HF Weather Fax
To decode HF FAX transmissions, you need an HF radio (shortwave radio) taht tunes SSB (single side band). You should use an audio cable to
connect the radio's audio output (line level or headphone jack) to your computer's microphone input, for best reception.
Start the app, and make sure that:
- GRAY is toggled on for grayline.
- 576 is toggled on.
- 120 is toggled on.
- HF is toggled on.
Tune your radio in USB mode to the weather fax frequency, making note of the fact that you usually need to
tune about 1.9 kHz lower in frequency than the listed carrier frequency in a schedule.
Fine tune your radio for proper reception of black and white picture elements.
When first using the app, it is normal for the received image to be slightly slanted,
due to slight timing variations between devices. This can be corrected by the timing adjustment
value, which is located at the top of the screen between the >- and >+ buttons.
The default value is zero. If the received image is slanting to the right, the timing adjustment
must be increased. Press the + button a few times.
If the received image is slanting to the left, the timing adjustment
must be decreased. Press the - button a few times.
Now wait several seconds, and see if new picture information for the fax is no longer slanted.
(previously received picture information will not be changed) Continue to make adjustments as
The timing adjustment value will be automatically saved and used each time you run the app, so
typically you only need to go through this procedure once.
When the Auto Save button is toggled on,
the stop tone of a fax will be used to save the received image. You can also tap on the
Save button to manually save an image. Images are saved to your Pictures directory.
Note that proper reception of fax images and detection of the start and stop tones requires
good quality reception. Weak signals, noise and other interference can make it difficult or
impossible to properly decode and display a fax image.
The start tones sent at the very beginning of a fax image transmission
are also used to align the left and right sides of the fax image. If they are not
properly received, the image will not be centered. Typically you will see a black bar running down the
middle of the screen somewhere when this happens. This will also happen if you tune in a fax transmission
after it has started, since the start tones will not have been decoded.
The solution is to get a better quality received signal on
your radio, as well as make sure the audio from the radio is getting into your computer without
distortion. Use of a direct cable connection is highly recommended.