One full transmission cycle can take between a few minutes and more than half an hour. 2 shows a complete FAX transmission with automatic picture transmission (APT) tones and – along with other things – a grey ‘wedge’ at the end. This enables the software to automatically recognize the correct values for RPM and IOC.įig. This ratio is usually 576 and is calculated from the diameter of the drum, usually 152mm, multiplied by the number of lines per mm.Īt the beginning of the transmission, standardized tones are transmitted. The ‘Index of Cooperation (IOC)’ determines the aspect ratio of the FAX image. The abbreviation ‘rpm’ stands for ‘revolutions per minute’ and refers to the mechanical method of FAX transmission, in which the original was mounted on a drum. However, occasionally, a rate of one (60 rpm) or 1.5 (90 rpm) is in use. For weather maps, this is almost always two lines per minute (120 rpm). What else do you need to know? The important thing is how many lines are sent per minute. Line drawings, such as weather maps, can thus be interpreted as frequency shift keying (FSK), with an 800Hz shift. More rarely, images from weather satellites are also broadcast in grey (‘halftone’).įor this purpose, FM is used, with a centre audio frequency of 1,900Hz: 1,500 Hz corresponds to white, and 2,300Hz represents black. Usually, all Fax charts (plus some text in English, Japanese, Thai, Russian or Korean) are transmitted in black and white. Thanks to the unique combination of robust modulation and the powerful pattern recognition of our eye-brain system, you can also read the results under conditions of poor reception quality. 1).Īpart from this, there are still a number of meteorological services disseminating weather charts, especially for ships at sea, with a power between one and 20kW.įree software makes decoding the signals a breeze and renders reception very comfortable. Nowadays, only the Japanese news agency KYODO transmits some text and pictures via HF FAX from stations in Japan Singapore does so too, to supply some cruise vessels with a fresh daily (Fig. With the advent of the Internet, FAX gave way to file transfer. At the reception end, things work the other way around, translating different tones into the transmitted picture.īy means of this technique, news agencies would distribute their pictures worldwide, via landline, sea cable, HF or even satellite. The key idea was to scan the original line by line, thereby translating the brightness fluctuations into electricity and thus modulating a transmitter. You can receive signals from Hawaii to Japan and from New Zealand to Seoul – daily and with free software. His technique, called ‘Radio-FAX’, is still popular today. On 12th December 1922, the American inventor Charles Francis Jenkins (1867-1934) was the first to successfully demonstrate transmitting pictures by means of radio waves. Alas, these stations have faded, but there still are many other Fax pictures to collect. Nils Schiffhauer DK8OK has an update on radio facsimile (FAX) transmissions, one of the most enduring modes of short wave utility signals, which you can receive and decode with little effort and to great effect.ĭo you remember? Around 25 years ago or so, you may have received weather and press facsimile (Fax) broadcasts from such exotic locations as Diego Garcia, Khabarovsk, Casey Base and Molodezhnaya in Antarctica or Beijing.
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