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.
entering any Wefax mode, the reception window opens, and optionnaly the
transmiot window. it is always possible to bypass this with the menu
Configuration.As for any modem, weather fax has its own tab in the configuration window.
Embedded vs floating display mode.There are two display modes for this modem:
- Embedded mode: This is the default mode, the normal receipt and transmit windows gets graphic (Like Hellschreiber mode).
mode: There are two seperate windows for transmission and
reception. This was the only available mode until fldigi 3.21.49.
Hide transmission window.This
option, allows to closed by default, the transmisson window, when
entering Wefax mode. In embededd 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.
Logging messages to ADIF fileEach 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
Frequency shift adjustmentThe default frequency is 800 Hz. However, it is possible to adjust for example to 850 Hz for Deutsche Wetter Dienst.
Maximum rows numberIt
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:
automatic mode (APT control), if an
image end is not detected, we can garantee that the result will take no
more than, for example, the size of two faxes. Typical faxes have about
1300 lines, so the max lines parameters can be tuned to, for example,
- In manual mode, where images are read continuously, this cuts the received images into chunks of equal size.
Destination directory for saved imagesReceived images are saved in the default folder
$HOME/.fldigi/images (Linux) or <defaultpath>/fldigi.files/images
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.
Monochrome imagesFax 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.
Transmitting an Image
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:
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 intentionaly disabled at the moment.
Receiving an image
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:
- Save: This allows to save the current image as a PNG file at any moment.
- Non-stop: 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.
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.
APT/Skip phasing: When in automatic mode, this allows to skip detection
steps. This is often necessary when the signal is not very good.
- Line: The number of the line currently received.
- Width: The image width in pixels. This is usually 1809, if LPM is 120.
Lines per minute: Typically 120, can be 60, 90, 120 or 240. This is
detected in automatic mode, but can be manually adjusted.
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.
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 modeThis
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
Manual reception mode (Non-Stop)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.
Input FIR filtersThere are three input Finite Impulse Response filters available. Here are their frequency characteristics:
Narrow filter, the default one, give the better results.
Narrow filter response
Middle filter response
Wide filter response
Centering an imageIf 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 slantIf 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.
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
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
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 - inherited from the Hamfax 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.
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 traditionaly method does not detect anything.
These two methods are interchangeable but used together for better detection.
AFC: Automatic Frequency ControlThis
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.
Noise eliminationThis option
eliminates short-lived noise, individual pixels with a different value
from their horizontal neighbours. 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 neighbours.
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.
Displaying received filesEach
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.