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:

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 "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".

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:
- Save: This allows to save the current image as a PNG file at any moment.
- Manual
mode/Abort: At startup, the modem goes into automatic mode, and the
text "Abort" is displayed. When clicking "Abort", this blanks the image
and resets the APT detection. When "Manual mode'" is clicked, no APT
detection is done. The Automatic/Manual mode is displayed in the
reception window label.
- Pause/Resume: At any moment, the image reception can be paused and resumed (State is displayed in the reception window label).
- Zoom: This allows to zoom in/out the image.
- FIR:
This allows to select an input FIR (Finite Impulse Response) filter.
Practically, the narrow filter (Default value) gives the best results.
The selected value is saved in the configuration file.
- Skip
APT/Skip phasing: When in automatic mode, this allows to skip detection
steps. This is often necessary when the signal is not very good.

When receiving an image, either in manual or automatic mode (APT control), other controls are displayed:- Max lines: This is the maximum number of lines admissible for a fax.
When this number is reached, an image file is autoatically saved and
the line counter returns to one. Typical fax have about 1300 lines.
- Line: The number of the line currently received.
- Width: The image width in pixels. This is usually 1809, if LPM is 120.
- LPM:
Lines per minute: Typically 120, can be 60, 90, 120 or 240. This is
detected in automatic mode, but can be manually adjusted.
- Slant:
This is used to adjust the slant of the image due to a clock
inaccuracy. This value is saved in the configuration file, so it is not
needed to reenter it each time.
- Center: This is used to manually adjust the horizontal center of the image, if it could not be detected in the phasing step.
- Auto:
When this button is set, the image will be automatically centered. This
process starts after some hundredth of lines are received, by shifting
left and right the image. It takes some time to converge.

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.