Analogue image transmission system (note: this is not the same as FAX with telephone modems which is a more sophisticated development of this technology). An example weather FAX (WEFAX) is shown above.
This is a simple analogue frequency modulation system which can convey an image using the amount of deviation to indicate a greyscale: the degree of darkness affects the amount of deviation from the rest frequency. In modern systems, the image is generated electronically and the HF FAX signal synthesised in a modem, but in the past a large rotating drum was used to mount the image which was raster scanned optically to produce the modulating waveform. A FAX transmission comprises a synchronisation phase (indicating line speed), a transmission phase (in which the image is scanned and transmitted) and a termination phase (in which the receiver is returned to the ready state).
This scheme is where most of the parameters used with HF FAX are derived:
Line speed. In lines per minute (LPM), this is the rate at which the image is raster scanned. The drum is rotated forwards (notionally) a line at a time. Typical drum speeds are 120 lpm.
Aspect ratio. Known as index of cooperation (IOC). This is the product of the drum diameter, d, and line density, l. For example, IOC = l x m = 152mm x 3.79 per mm = 576. The IOC must be the same at the transmitter and receiver for correct operation. Hence the name.
Systems heard on air include:
HF FAX Weather-FAX, Radio Amateurs and less often Press-FAX
HF FAX signals are normally transmitted using SSB modulation.
Amateur Radio operators use FAX: 120/288, 120/264 and HamColor 360/204 and can be heard in the following frequency bands: 3730-3740 kHz, 7035-7045 kHz, 14225-14235 kHz, 18105-18115 kHz, 21335-21345 kHz, 24925-24935 kHz, 28675-28685 kHz.
A FAX-like mode developed during World War II by the German engineer Rudolf Hell (Hellschreiber = Hell's writer). The system sends text as an image using a variety of modulation techniques (Feld-Hellschreiber is the original on-off keyed system; Concurrent Multi-tone Hellschreiber sends using individual tone frequencies in parallel to represent columns of pixels building up character images; Sequential multi-tone uses single tone frequencies at either one of five or one of seven frequencies; FSK Hellschreiber uses two-tone frequency shift keying instead of OOK used in Feld Hellschreiber).
Example Hellschreiber signal received using
Nino Porcino's Windows software implementation of this ancient
mode. A rather
weak signal received on the 14MHz band using an AOR AR-7030
receiver.
Hellschreiber can be heard (very occasionally) being used by amateur radio operators around 14060 kHz. In Europe, there are skeds Tuesdays on 7037 kHZ LSB at 2000 UTC and 3580 kHz 2100 UTC.
