OS/2 Shareware BBS: 8 Other/08-Other.zip/rafax11.zip/RADIOFAX.HLP

OS/2 Help File | 1996-06-12 | 28KB | 658 lines

ΓòÉΓòÉΓòÉ 1. Introduction ΓòÉΓòÉΓòÉ

This program receives weather faxes and other radio faxes from a High Frequency
(Short Wave) radio. The radio requires Single Side Band (SSB) or a Beat
Frequency Oscillator (BFO).

The computer requires OS/2 MMPM/2 and a supported sound card.

To get started browse the following sections :

     Physical configuration
     Tuning the radio
     Tuning the computer
     Possible problems


ΓòÉΓòÉΓòÉ 2. Getting Started ΓòÉΓòÉΓòÉ

This program is designed for novice users. Jargon is kept to a minimum. The
following sections should get you started in receiving the standard weather
faxes.


ΓòÉΓòÉΓòÉ 2.1. Physical configuration ΓòÉΓòÉΓòÉ

Use either a microphone :

Or an inline connection :

to the microphone or inline socket of the sound card.

When you first start this program a Set Initial Parameters dialogue should be
displayed. If not then on the menu line, select Options, then select Set
Initial Parameters. The Set Initial Parameters dialogue is displayed. Click on
the appropriate Audio Input radio button. Click on OK


ΓòÉΓòÉΓòÉ 2.2. Tuning the radio ΓòÉΓòÉΓòÉ

Tune the radio to a weather fax frequency. Choose a single side band, usually
upper, or tune the beat frequency oscillator, usually negative. You should hear
a sound like crickets.

     What radio fax sounds like


ΓòÉΓòÉΓòÉ 2.3. Tuning the computer ΓòÉΓòÉΓòÉ

On the menu line, select Window.  Select Show spectrum.

A small window will appear. This window displays the audio frequency spectrum
of the incoming fax.

Select Record.  Select Start now. After a few seconds black lines appear in the
spectrum window. The red line is the division between black and white parts of
the incoming weather fax. There may be two peaks on the black lines if a fax is
being received on the radio, one for black and the other for white. If a
greyscale picture (an audio frequency between the black and white extremes
represents a shade of grey) is being received, the black lines look like a blob
shaped by the particular picture.

Adjust the volume on the radio so that the VOL bar at the top of the spectrum
window is about a third of the way across. This volume bar indicates the
amplitude of the audio. Almost no bar indicates insufficient audio from the
radio. A near full bar indicates too much audio and likely signal distortion.
You may need an OS/2 mixer applet for your specific sound card to increase the
Microphone or linein gain sufficiently.

The aim of the next exercise is to position the red line between the two peaks
(or somewhere within the blob for greyscale). If you have a BFO you can tune it
to reposition the peaks around the red line. Or you can use the + and - keys to
move the red line up and down. Note that the spectrum window only refreshes
after every few seconds. If you want faster update select Record/Fast update.
The best results may be got tuning the BFO so that the white peak is at 2300
Hz, the black at 1500 Hz and the red line at 1900 Hz.

With the red line positioned between the two peaks, a picture should be
unfolding in the main window. For a black and white picture, the foreground is
usually black and the background white. If this is not the case the radio is
tuned to the oposite side band. Select  Record/Negative or Positive to correct
the colours.

The two peaks of the spectrum may be hard to see if there is some high
frequency interference or computer generated noise. Move the red line up and
down until you get the best picture;

When you want to stop receiving, select Record then select Stop.

Also refer to :

     Alignment/Align
     Record/Start on sync
     Record/Auto sync and save


ΓòÉΓòÉΓòÉ 2.4. Possible problems ΓòÉΓòÉΓòÉ

Gaps between individual lines in the spectrum.

The audio volume from the radio is too low. You may need an OS/2 mixer
application, for your particular sound card, to set the input gain level.
Adjust the gain and radio volume so that the volume bar in the Spectrum window
is about one third the way across the window.

Many peaks in the spectrum.

The audio volume from the radio may be too high and clipping the signal. Adjust
the input gain (mixer) and radio volume so that the volume bar in the Spectrum
window is about one third the way across the window.

