1 May 2015


HF-FAX are usually transmitted in single sideband according to the ATP (Automatic Picture Transmission) format, 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 (1.9 kHz below the carrier frequency for USB, above for LSB), the signal shares some characteristics with SSTV, with black at 1500 Hz and peak white at 2300 Hz.

The start tone triggers the receiving system. It was originally meant to allow enough time for the drum of mechanical systems to get up to speed. It consists of rapid modulation of the video carrier, resulting in a characteristic rasp-like sound.
The start tone provides the Index Of Cooperation (IOC) which must 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 (known sometimes as the factor of cooperation), divided by 'piGreco'. Usually the IOC is 576.

The phasing signal, consisting of a periodic pulse, synchronizes the receiver so that the image will be centered on the paper.

 Usually, 120 lines per minute (LPM) are sent (For monochrome fax, possible values are: 60, 90, 100, 120, 180, 240. For colour fax, LPM can be: 120, 240[1]). 

The stop tone at 450 Hz, optionally followed by black, marks the end of the transmission.

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