The SSTV is an acronym, stands for Slow Scan TeleVision. This mode is a thinband TV system, which was developed by a group of radio amateurs led by Copthorne McDonald - VE1BFL - in 1958. The bandwidth of the system at the picture resolution was choosen in mind of the SSB enabled radio systems of current radio amateurs. Since the transmission of the motion wasn't important, by reducing the scan speed of the normal TV pictures the bandwidth lowering was reached. The 3MHz bandwidth TV sign turned into 3 kHz, so it consumes less space as the well known FSTV system.
The line and picture synchron sign has to be changed, this resulted in the following. On 50Hz based areas the line sync is 16.66 Hz and the picture sync is 0.139 Hz, on 60Hz areas the line is 15Hz and the picture is 0.125 Hz. So the picture time is 7.2 secunds and 8 secunds. The bandwidth of the sign is max. 2.5 kHz. The pictures size is 1:1, the scanning process of the image is done from left to right and from above to the bottom. In one row there is 128 pixels, and one picture contains 120 lines. The synchron sign is 1200 Hz, the black point is 1500 Hz, the white point is 2300 Hz. These are the properties of the 8 sec black and white SSTV picture.
The sign received by the SSB receiver is a sound frequency converted from videosign and it is sent to a monitor, which has a special part called long backlighting cathod ray tube, which helps in storing the picture. This was able to still show the first lines after the picture was drawn on the screen. The amount of time the picture was stored depended on the backlighting effect of the tube. This is how they solved the storing of pixels for the picture time. The monitor was built using electron tubes, later transistors and integrated circuits.
The scanning of the image was done by slow scan camera, sampling camera and flying spot scanner. Mechanical picture scanner was used too. By mixing the line and picture frequency to the video sign the SSTV sign resulted. In this way the picture sign connected to the SSB transmitter's microphon input could be transmitted.
For the better and finer image resolution they increased the picture time and increased pixel number transmitted in one line. Soon we were able to work in 16, 32, 36 sec black and white pictures. The equipements were made in analog systems by radio amateurs and companies. The better picture quality, storage, better scanning was enabled by the digital technology. The next step was the development of the so called "Digital Converters". The analog SSTV signs were converted to digital with the required circuits, and then it was stored. The required transformations were applied which was needed in further processing the image, and it was read from the memory, converted to analog signs, and displayed on an FSTV monitor. The camera produced analog sign was digitalized in the same way, stored in memory, and sent to the microphone input of the SSB transmitter.
The color picture transmission was enabled by the digital converters. One color image occupied three memory. One for the red R, green G, blue B. The picture is read by lines and pictures, we call it line or picture selecting system. For receive a small sized TV is enough, with some minor modification. For sending a color or black and white camera was used, with latter color filter had to be applied. The picture time of the color ones were three times as much as the black and white ones: 24, 48, 72, 96 sec.
For the correction of noise caused picture modifications, information loss, color errors, and for higher resolution new SSTV modes started to develop. The image size ratio was 4:3. The pixels per lines and the number of lines per picture was increased to 256. The system became free run synchornized, this allows the automatic correction of synchron loss. Some new modes that are used ever since: Martin, Scottie, ATV.
In Hungary the first SSTV article was released in 1972 by HA7RH. He described the features of this mode, that was unknown for lots of radio amateurs. A lot of good articles were released in the Radiotechnika magazine, ranging from the simple analog to the advanced digital converters.
Many thanks to HA7LF and HA6VK amateur friends, who helped a lot in the progress of educating radio amatuers and there are so many of them on the radio bands.
The radio amateur bands are very narrow, but within this an even thinner band was marked for SSTV mode. The general frequencies are: 3730 kHz, 7040 kHz, 14230 kHz, 14233 kHz, 21340 kHz and 28680 kHz and on 2 meters 144500 kHz +/- 5kHz.
Contests are held in this mode too, for example: in March the DL GARTG, the USA IVCA world wide contest, in May OZ, in August the JA JASTA contest. The exact date for these events can be found on the internet at the releated country's SSTV news.
The SSTV transmission cannot only be done with above mentioned equipement, there are quite a few computer based application exist that allow the use of this mode. There is no need to build complicated equipements to exchange pictures with other stations. From the available programs the user can choose the one that he likes the best. Of course some is hardware dependent, but the currently available computers don't cause a problem. By mentioning the URL of some programs I hope I can help my radio amatuer friends who decided to try this mode of communication. It is a great pleasure for those who haven't tried this mode yet.
SSTV programs can be downloaded from the following URLs:
Help files and resources are available at the download pages, or within the application.
I guess this is what fits into a nutshell, but if I can help in anything please contact me by email and I happily answer all questions.
Thanks for reading! ------------ ------------ HA1ZH Zoli