HF-Fax Weather Chart

HF radiofacsimile (fax) is an old, analog mode, using a frequency-modulated tone sent to a USB transmitter. It deviates plus or minus 400 Hz from the channel offset center at 1900 Hz, higher being whiter. Weather chart faxes, like this one, have a white background, which creates the yellow line we see at 2300 Hz, and hear as a rather piercing shreik, punctuated twice a second by scratchings from insertion of the black pixels for that line, followed by its black/white sync pulse.

This plot shows the fax machine hitting a latitude mark, sending three lines of pure black, dramatically changing the sound. Three 1500-Hz beeps are sent, each with its white pip for sync, before the screech resumes on the white paper.

To hear HF fax make some different noises, listen while the machine is sending a satellite picture or, better still, a test pattern.