1.Introduction
WEFAX Mode is a Guest mode for the decoding, displaying and saving
of Wefax and Radiofax images, produced by processing the audio
samples stream from the SSB demodulator.
2. Features
Software AF/FM
Frequency Discriminators
WEFAX Mode carries out demodulation of the Audio
Frequency-Modulated Wefax signal with fairly simple DSP FM
discriminator algorithms. There are two such FM discriminators, an
I/Q Phase Detector and a Bi-Level Tone detector, one of which can
be selected from the "FM Detector" popup menu item. The default is
the I/Q Phase Detector which is usually more suitable for general
use, as the Bi-level detector posterizes the Wefax image, and is
mostly useful for receiving stations that transmit a white|black
image only.
The I/Q Phase Detector Estimates the frequency of the incoming Wefax audio signal by calculating the instantaneous phase angle of the audio signal. This is done by saving audio samples from the Demodulator in a ring buffer with a length in samples that represents a quarter cycle of the center frequency of the Wefax signal. This would normally be 1900 Hz (the range of Wefax signal frequencies is 1500 Hz to 2300 Hz) but because the sample rate of audio is 48 kHz, a frequency of 2000 Hz is used as the center frequency. This gives us 24 samples per cycle so that the quarter cycle is a round 6 samples long. Samples spaced 6 positions apart in the ring buffer would be in quadrature at the center frequency and for frequencies above and below this, the phase angle will vary accordingly and this can be used to calculate frequency.
The Bi-level detector works by implementing two Goertzel tone detectors, one on the White frequency (2300 Hz) and one on the Black frequency (1500 Hz). By comparing the signal levels as measured by the Goertzel detectors, the frequency of the Wefax carrier signal can be estimated. These level comparisons are limited to only 5 thresholds, therefore the Bi-level detector produces a posterized Wefax image.
The calculated frequency of the input AF signal is scaled and threshold-ed so the a value of 0 to 255, for the brightness level of the relevant Wefax image pixel can be calculated and used to create the Wefax image. The standard frequency range for transmitting Wefax and Radiofax images is 1500 Hz for black and 2300 Hz for white. Here is the complete specification for the Wefax image transmission process, as generally accepted by most Wefax stations:
| Signal | Duration | IOC576 | IOC288 | Remarks |
|---|---|---|---|---|
| Start tone | 5s | 300 Hz | 675 Hz | 200 Hz for color fax modes. |
| Phasing signal | 30s | White line interrupted by a black pulse. | ||
| Image | Variable | 1200 lines | 600 lines | At 120 lpm. |
| Stop tone | 5s | 450 Hz | 450 Hz | |
| Black | 10s |
"Waterfall" display with
CAT
WEFAX Mode has a "Waterfall" (audio Spectrum) display derived from
an IFFT of the receiver's audio output. The Waterfall display has
two vertical white lines at each side of window, which indicate the
position of the White and Black frequencies. The position of these
lines depends on the specified White and Black frequencies
(normally 2300 Hz and 1500 Hz). Wefax/Radiofax signals are mostly
concentrated around the White frequency, so the Spectrum display
usually shows a band of signals either side of this frequency. For
proper tuning of a Radiofax signal, this spectrum band should be
centered on the White frequency line at the right of the Spectrum
display. Most Wefax/Radiofax stations on the short waves specify
the carrier frequency, while the modulation is Upper Side Band
(USB) and it is centered in the middle between the white and black
frequencies, e.g. (2300 + 1500) / 2 = 1900 Hz. So the receiver must
be tuned to 1.9 KHz lower than the specified carrier frequency. For
example DDK3 transmits on 7880 KHz so the receiver must be tuned to
7878.1 KHz USB. Some stations, like SVJ4 in Athens, specify the
receiver dial frequency, e.g. 8105.0 KHz, which gives the right AF
output frequencies already.
