usually a minimum of 14.4 kbit/s.
|ITU Standard||Released Date||Data Rates (bit/s)||Modulation Method|
|V.29||1988||9600, 7200, 4800||QAM|
|V.17||1991||14400, 12000, 9600, 7200||TCM|
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 and inkjet 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.
At the receiving end, much research has occurred into how to more efficiently process incoming faxes, now that digital storage is much cheaper than it was in the 1970s, and junk faxes have become a common problem (and an enormous waste of paper).
Some high-end communications servers do not automatically print out all incoming faxes, but instead integrate them into a single in-box along with other forms of store and forward communications like email and voice mail (see unified messaging ).
As a designer for the Radio Corporation of America (RCA), in 1924, Richard H. Ranger invented the wireless photoradiogram, or transoceanic radio facsimile , the forerunner of todayâ€™s "Fax" machines. A photograph of President Calvin Coolidge sent from New York to London in November 1924 became the first photo picture reproduced by transoceanic radio facsimile. Commercial use of Rangerâ€™s product began two years later. Radio fax is still in common use today for transmitting weather charts and information.
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