WEFAX

This modem is able to receive and transmit HF-Fax images, traditionally used for weather reports.

You can refer to the wikipedia article  Radiofax for more detailed information.

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.

Transmitting an Image

To open the transmit window, you must of course select one Wefax modem, and then right-click on the transmit (blue) window:

How to open the transmit window


Then, the transmit window just opens. This is the same logic as sending MFSK images.

Empty transmit window


Then, you must open an image file using the button "Load". The image is then displayed, for example like that:

Transmit window, with a file.

Now, to start the transmission, you just need to click "Xmt Grey" 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...)

Receiving an image

To go into reception mode, one must click the "View" menu tab, and select "Weather Fax Image".

Finding the reception window



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:

Reception window when not receiving.


When receiving an image, either in manual or automatic mode (APT control), other controls are displayed:Reception window

APT control reception mode

This mode uses the APT start and STOP frequencies to detect the beginning and end of an image. Additionnaly, 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.

Manual reception mode

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.

Centering an image

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.

Picture with a slant

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.

Automatic centering.

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 derivativatives 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 constrasted details.

Image detection based on signal power.


The APT control - inheritated from the Hamfaw signal, does not work very well when the image is noised. On the other hand, fldigi provides ways to evalute 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 traditionnaly method does not detect anything.

These two methods are interchangeable but used together for better detection.

Saving an image

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'. The PNG images  received some extra text comments which can be displayed, for example, with GIMP.

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