Picture jumps horizontally.

Buffers from the sound card occasionally disappear. I have seen this on Sound
Blaster cards recording 8 bit samples. If you have a 16 bit Sound Blaster card
set Bits per sample (Options menu) to 16.

Picture looks noisy.

Check that the input gain and radio volume is adjusted so that the volume bar
in the Spectrum window is about one third the way across the window. Try
selecting 64 greys in case the picture is grey scale. If the picture still
looks noisy, listen to the monitor then compare to What radio fax sounds like.

Noise could come from the computer, monitor, other appliance or atmospheric
conditions. You should use an outside antenna and the lead connecting it to the
receiver should be shielded.

Message - Program not keeping up with Fax input.

This message occurs when half the allocated memory for the transfer of sound
information from the sound card to the program is used up. Try to reduces
processor load and swapping activity from other applications.

Try only using the spectrum when tuning, reducing the dots per line, reducing
the sampling rate, reducing the grey scale or using black and white when grey
scale pictures are not being transmitted.

Radiofax fails while Grey definition is set to 16

Some VGA drivers have a problem processing 4 bit bitmaps (16 greyscale). Use 64
greyscales instead or try and obtain a recent video driver.

Picture is skewed.

The sound card and driver may not be sampling exactly as specified. Use
Alignment/Skew to slope the picture back to the left or right while a picture
is being received.


ΓòÉΓòÉΓòÉ 3. Menu items ΓòÉΓòÉΓòÉ

Only valid menu items will be enabled at a particular time. A few items, for
which facilities is not yet written, are permanently disabled.


ΓòÉΓòÉΓòÉ 3.1. File/New ΓòÉΓòÉΓòÉ

Clears the current picture without saving it. The Record menu items will then
be enabled.


ΓòÉΓòÉΓòÉ 3.2. File/Create viewing window ΓòÉΓòÉΓòÉ

Creates a new window titled "View Faxes" for the purpose of displaying saved
faxes while a fax is being received in the primary window. Any number of these
windows can be created.


ΓòÉΓòÉΓòÉ 3.2.1. View Faxes Window ΓòÉΓòÉΓòÉ

This window was created by the File/Create viewing window menu item. It is used
primarily to display a saved fax (use File/Open) while the primary window is
busy receiving a new fax. If you want to compare a number of pictures, create a
number of View Faxes windows.


ΓòÉΓòÉΓòÉ 3.3. File/Open ΓòÉΓòÉΓòÉ

Displays a dialogue from which you can select one or more saved faxes from a
list. The selected faxes will then be displayed in progression.

If the disk is formatted with FAT then each list entry displays the long file
name from the Extended Attribute followed by the FAT file name.

Refer to Open Radio Faxes Dialogue.


ΓòÉΓòÉΓòÉ 3.4. File/Save ΓòÉΓòÉΓòÉ

Saves the fax, currently displayed in this window, on the hard drive. The file
path from the current directory will be \RFAXES\FAX on 1995-01-25 at 02-30.BMP
where :

     1995 is the year
     01 is the month, January
     25 is the day of month
     02-30 is the Coordinated Universal Time in hours(24) and minutes.

 This is the date and time the fax started. The date format enables the files
 to be listed cronologically when sorted in alphabetic order.

 This is an OS/2 HPFS long file name. If the disk is formatted with FAT then a
 file name FAXnnnnn.BMP is generated and the long name is stored in the
 extended attributes.


ΓòÉΓòÉΓòÉ 3.5. File/Save as ΓòÉΓòÉΓòÉ

Saves the fax, currently displayed in this window, on the hard drive. You
specify the path and file name. You can use a long file name even if the disk
is formatted with FAT. The long file name will be stored in the extended
attributes.


ΓòÉΓòÉΓòÉ 3.6. File/Delete ΓòÉΓòÉΓòÉ

Deletes the saved copy of the fax, displayed in this window, from the hard
drive.


