"The FAX and SSTV Corner"

SSTV - Fundamentals

The start

SSTV has been developed by radio amateurs inn order to transfer still pictures within the bandwidth of a voice channel. Slow Scan TV depends on frame and line sync pulses like all television methods. Because of the narrow bandwidth (max. 3 KHz) it needs very slow scanning for only still pictures without additional information like sound or so. In the beginning SSTV was only black and white at a resolution of 120 pixels on 120 lines, that needed seven to eight seconfs per frame. By doubling the lines or the pixels per line or both together the transfer time rose to 16 and 32 seconds.

Color

Trying to transfer colored pictures first there were experiments using frame sequential procedures with seperated red, green and blue extracts. These were transmitted in black and white one by one and compiled again into the color picture at the receiving end. This way the complete picture color was visible only after transfer of the third extract. The german slow scan pioneer Volker Wraase, DL2RZ, developed a line sequential procedure that represents the presently used standard in an adapted form. Here the color picture is sliced to three extracts that are transmitted line by line in red, green and blue portions. Assortment of these extracts to their respective frame store at the receiver is realised by a modified sync pulse at the start of the red line.
Essential developments were made by the british radio amateurs Eddie ("Scottie") Murphy, GM3SBC, and Martin Emmerson, G3OQD, who wanted to make color SSTV less prone to interference. The amount of sync signals was reduced, so the pictures are transfered nearly running free. These new procedures were accepted as so-called "new modes" or "free run modes" relatively fast amoung the slow scanners. Today the mostly used SSTV standard derives from the work of G3OQD. Out of his four modes the "Martin 1" is prefered around the world. The color sequence is green - blue - red (GBR) opposite to the common red - green - blue (RGB). The transfer of a "Martin 1" picture takes 114 seconds and is a well balanced compromise between durance and resolution.

How to get qrv

To realise slow scan operation you can take special SSTV devices with all functions and interfaces for storing and transfering pictures, or you take your home computer. With this and various available special software and different converter types the expenses are relatively small.

Which frequencies are in use for SSTV?

The calling frequencies for SSTV on hf are 3730, 7040, 14230, 21340 and 28680 KHz with daily activities. At the same frequencies there are meeting FAX friends too, collisions and misunderstandings can be prevented if you announce your mode before transmitting.

More info

If you want to engage in this attractive mode you can find out alot in the book "Das FAX/SSTV-Praxisbuch fuer Funkamateure" from DARC-Verlag. HF section of DARC, in charge of SSTV: K. D. Koch, DF6DR
(PS: english language book: "Slow Scan Television Explained" by Mike Wooding, G6IQM (BATC), Great Britain; actual infos on SSTV in all PR mailboxes)
73 Klaus, DL4KCK@t-online.de

Facsimile/FAX - Basics

Look back

Facsimile telegraphy is one of the oldest telegraphy modes. Already before the first use of Morse code in Europe (1848) Alexander Bain invented a copy telegraph (1843). Practical use of that followed much later.
After 1920 the picture telegraphy was developed further in USA and Europe by well known companies like RCA, Bell, Muirhead, Siemens and Hell until todays photo telegraphy in black and white or even in three color extracts transfered one after another.

 

Functioning

With classic style facsimile transmission a paper sheet is scanned by a light beam generating an electrical signal in amplitude modulation. The sheet is scanned on a rotating drum line by line similar to television. The picture signal is modulated onto a sound subcarrier and demodulated at the receiving end. There the gained signal controls the writing head in a FAX machine. Fax machines are classified in different groups. The analog types of CCITT group 1 and 2 are usable in amateur radio FAX traffic (after some modifications).
The quality of picture transmissions depends upon the number of pixels per line, number of lines per millimeter and the number of lines per minute (transfer time). There are international standards like 60, 90, 120 and 240 lines per minute (lpm) and frequency modulation on a 1900 Hz subcarrier with plus/minus 400 Hz maximum deviation.
Wheather satellite transmissions on VHF/SHF have amplitude modulation on a 2400 Hz subcarrier that is sent in FM with plus/minus 20 KHz deviation peaks. Synchronisation of FAX transmitter and receiver is done only at start of scanning, after that they run free or crystal controlled in sync. New arrangements have been met after development of modern computer based FAX techniques.
A color FAX method called "JV-color" with 360 lines per minute by DK8JV is similar to color SSTV, but gives higher resolution. In a package with SlowScan and wheather satellite decoding extensions it promoted as "JVFax" PC program the international spreading of picture modes in amateur radio.

 

How to get on air

To transfer FAX in amateur radio you do not need to modify the usual transceivers. On hf the mode J3C is used (FAX in SSB). The picture polarity is set by choice of the sideband.
Following CCITT recommendations picture white should always be at a higher frequency than the (middle) subcarrier frequency. The normal audio input and output (micro and loudspeaker) of transceivers can be used to connect analog fax machines or personal computers.
With using computers and the various software supply together with different converter types the expenses can stay low.

 

Which frequencies are used

Calling frequencies for FAX on hf are 3730, 7040, 14230, 18110, 21340, 24930 and 28680 KHz. On 160 and on 30 m no FAX traffic is allowed. SSTV traffic on the same frequencies dominates the picture modes on the hf amateur bands.

 

More info

If you want to engage in this attractive mode you can find out alot in the book
"Das FAX/SSTV-Praxisbuch fuer Funkamateure" from DARC- Verlag.
HF section of DARC, in charge of FAX: K. D. Koch, DF6DR

73 Klaus, DL4KCK


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