Known alternatively as radiofax, hf fax and weatherfax these transmissions were the forerunner to SSTV transmissions and are intended for marine navigation and weather forecasting. Using the short-wave HF band transmission they still enable information to be received by vessels out of range of modern internet communications. Some Japanese stations even use the technology to transmit newspaper images. More details of the technology and the history at http://en.wikipedia.org/wiki/Radiofax
These transmissions use single sideband and 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. With correct tuning (1.9 kHz below the carrier frequency for USB, above for LSB), the signal shares some characteristics with SSTV, with black at 1500 Hz and peak white at 2300 Hz.
It is fairly simple to decode using a computer sound card and there are many software solutions available, however most are commercial products and have severely limited trial versions. I have used the free WXFAX software, intended primarily for weather satellite decode (more on that at a later date) it can be configured to decode the FM-120 signal. Details here http://www.newearth.demon.co.uk/radio/hfwefax1.htm
Some more useful links about the broadcasts at Marine Weatherfax NOAA broadcasts including a schedule of internal transmissions and stations (pdf) and also the German Deutscher Wetterdienst
Here are a few decodes made over the last few days on my Picasa Web Album
|HF Weather faxes|
|From Please Ignore|
To decode the signal require a SSB receiver I currently have in my possession a Alinco DJ-10X which has never excelled at HF frequencies and is very susceptible to overload and computer interference (it is more a VHF/UHF scanner) and has proved problematic in the past when using any data decoding using a sound card (suspect due to the heavily filtered audio output) but has been surprisingly good over the last few days. I am not using a dedicated HF antenna, but hasn't been too bad since the signal is coming from Hamburg (on 7880kHz) and is fairly strong in the morning but performance falls off during the day.
|From Please Ignore|