ΓòÉΓòÉΓòÉ 3.7. File/Print ΓòÉΓòÉΓòÉ

Prints the displayed fax on the default printer. The height and width
dimensions will be as displayed on the screen. You can change the dimensions
selecting Window/Scale to window and resizing the window. Use the settings on
your OS/2 print icon to change the default printer and the page orientation.


ΓòÉΓòÉΓòÉ 3.8. Alignment/Align ΓòÉΓòÉΓòÉ

This facility is required when the sync pulses have not been picked up at the
start of the fax.

After selecting this item the  pointer  is displayed in this window. Align the
vertical line of the pointer where you want the left hand edge of the picture
to be. Then click mouse button one. The picture will be re-aligned and the
normal pointer restored.

If you want to cancel the align, either hit Esc or select Align again.

This facility also works while a fax is being received into the current window.


ΓòÉΓòÉΓòÉ 3.9. Alignment/Skew ΓòÉΓòÉΓòÉ

Usually a sound card has a slight inaccuracy in its sampling rates. This
results in a picture that slants or skews to the right or left. Use this
facility to correct this problem.

While a picture is being received, select this item.  The pointer icon will
change. Position the centre of the icon on the top of a line that should be
vertical and click mouse button one.  Move the pointer down the line and click
again.  The picture should straighten up. If not quite right, do it again.

This correction is remembered until the sampling rate, bits per sample or lines
per minute is changed.


ΓòÉΓòÉΓòÉ 3.10. Record/Auto sync and save ΓòÉΓòÉΓòÉ

Select this item to automatically start, stop and save faxes. Select it again
to revert to manual operation. This item can be selected while a fax is being
received. It will then detect the end of the fax, stop recording, save it and
wait for the next fax, commence recording and so forth.

This will not work consistently for noisy pictures or incorrect tuning. Refer
to Tuning the computer.


ΓòÉΓòÉΓòÉ 3.11. Record/Start on sync ΓòÉΓòÉΓòÉ

Waits for the sync pulses at the start of a fax and then starts receiving the
fax. The fax will be correctly left aligned in the window. This will not work
consistently for noise pictures.


ΓòÉΓòÉΓòÉ 3.12. Record/Start now ΓòÉΓòÉΓòÉ

This facility lets you start recording a fax after the transmission has
started. It is useful for tuning in fax signals on the radio.

It starts receiving now, irrespective of what is being received on the radio.
Use the Alignment/Align menu item to align the fax in the window.


ΓòÉΓòÉΓòÉ 3.13. Record/Stop ΓòÉΓòÉΓòÉ

Stops receiving faxes.


ΓòÉΓòÉΓòÉ 3.14. Record/Fast update ΓòÉΓòÉΓòÉ

If this item is checked, the fax picture and the spectrum display will update
more frequently. This will give faster feedback for tuning in a picture but
possibly less clarity.


ΓòÉΓòÉΓòÉ 3.15. Record/Negative or Positive ΓòÉΓòÉΓòÉ

If the radio is tuned to the wrong side band (for black and white, the
background is black and the foreground white) select Negative. Select Positive
to revert to normal.


ΓòÉΓòÉΓòÉ 3.16. Record/Monitor ΓòÉΓòÉΓòÉ

If this item is checked, the sound card will output whatever is input. This is
useful while you are tuning.


ΓòÉΓòÉΓòÉ 3.17. Record/Auto tune from sync pulses ΓòÉΓòÉΓòÉ

Select this option to automatically reposition the red dividing line between
the frequencies of the black and white signals as determined during the
syncronizing pulses at the beginning of each picture.

Warning,choosing this option when reception is poor can cause the red dividing
line to be positioned incorrectly.

If reception is good, this option is useful for radios that drift in frequency.


ΓòÉΓòÉΓòÉ 3.18. Image/Rotate 180 ΓòÉΓòÉΓòÉ

The picture is rotated 180 degrees.


ΓòÉΓòÉΓòÉ 3.19. Image/Rotate 90 ΓòÉΓòÉΓòÉ

The picture is rotated 90 degrees clockwise.


ΓòÉΓòÉΓòÉ 3.20. Image/Negative ΓòÉΓòÉΓòÉ

White is changed to black and black to white. The greys are also reversed.


