Fax

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.

Capabilities

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 kilobits 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.

Related Topics:
Resolution - Bit - ITU-T - CCITT

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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.

Related Topics:
Pixel - A4 - Compressed - Huffman code

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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).

Related Topics:
Modified Huffman - Modified READ - T.4 - Modified Modified READ - T.6

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

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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.

Related Topics:
Modem - Handshake

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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.

Related Topics:
Thermal printer - Thermal transfer printer - Inkjet printer

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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.

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