Status
Indicators:
There are three Status Indicator icons in the WEFAX Mode Control
Frame:
o The "Start Tone" Indicator shows a green "LED" icon when
WEFAX is listening for a Wefax/Radiofax Start tone. If this stage
in the decoding of Wefax images is skipped by the user, this
indicator shows a red "cancel" icon and if a start tone is
detected, it shows a green "tick" icon.
o The "Sync Pulse" Indicator shows a green "LED" icon when
WEFAX is synchronizing itself with the Phasing Pulses transmitted
by the station. If this stage in the decoding of Wefax images is
skipped by the user, this indicator shows a red "cancel" icon and
when synchronizing is complete, it shows a green "tick" icon.
o The "Decoding" Indicator shows a green "LED" icon when
WEFAX is decoding the Wefax/Radiofax image from the incoming signal
from the receiver. If this stage in the decoding of Wefax images is
skipped by the user, this indicator shows a red "cancel" icon and
when decoding is completed (a Stop Tone has been detected), it
shows a green "tick" icon.
o The "Save Img" Check Box can be used to disable or enable
Saving of the decoded Wefax image. The "Save Img" Indicator shows
the status of the Wefax image Save functionality. If saving is
disabled then the "x" icon is shown. Otherwise a green "LED"
indicates that saving is enabled and a green "tick" icon indicates
that image saving to file has been successful. A red "LED"
indicates a problem with image saving.
o At the right of the Wefax Control frame, a Level Bar
indicates the level of either the Start or Stop signal, depending
on the current operation. The Level Bar indicates Green color if
the signal is above the "Detected" threshold, Orange color if the
signal is between the "Detected" and "Activate" threshold and Red
if the Start/Stop signal is not detected. WEFAX expects the level
of the Start/Stop signal to first rise into the Green sector and
then fall below the Orange sector (Hysteresis), before it is
considered as successfully detected.
Image Slant
Correction:
Unfortunately Wefax/Radiofax stations do not follow a consistent or
standard format of image transmission and this makes it difficult
to decode images in a uniform way. Some stations transmit a regular
Phasing (Sync) pulse at the start of each image line. This pulse is
usually about 55-60 pixels long and it is black color on a mostly
white line. WEFAX can use this pulse to synchronize decoding of
images so that there is little or no slanting of the image content,
due to slight timing differences between the transmitting and
receiving stations. Some stations just do not transmit in-image
phasing pulses and others transmit shorter or intermittent pulses,
making it impossible to actively synchronize sender and receiver.
In this case WEFAX can de-slant images pre-emptively by applying a
correction factor, calculated from measuring the slant of a saved
image. This de-slanting factor is in (pixels/1000 lines of image)
and it can be specified either permanently in the Stations Treeview
Window (and saved to the Stations file) or set before decoding in
the Fix Slant spin button. Positive figures compensate for slanting
to the left and negative figures for slanting to the right.
The slant correction factor is calculated as follows: First receive and save a wefax image, one that contains a vertical line or the trace of the phasing pulse, if transmitted, and then open the Wefax image in and image editor like the Gimp. Measure the horizontal position in pixels of the vertical line at the top and bottom of the image and find the difference (in pixels). Then measure the vertical position of the line at the top and bottom of the image (this is the height of the line in pixels). Divide the slant of the vertical line with the height and multiply by 1000, to calculate the slant correction factor in pixels/1000 lines. If the image is slanted to the right, the correction factor is negative, otherwise positive. This value can then be entered in the stations file, either using the stations Treeview window or if necessary using a text editor.
Status Text
Messages:
The "WEFAX Messages" text view port prints various messages from
WEFAX Mode regarding the status of various operations, error
conditions and other information relevant to WEFAX's operation.
Pending operations are shown in black, successful completions in
green, canceled operations in orange and error messages in red.
Wefax/Radiofax
Stations List
The Stations Treeview Window presents a list of Wefax/Radiofax
stations and some of the relevant parameters for decoding images.
For each station row, there are the following columns:
o The Name of the station.
o The Frequency in Hz it transmits on, including a 1900 Hz
compensation to take into account the use of USB mode, and
frequency shift from 1500 Hz to 2300 Hz (center frequency 1900
Hz).
o The RPM (transmission speed) value in lines/min.
o Image resolution in pixels/line.
o The IOC (Index Of Co-operation) value.
o The Image Slanting correction factor in pixels/1000 lines.
When a station is selected from the Bookmarks, the Receiver
frequency will be set to the Station frequency and the proper
settings (bandwidth, BFO frequency) will be set up. The
Wefax/Radiofax decoding parameters are also entered into the
relevant items in the pop-up Menu. To navigate the Treeview list,
the Up and Down buttons can be used, as well as the Up/Down arrow
keys in the keyboard.