ΓòÉΓòÉΓòÉ 3.21. Image/Crop ΓòÉΓòÉΓòÉ

This facility cuts out a rectangle from the picture and makes it the new
picture.

After selecting this item, a vertical and a horizontal line follow the pointer.
Position them on two sides of the intended rectangle and click mouse button
one. Now move the pointer until the intended rectangle is boxed and again click
mouse button one.

Hit Esc to cancel the operation.

Note, when the pointer exits the window the picture will scroll.


ΓòÉΓòÉΓòÉ 3.22. Image/Undo all crops ΓòÉΓòÉΓòÉ

Restores the picture to before the cropping operations.


ΓòÉΓòÉΓòÉ 3.23. Options/Set initial parameters ΓòÉΓòÉΓòÉ

Presents a dialogue to sets basic configuration parameters. Refer to Set
initial parameters dialogue.


ΓòÉΓòÉΓòÉ 3.24. Options/Set parameters ΓòÉΓòÉΓòÉ

Presents a dialogue to set parameters such as lines per minute and dots per
line. Refer to Set parameters dialogue.


ΓòÉΓòÉΓòÉ 3.25. Window/Scale to window ΓòÉΓòÉΓòÉ

Rescales the picture to fit the size and shape of the window. Therefore the
whole fax can be seen. Using this facility, the window frame dimensions can be
adjusted to change the vertical and horizontal proportions of the picture.

This facility is not enabled while the fax is being received.

Select again to disable scaling.


ΓòÉΓòÉΓòÉ 3.26. Window/Show spectrum ΓòÉΓòÉΓòÉ

Creates a small window to display the audio frequency spectrum of the incoming
fax. Select again to delete the window. Refer to Tuning the computer for use of
the spectrum window.


ΓòÉΓòÉΓòÉ 3.27. Show/Pause ΓòÉΓòÉΓòÉ

Select to pause a slide show type presentation of faxes. Select again to
continue the presentation. Refer to Open Radio Faxes Dialogue.


ΓòÉΓòÉΓòÉ 3.28. Show/Forward ΓòÉΓòÉΓòÉ

Displays the next fax in a slide show type presentation. Refer to Open Radio
Faxes Dialogue.


ΓòÉΓòÉΓòÉ 3.29. Show/Backward ΓòÉΓòÉΓòÉ

Displays the previous fax in a slide show type presentation. Refer to Open
Radio Faxes Dialogue.


ΓòÉΓòÉΓòÉ 3.30. Show/Continuous ΓòÉΓòÉΓòÉ

Presents the current slide show continuously.


ΓòÉΓòÉΓòÉ 3.31. Show/Replay ΓòÉΓòÉΓòÉ

Re-starts the current slide show type presentation from the beginning or the
end depending on whether forward or backward were selected. Refer to Open Radio
Faxes Dialogue.


ΓòÉΓòÉΓòÉ 4. Set Initial Parameters Dialogue ΓòÉΓòÉΓòÉ

Sets basic configuration parameters.


ΓòÉΓòÉΓòÉ 4.1. Audio Input ΓòÉΓòÉΓòÉ

The sound card input has a Line in and a Microphone jack. Choose as you have
cabled.


ΓòÉΓòÉΓòÉ 4.2. Set Coordinated Universal Time ΓòÉΓòÉΓòÉ

Set the time difference between the time set in the computer's clock and the
Coordinated Universal Time. If you are located east of the International Date
Line and west of Greenwich, England then you are ahead of UTC. Otherwise you
are behind.


ΓòÉΓòÉΓòÉ 4.3. Save compressed bitmaps ΓòÉΓòÉΓòÉ

Check this to save compressed bitmaps. This will save considerable disk space.
However, be aware, I have found one video driver which was unable to read
compressed bitmaps.


ΓòÉΓòÉΓòÉ 5. Set Parameters Dialogue ΓòÉΓòÉΓòÉ

Sets parameters that describe the basic characteristics of the incoming fax.


