Fldigi Users Manual
4.2.00
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This modem is able to receive and transmit HF-Fax images, traditionally used for weather reports.
More technical information is available on the wikipedia article Radiofax.
Two modes are implemented IOC=576 or 288. The focus is made on black-and-white images, color mode is still experimental.
Many frequencies are available at http://www.hffax.com/ for example.
As for any modem, weather fax has its own tab in the configuration window.
Each time an image is saved, it is possible to log this event, with the frequency and reception time, to the Adif file. This option is disabled by default.
The default frequency is 800 Hz. However, it is possible to adjust for example to 850 Hz for Deutsche Wetter Dienst.
Wefax transmissions are USB, with the center of the frequency modulated audio signal center at 1900 Hertz. This is the default value.
It non-continuous (Non non-stop) reception mode, an image is automatically saved when it has more than this number of lines (Default 2500 lines). Once this number of rows is reached, the image is saved and a new image is read with the same parameters. This feature has two applications:
The Wefax signal has start, stop, and phasing components to the transmission. The decoder searches for the start signal, the stop signal and the phasing signal. The phasing signal is used for auto alignment of the image. You can specify the start, stop, and interval for the alignment.
Wefax images have a strong line to line correlation. This correlation is used to detect a signal loss to prevent printing noise. The image display is stopped when the correlation falls below the threshold as detected over the specified number of lines.
Select an input FIR (Finite Impulse Response) filter:
There is rarely any QRM on a government WEFAX broadcast, but the filter selection might improve the decoded signal to noise ratio. Start with the medium filter setting.
Received images are saved in the default folder
$HOME/.fldigi/images (Linux) or <defaultpath>/fldigi.files/images (Windows).
Additionally, they can be saved manually, at any time, using the button 'Save'.
A blank image is initially displayed. Several controls are available:
Each time the end of an image is detected, an image file is created and stored in the default Wefax image folder. Open the folder with the system file finder using the menu item "File / Folder / WEFAX images...". The images are saved as a Portable Network Graphics file, with the extension ".png". Use the system image viewer to view the images.
To open the transmit window, select the menu item "View/Weather Fax Image TX"
Load an image file using the button "Load".
Click "Tx B/W" to start the transmission. During transmission, image reception is paused. The window will display each image line as it is sent. Please be patient, this may take a while. You might note that FlDigi status line displays the estimation transmit time, and the current stage (Start, phasing etc...).