WEFAX Mode User
Interface
The main WEFAX's window has the following widgets:
1. The Wefax/Radiofax Image Viewport: This is the main part of the WEFAX Mode window and it is used to display the Wefax/Radiofax image as it is decoded progressively. Right clicking on the window opens the main Pop up Menu. When image decoding is over, the displayed image is saved in a PGM file format in ~/.lhpsdr/wefax/images/ folder. The Wefax image can be processed, while it is being received, either by contrast stretching or bi-level (black-white) threshold. These functions are selected from the "Enhance Image" item in the Pop up Menu.
2. The Signal Input Display: This is the small window at the bottom left of the WEFAX Mode window, with the dark green background. It can display either the output of the FM discriminator (e.g. the incoming Wefax signal) or the amplitude of the AF signal supplied by the radio receiver.
3. The Spectrum/Waterfall Display: This is to the right of the Signal display and it has a black background. It is used to display the Spectrum of the incoming Wefax signal, as produced by an IFFT function, in the form of a "waterfall". This display can be used to tune the Receiver to the right frequency. The Wefax signal's spectrum is normally concentrated around the White frequency of 2300 Hz and it produces a fairly wide strip down the Waterfall. By clicking on this strip, it is possible to tune in the Receiver, fairly accurately.
4. The WEFAX Control Frame: This frame contains a couple of buttons, one for starting WEFAX and one for skipping between operating modes of WEFAX. The "STANDBY" button puts WEFAX in the Receive mode, whereby the button label indicates "RECEIVE" in green and the top "Start Tone" red indicator changes to green. In this condition WEFAX waits for a Wefax Start tone, to initiate the image decoding process. The "Skip" button causes WEFAX to skip forward to the next mode, e.g. from waiting for a Start tone to Synchronizing with the Phasing pulses, or from this mode to Decoding the Wefax image. This makes it possible to bypass missed stages in the Wefax image transmission sequence. The appearance of the indicators shows Non-Function (Red), Operating (Green), Skipped (Red X) and Finished (Green tick).
There is also a vertical Level Gauge at the right side of the frame. This gauge indicates the output of the Start Tone detector when WEFAX is waiting for one, or the output of the Stop Tone detector while an image is being decoded. I added this gauge as there were some problems detecting start/stop tones and I wanted an indicator to help me investigate.
5. The Messages Text Viewport: Various messages are printed here by WEFAX to indicate current status, operations like saving images, error conditions etc.
6. The pop-up Menu: This opens on a right-click on the
Wefax Image window. It has the following items:
o Bookmarks List: Opens the Stations List Treeview window
from which a Wefax/Radiofax station can be selected by clicking on
a Treeview row. It is also possible to move between stations by
clicking the Up or Down buttons or the Up/Down arrow keys on the
keyboard. The Delete button removes a row, whereas the New button
creates a new row, which can be filled either manually or by middle
clicking on the row. In this case the row will be filled with the
currently used values of the Wefax parameters and the Receiver's
frequency will also be set.
o FM Decoder: Select one of the two available Wefax carrier
signal decoders, the Zero Crossing detector or the Bi-level Tone
Detector.
o RPM: Select the RPM (Lines/min) value used by the station.
Default is 120 RPM (lines/min).
o Resolution: Select the image resolution in pixels/line.
The default is 1200 pix/line.
o IOC Value: Select the IOC (Index of Co-operation) value of
the station. The default is 576.
o Phasing Lines: Select the number Phasing lines to examine
at the start of image transmission. There is no universally
followed standard unfortunately, with some stations transmitting as
few as 10 phasing lines and others up to 60. The default of 20
lines will not work for stations transmitting less than this. As of
version 1.7, the number of phasing lines to listen for can be
specified and saved in the Stations List Treeview window. This will
override any selection made from the popup menu, when a station is
selected from the list.
o In Image Phasing: Enable synchronization with in-image
phasing pulses transmitted by some stations.
o Image Enhancement: Select "Normalize Image" to stretch the
contrast of the image as it is being received or "Bi-level Image"
to threshold the image to only Black or White values.
o Image File Format: Select the Image File format: JPEG, PGM
or Both, if needed. WEFAX has a simple built-in JPEG encoder for
gray scale images that can reduce the image file size considerably.