ΓòÉΓòÉΓòÉ 5.1. Lines per minute ΓòÉΓòÉΓòÉ

Most weather faxes come at 120 lines per minute. This rate is exact and is used
to align the picture down the screen. Other radio faxes may have other rates.


ΓòÉΓòÉΓòÉ 5.2. Dots per line ΓòÉΓòÉΓòÉ

The audio out of the radio is a continuous signal. It is represented on the
screen and printer by a finite number of dots. It is up to you how many dots
you want to use. More dots provide more accuracy, but require more computer
overhead. Also if the Scale feature is off, a high number of dots yields a
picture too wide for its height. You can experiment with this parameter.


ΓòÉΓòÉΓòÉ 5.3. Index of Correlation ΓòÉΓòÉΓòÉ

You may set the index of correlation to create a picture (if the scale feature
is off) that has the correct horizontal and vertical dimension. Weather fax
usually use an index of correlation of 576.

Note, this can create a large picture using much memory and disk space.


ΓòÉΓòÉΓòÉ 5.4. Max lines in picture ΓòÉΓòÉΓòÉ

Sometimes, due to noise, the end of the fax is not detected. This parameter
stops the fax reception after the set number of lines.


ΓòÉΓòÉΓòÉ 5.5. Grey definition ΓòÉΓòÉΓòÉ

The number of shades of grey the signal is translated too.

Warning,choosing 256 greyscale on a display that is limited to 256 colours will
turn the rest of the desktop to greys while the picture is displayed (or
re-displayed in the future).

Warning, some VGA drivers have problems with 16 greyscale. Choose 64 greyscale
or try to obtain a recent video driver.


ΓòÉΓòÉΓòÉ 5.6. Bits per sample ΓòÉΓòÉΓòÉ

Sample sound at 8 bit or 16 bits per sample. Generally choose 8 bit for lower
overhead.

One sound card loses data on 8 bit (picture jumps horizontally) but works on 16
bit.


ΓòÉΓòÉΓòÉ 5.7. Samples per second ΓòÉΓòÉΓòÉ

The rate at which the audio sound from the radio is sampled into digital
amplitude values. The higher the rate, the greater the processor overhead. You
can experiment with this.


ΓòÉΓòÉΓòÉ 6. Open Radio Faxes Dialogue ΓòÉΓòÉΓòÉ

Displays faxes saved on disk. You can select the disk and directory. One fax
can be selected from the list (double click the entry). Or several faxes can be
selected from the list (hold down the Ctrl key while selecting) for a slide
show type presentation.


ΓòÉΓòÉΓòÉ 6.1. Select the disk ΓòÉΓòÉΓòÉ

Select from the drop down list.


ΓòÉΓòÉΓòÉ 6.2. Select the directory ΓòÉΓòÉΓòÉ

Select from the drop down list.


ΓòÉΓòÉΓòÉ 6.3. Select one or more ΓòÉΓòÉΓòÉ

Highlight one or more faxes from the list then click OK.


ΓòÉΓòÉΓòÉ 6.4. Delay in seconds ΓòÉΓòÉΓòÉ

The time, in seconds, between each fax shown in the slide show type
presentation.


ΓòÉΓòÉΓòÉ 7. General information ΓòÉΓòÉΓòÉ

The following sections give a simple description of facilities used in weather
faxes.


ΓòÉΓòÉΓòÉ 7.1. Weather faxes ΓòÉΓòÉΓòÉ

Weather faxes are weather maps, cloud maps and other charts that are
transmitted by meteorological departments around the world, for ships and
aircraft and others affected by air and sea conditions.


ΓòÉΓòÉΓòÉ 7.2. Weather fax station frequencies ΓòÉΓòÉΓòÉ

Listed are a few frequencies (MHz) on which fax transmissions can be found.
Other frequencies and transmission times may be found by contacting your local
Meteorological bureau or on CompuServe or Internet.

ΓöîΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÉ
ΓöéArgentina,  Γöé5.7775, 11.48          Γöé
ΓöéBuenosAires Γöé                       Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéAustralia,  Γöé5.1, 11.05, 13.92,     Γöé
ΓöéMelbourne   Γöé20.469                 Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéAustralia,  Γöé2.628                  Γöé
ΓöéCanberra    Γöé                       Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéAustralia,  Γöé5.755, 7.535, 10.555,  Γöé
ΓöéDarwin      Γöé15.615, 18.060         Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéEngland,    Γöé2.374, 3.652, 4.307,   Γöé
ΓöéLondon      Γöé6.446, 16.912          Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéChina,      Γöé7.42, 11.42, 18.94     Γöé
ΓöéShanghai    Γöé                       Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéJapan, TokyoΓöé3.365, 5.404, 7.305,   Γöé
Γöé            Γöé9.438, 9.970, 13.597,  Γöé
Γöé            Γöé18.22                  Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéNew Zealand,Γöé5.8066, 9.4586,        Γöé
ΓöéAuckland    Γöé13.5501, 16.3401       Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
ΓöéReunion,    Γöé8.176, 16.335          Γöé
ΓöéSaintDenis  Γöé                       Γöé
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ΓöéUSA, NorfolkΓöé3.357, 8.08, 10.865,   Γöé
Γöé            Γöé15.959, 20.015         Γöé
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ΓöéUSA,        Γöé4.346, 8.682, 12.73,   Γöé
ΓöéSanFranciscoΓöé22.527                 Γöé
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ΓòÉΓòÉΓòÉ 7.3. SSB/BFO ΓòÉΓòÉΓòÉ

Faxes comprise audio frequencies of 1500 Hz for black and 2300 Hz for white,
and in between if there are greys. The audio signal modulates a high frequency
(short wave) carrier producing an amplitude modulated (AM) signal. An AM signal
contains the carrier frequency and two side bands. For transmission, one side
band and the carrier are suppressed, resulting in a single side band (SSB)
signal.

Therefore, to regain the 1500 and 2300 Hz audio, the radio receiver must
re-insert the carrier and other side band. This is done using a Beat Frequency
Oscillator (BFO) or Single Side Band (SSB) mode.


ΓòÉΓòÉΓòÉ 7.4. Makeup of a fax ΓòÉΓòÉΓòÉ

The audio tones from the radio, if tuned precisely, will be 1500 Hz for black
and 2300 Hz for white, and in between for greys.

The fax begins with about 60 sync pulse lines. Each pulse is a line of black
terminated by a short period of white. The left hand edge of the picture is the
start of the black.

Most Weather faxes are sent at exactly 120 lines per minute. Each line takes
exactly half a second. The picture after the sync pulses is aligned based on
this. If you tape record the audio and play it back to this program, you will
find the slight inaccuracy in the speed of the tape will cause the picture to
skew.

The fax is terminated by a few lines of alternate black and white spaces.


ΓòÉΓòÉΓòÉ 7.5. How the program works ΓòÉΓòÉΓòÉ

The sound card samples the analogue audio input into 8 or 16-bit digital values
at the specified rate (22050/sec by default).

One thread of the program uses MMPM/2 playlist to fill buffers.

Another thread takes each buffer, breaks it up according to the number of dots
per line, determines audio frequency and whether each dot is black or white,
and updates a shadow bit map.

A third thread updates the window on the screen from the shadow bit map.

A fourth thread displays the frequency spectrum.


ΓòÉΓòÉΓòÉ 7.6. Coordinated Universal Time (UTC) ΓòÉΓòÉΓòÉ

Is the time in Greenwich, England. It is sometimes referred to as Greenwich
Mean Time (GMT). It is used as a worldwide standard time for worldwide
facilities such as short wave radio. Most times displayed on the faxes will be
UTC.


ΓòÉΓòÉΓòÉ 7.7. High Frequency (Short Wave) ΓòÉΓòÉΓòÉ

Is the range of radio frequencies that most easily travel long distances by
bouncing off the ionisphere and the ground. However they are subject to noise
interference, echoes (signals travelling different paths and arriving at
different times), and diurnal variations.

Meteorological stations transmit on a number of HF frequencies. Choose the best
one for the time of day. These stations also transmit maps for determining the
best frequency to use for a location and time.