Radiofax
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Radiofax, also known as HF Fax due to its common use in the HF bands (shortwave), is an analogue mode for transmitting images in greyscale.
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Transmission details
Radiofax is transmitted in single sideband and uses frequency modulation. The signal shifts up or down a given amount to designate white or black pixels. A deviation less than that for a white or black pixel is taken to be a shade of grey.
Usually, 120 lines per minute (LPM) are sent (though some stations use 60 LPM or other values). A value known as the index of cooperation (IOC) must also be known to decode a radio fax transmission - this governs the image resolution, and derives from early radio fax machines which used drum readers, and is the product of the total line length and the number of lines per unit length (known sometimes as the factor of cooperation), divided by π. Usually the IOC is 576.
History
- 1911 The first amplitude modulator for fax machines is patented, permitting transmission via telephone lines.
- 1913 Edouard Belin's Belinograph
- 1922 The first transatlantic facsimile services was provided by RCA.
- 1922-1925 RCA faxes photos across the Atlantic in six minutes; AT&T, RCA and Western Union develop "high-speed" fax systems. Dr Arthur Korn's facsimile system is used to transmit, by radio, a photograph of Pope Pius XI from Rome to Maine, USA. The picture is published the same day in the New York World newspaper -- a major feat in an era when news pictures crossed the ocean by ship.
- 1925 AT&T wirephoto starts operations
- 1926 RCA radiophoto starts operations
- 1926 Rudolf Hell introduced the Hellschreiber.
- 1927 first Siemens-Karolus-Telefunken facsimile between Berlin and other European City’s
- 1939 First Daily Newspaper by Radio Facsimile. More than 1,000 U.S. households are experimentally equipped with fax receivers that electronically print morning newspapers overnight.
- 1941 Fax is enlisted by the military to transmit maps, orders and weather charts during World War II.
- 1947 Alexander Muirhead's fax
- 1960 First SSTV test transmissions in the USA
- 1972 First SSTV transmissions in Germany