HF Fax is similar to the RTTY mode with a shift of 800 Hz, the white at 2300 Hz and the black at 1500 Hz, except it concerns RX/TX of pictures either in black and white or in gray levels.
In HF Fax, such stations as meteorological transmissions of official stations exist, for example:
* Bracknell Meteo on 2618.5, 4610 or 8040 Khz,
* RN London on 3652, 4307 or 6446 Khz,
* Hambourg Meteo on 3855, 7880 or 13882.5 Khz.
There are two types of fax: "black and white" or with 8 grey scales, but only the first seems to be used.
In " black and white" fax, two AF frequencies are transmitted: one at low frequency (1500 Hz), the other at high frequency (2300 Hz), the first tone corresponding to black and the second to white. These two frequencies, separated by a shift (800 Hz in SW), can effectively modulate the RF when transmitted (AFSK) or be artificially produced by a simple shift of RF frequency (FSK). For the listener, they both sound the same.
The listening is on USB, the user must adjust the receiver frequency 1900 Hz ((1500+2300)/2) below the transmitter frequency (for example, BRACKNELL METEO on 2616,6 KHz).
In grey scales, frequencies are located between 1500 and 2300 Hz.
The scan speed or number of lines produced per minute (lpm) may be of 60, 90 ou 120. In Western Europ, the speed is 120 lpm. In Eastern Europ, the speeds are 60 and 90 lpm.
The indice of co-operation (IOC) is an exploration characterictic of the transmitted picture. This indice corresponds to the ratio between the cylinder diameter to the scan step (between two successive lines). In practice, the IOC defines the maximum transmission duration. For example, for 120 lpm, the duration will be of 9.4 minutes for an IOC of 288 and 18.8 minutes for an IOC of 576. For 60 lpm, durations would be doubled.
The official stations use IOC of 288 and 576 and rather this second value, but as the maximum duration is rarely used, an IOC of 288 may be utilized.
Hams use an IOC of 288 and a speed of 120 lpm. Pictures are transmitted either in "black and white" or in grey scales.
The fax transmissions are preceded and followed by "APT" signals (alternation of black and white). The first "APT" signal could be used to determine the IOC
Mode Audio Sample
FFT Window Screenshot