Radiofax or Weather Fax is
transmitted in
single sideband and uses
frequency modulation. The signal shifts up or down a given amount to
designate white or black pixels. A
deviation less than that for a white or black pixel is taken to be a
shade of grey. With correct tuning (1.9kHz below the carrier frequency
for USB, above for LSB), the signal shares some characteristics with
SSTV, with black at 1500Hz and peak white at 2300Hz.
Usually, 120 lines per minute (LPM) are
sent (though some stations use 60 LPM or other values). A value known as
the index of cooperation (IOC) must also be known to decode a
radio fax transmission - this governs the image resolution, and derives
from early radio fax machines which used drum readers, and is the
product of the total line length and the number of lines per unit
length. Weather Fax now is available on
LCD
display products, and no longer requires "stinky" aluminum anodized
paper to be produced and displayed.
