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Fax (short for 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 kilobit per second or higher. The transferred image formats are called 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 1728x1145 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.

Several different telephone line modulation techniques are used by fax machines. They are negotiated during the modem handshake. Today, only the fastest 14000 bit/s modulation is used, normally.

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