Radiofax
From Wikipedia, the free encyclopedia
- This article is about the image transmission mode. For the radio station, see Radiofax (radio station).
Radiofax, also known as Weatherfax and HF fax due to its common use in the HF bands (shortwave), is an analogue mode for transmitting images in grayscale. It was the predecessor to slow-scan television (SSTV).
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[edit] Transmission details
Radiofax is transmitted in single sideband and uses frequency modulation (emission mode F3C). 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. With correct tuning (1.9kHz below the carrier frequency for USB, above for LSB), the signal shares some characteristics with SSTV, with black at 1500Hz and peak white at 2300Hz.
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.
[edit] APT
The APT format (automatic picture transmission) permits unattended monitoring of services. It is employed by most terrestrial weather facsimile stations as well as geostationary weather satellites.
- The start tone triggers the receiving system. It was originally meant to allow enough time for the drum of mechanical systems to get up to speed. It consists of rapid modulation of the video carrier, resulting in a characteristic rasp-like sound.
- The phasing signal, consisting of a periodic pulse, synchronises the receiver so that the image will be centered on the paper.
- The stop tone, optionally followed by black, marks the end of the transmission.
| Signal | Duration | IOC576 | IOC288 | Remarks |
|---|---|---|---|---|
| Start tone | 5s | 300Hz | 675Hz | 200Hz for colour fax modes. |
| Phasing signal | 30s | White line interrupted by a black pulse. | ||
| Image | Variable | 1200 lines | 600 lines | At 120 lpm. |
| Stop tone | 5s | 450Hz | 450Hz | |
| Black | 10s |
[edit] 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: W9XZY St. Louis delivers 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
[edit] Stations
Today, radiofax is primarily used worldwide for the dissemination of weather charts, satellite weather images, and forecasts to ships at sea. The oceans are covered by coastal stations in various countries.
In the United States, fax weather products are prepared by a rather bewildering alphabet soup of offices, branches, and agencies within the National Weather Service (NWS) of the National Oceanic and Atmospheric Administration (NOAA). Tropical and hurricane products come from the National Hurricane Center (NHC) and Tropical Prediction Center (TPC), both in the Tropical Analysis and Forecast Branch (TAFB). They are broadcast over US Coast Guard communication stations NMG, in New Orleans, LA, and NMC, the Pacific master station on Point Reyes, CA. After Hurricane Katrina damaged NMG, the Boston Coast Guard station NMF added a limited schedule of tropical warning charts. NMG is back at full capability, but NMF continues to broadcast these. All other products come from the Ocean Prediction Center (OPC) of the NWS, in cooperation with several other offices depending on the region and nature of information. These also use NMG, NMC, and NMF, plus Coast Guard station NOJ in Kodiak, AK, and Department of Defense station KVM70 in Hawaii.
Ever since the RMS Titanic dramatized the dangers of icebergs in the North Atlantic, an International Ice Patrol has also originated weather data, and its charts are broadcast by the Boston station during the prime iceberg season of February through September, using the callsign NIK.
A major producer of Canadian radiofax is the Canadian Forces METOC (Meteorology and Oceanography Centre) in Halifax, NS, using the communication station CFH. Charts are sent on the hour, then the station switches to radioteletype (RTTY) for the rest of the period.
CBV, Playa Ancha Radio in Valparaiso, Chile broadcasts a daily schedule of Armada de Chile weather fax for the southeastern Pacific, all the way to the Antarctic. Also in the Pacific, Japan has two stations, as does the Bureau of Meteorology in Australia. Most European countries have stations, as does Russia.
There's one holdout for news via radiofax. This is Kyodo News in Japan. It broadcasts complete newspapers in Japanese, often at 60 lines per minute instead of the more normal 120. This is because of the greater complexity of written Japanese. A full day's news takes hours to transmit. Kyodo uses JJC, in Tokyo, and a station in Singapore.