The JPEG file format is the default.
o Quit: Quits WEFAX Mode.
Configuration and
set-up
After compilation and installation, the
~/.lhpsdr/wefax/wefax.config run time configuration file MUST be
edited for proper operation! To do this, load
~/.lhpsdr/wefax/wefax.config into your favorite editor and make
sure that you review ALL the configuration fields in the file and
that you properly edit all the ones that must be edited. There are
more detailed instructions in the file itself so please follow them
for a proper set-up.
Using the Stations
Treeview List
The Stations Treeview Window opens when the "Station List" menu
item is activated. The Stations Treeview can be used to select the
desired Wefax/Radiofax station and to enter the decoding parameters
into the relevant menu items. As well as selecting stations and
entering decoding parameters, the Treeview can be used to edit all
data in the columns of a selected row and save all Treeview data to
the "stations" file in the ~/.lhpsdr/wefax directory. The following
are the columns available in the list:
STATION NAME: The name of the Wefax station in the row.
FREQUENCY: The frequency of the station in Hz. This is normally
1900 Hz below the specified carrier frequency. This needs to be
fine-tuned by tuning the Receiver, so that the trace of the White
frequency (2300 Hz) in the Waterfall display is under the vertical
white line at the right of the display. Clicking near the trace of
the White frequency will automatically tune the receiver to the
right frequency. This setting can be saved to the Stations List by
middle click on the selected Station row.
S/B: The side band used by the Wefax station. This should normally
be Upper Side Band but on a couple of occasions I heard one station
using LSB, so I added this column.
RPM: The RPM or Line Transmission Rate of the station. This is
normally 120 RPM or Lines/min but some stations (Kyodo News) use 60
RPM also.
PIX/L: The resolution of images in pixels/line. This is normally
1200 pix/line.
IOC: The Index of Cooperation. This is normally 576.
PHL: The number of phasing lines to listen for, after the Start
Tone. The default is 20, but some stations (the Hellenic Met
service) use only 10 but since 2-3 line may be missed at the start,
due to a delay in the Start Tone detector, it is necessary to
specify 10 for this station. For most others, a value of 40 can be
specified if desired, since usually about 60 phasing line are
sent.
SKEW: The image slant correction factor in pixels/1000. This is
needed to compensate for the difference in timing between the
sending and receiving stations. The right value must be found as
described previously.
Receiving Wefax/Radiofax
signals
Wefax/Radiofax signals are transmitted on the short waves by
stations operating in the USB mode and with a Frequency Modulated
output, in a Slow Scan TV format. White color is represented by a
2300 Hz signal and Black color with a 1500 Hz signal. The
transmission begins with a few seconds of a Start Tone and then a
number of Phasing (Synchronizing) lines are sent. These are used to
synchronize the start of image lines at the receiving end with
those of the transmitting end. The Phasing lines are black with a
short white part at the beginning, and they are then followed by
the image lines, with the height of the image (the number of image
lines) being variable. Some stations transmit phasing pulses at the
start of image lines, but these are short black lengths in mostly
white lines. Unfortunately not all stations follow the accepted
standards for Wefax transmissions, making it rather difficult to
apply regular algorithms for sync detection.
To receive a Wefax/Radiofax station, set up the Receiver Mode to RFAX-U (Radio FAX - Upper Sideband). Tune in the Wefax station so that the trace of the White signal (2300Hz) is under the right vertical white line in the Waterfall. Also, by clicking on the White signal trace in this display, the Receiver will be tuned fairly accurately to the right frequency. If WEFAX Mode is started by clicking on the STANDBY button, it will wait until a Start Tone is received and when the Phasing Pulse signals begin, it will synchronize itself and enter the Image Decoding mode. The Wefax image is decoded line by line and displayed in the Image window. When a Stop Tone is received, WEFAX saves the decoded image and again it waits for the next Start Tone. If left to its own devices, it will continue in the way, automatically decoding and saving images.
4. Bugs and deficiencies
I have fixed whatever bugs I came across testing WEFAX Mode, but
there may be some hiding, waiting for the right conditions to
appear.
5. Copying This software package is released under the GNU Public License. Please see the COPYING file for more details.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.