HF Radiofacsimile

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.

Here is one entire fax chart sent by KVM70 in Hawaii. Vertical divisions are every 30 seconds.

Parallel lines at beginning and end are APT (Automatic Picture Transmission) tones. These show on the decode as alternating black and white vertical bars. We then see the 30-second preamble, which is a specified number of lines with white pips on black followed by the standard black pips on white. This is a weather chart fax, so the typical FFT plot will show it as various types of noise around white and black. Individual black pips are too narrow to show here. 10 to 12 minutes is a typical transmission time for a normal sized weather chart at 120 lines per inch.

Here is a composite showing a satellite image with grey tones (left) and a weather chart (right). Both were sent from Hawaii in early 2011. Vertical divisions here are 30 seconds. Note how the satellite photograph shows most of its information in the center areas, shifting as cloud positions change. Satpics sound dramatically different on the air.

All plots made with GRAM.EXE and Spectrum Lab.