FLDIGI Users Manual 4.0
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.
When entering any Wefax mode, the reception window opens, and optionally the transmit window. it is always possible to bypass this with the menu bar:
As for any modem, weather fax has its own tab in the configuration window.
There are two display modes for this modem:
This option, allows to closed by default, the transmission window, when entering Wefax mode. In embedded mode, this means that the entire window is used for reception (Most common mode). In floating mode, this implies that the transmission window is not opened by default.However, it is always possible to manually open or close the transmission window at any time.
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.
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:
Received images are saved in the default folder
$HOME/.fldigi/images (Linux) or <defaultpath>/fldigi.files/images
Additionally, they can be saved manually, at any time, using the button 'Save'. The PNG images received some extra text comments which can be displayed, for example, with GIMP.
Fax images are monochromes and are saved as such by default. However, it is possible to bypass this parameter and save them as color RGB images.
To open the transmit window, you must of course select one of the two Wefax modems, and then right-click on the transmit (blue) window:
Then, the transmit window just opens. This is the same logic as sending MFSK images.
Then, you must open an image file using the button "Load". The image is then displayed, for example like that:
Now, to start the transmission, you just need to click "Tx B/W" for black-and-white images, etc... 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...); Color transmission ('Tx Color') is intentionally disabled at the moment.
To enter reception mode, one can click the "View" menu tab, and select "Weather Fax Image".
At this time, the reception window opens. A big blank picture is visible, this is where the received image will be displayed. Several controls are available:
When receiving an image, either in manual or automatic mode (APT control), other controls are displayed:
This mode uses the APT start and STOP frequencies to detect the beginning and end of an image. Additionally, it tries to detect the phasing signal - a wide black band - to detect the center of the image. This method is helped by the estimation of the signal power of these frequencies.
In this mode, the image is continuously read and displayed. When the maximum number of lines is reached, the image is saved and blanked, and the line counter returns to one.
There are three input Finite Impulse Response filters available. Here are their frequency characteristics:
Narrow filter, the default one, give the better results.
If the phasing was not automatically detected, the modem could not deduce the beginning of an image. The result is an image which is horizontally shifted. To correct this, one can use the "Center" slider.
If either the send, receive or both ends of the transmission are using an uncalibrated sound card whose sampling rate is not an exact multiple of the sample rate the resulting picture at the receive end will appear slanted. The degree of slant is directly related to the accumulation of the frequency error at both ends of the transfer. Stations wishing to receive Weather fax pictures should calibrate their sound card.
The WWV calibration mode is used to measure and set the parts per million (ppm) correction factor for the sound card.
Your sound system may be fully corrected, but the sending station may have an uncorrected sound card. You can usually correct for small errors during reception by using the slant slider. Its value (Typically between - 0.005 and 0.005) will be stored in fldigi configuration parameters.
If the phasing signal could not be used for centering the image, the program waits for a string image signal anyway to go into reception mode, but it sets an internal flag allowing to automatically center the image. This feature can be freely enabled and disabled at any moment. it works by detecting a wide vertical band of about hundred pixels, where the sum of the contrast as the lowest among the complete image width.
That is: It computes for each row and each pixel, the absolute value of the horizontal derivate. It then sums these derivatives pixel-wise, row by row. Then, it computes an average of about hundred pixels along this single row. The column which has the lowest averaged contrast is considered to be the image margin, which is then shifted on the left of the window.
This method takes some time to stabilize, because at the beginning, there are many areas of the image, without details. It gets stable at the end, when only the margin stays with few contrasted details.
The APT control - inherited from the Hamfax signal, does not work very well when the image is noised. On the other hand, fldigi provides ways to evaluate the signal power on a given bandwith. This is used because APT control relies on the emission on specific frequencies.
Therefore, in the APT start and phasing loops, when check for the presence of strong signals associated to APT control. This information is used to take a decision when the traditionally method does not detect anything.
These two methods are interchangeable but used together for better detection.
This option controls the frequency on the complete spectrum width. After several hunderads of lines have been correctly loaded (That is, with a high line-to-line correlation), the AFC locks until the frequency or mode is manually changed.
This option eliminates short-lived noise, individual pixels with a different value from their horizontal neighbors. They are modified using median values. This is based on the fact that no line should be narrower than two pixels, otherwise such an image would not be broadcast, because not correctly readable. It is therefore impossible to have one single pixel, simultaneously very different from its left and right neighbors.
Faxes can be stored as binary or grey level images. The cutoff level between black and white (Default 128) can be adjusted. No information is lost until the image is saved, therefore this level can be freely modified until image end. The purpose of this option is to save disk space.
Each time the end of an image is detected, an image file is created and its name appears on the file list of the reception window. By clicking on a file name, it is displayed in the transmit window.