A technical term for SSTV is narrowband television. Broadcast television requires huge 5, 6 or 8 MHz wide channels, because it transmits 25 or 30 pictures per second (in the NTSC, PAL or SECAM systems), but SSTV usually takes up to only 3 kHz of bandwidth. It is a much slower method of still picture transmission, usually lasting from about eight seconds to a couple of minutes.
SSTV was used to transmit images of the far side of the Moon from Luna 3 .
The first space television system was called Seliger-Tral-D and was used aboard Vostok. Vostok was based on an earlier videophone project, it used two cameras, with persistent LI-23 iconoscope tubes. Its output was 10 frames per second at 100 lines per frame video signal.
SSTV originally required quite a bit of specialized equipment. Usually there was a scanner or camera, a modem to create and receive the characteristic audio howl, and a cathode ray tube from a surplus radar set. The special cathode ray tube would have "long persistence" phosphors that would keep a picture visible for about ten seconds.
There are a number of different modes of transmission, but the most common ones are Martin M1 (popular in Europe) and Scottie S1 (used mostly in the USA). Using one of these, an image transfer takes 114 (M1) or 110 (S1) seconds. Some black and white modes take only 8 seconds to transfer an image.
|AVT||Ben Blish / AEA||8||BW or 1 of R, G, or B||8 s||128×128|
|16w||BW or 1 of R, G, or B||16 s||256×128|
|16h||BW or 1 of R, G, or B||16 s||128×256|
|32||BW or 1 of R, G, or B||32 s||256×256|
|Martin||Martin Emmerson||M1||RGB||114 s||240¹|
|Robot||Robot SSTV||8||BW or 1 of R, G or B||8 s||120|
|12||YC||12 s||128 luma, 32/32 chroma × 120|
|24||YC||24 s||128 luma, 64/64 chroma × 120|
|32||BW or 1 of R, G or B||32 s||256 × 240|
|36||YC||36 s||256 luma, 64/64 chroma × 240|
|72||YC||72 s||256 luma, 128/128 chroma × 240|
|Scottie||Eddie Murphy||S1||RGB||110 s||240¹|
The mode family called AVT (for Amiga Video Transceiver) was originally designed by Ben Blish (N4EJI, then AA7AS) for a custom modem attached to an Amiga computer, which was eventually marketed by AEA corporation.
The Scotty and Martin modes were originally implemented as ROM enhancements for the Robot corporation SSTV unit. The exact line timings for the Martin M1 mode are given in this reference.
The Robot SSTV modes were designed by Robot corporation for their own SSTV unit.
All four sets of SSTV modes are now available in various PC-resident SSTV systems and no longer depend upon the original hardware.
AVT is an abbreviation of "Amiga Video Transceiver", software and hardware modem originally developed by "Black Belt Systems" (USA) around 1990 for the Amiga home computer popular all over the world before the IBM PC family gained sufficient audio quality with the help of special sound cards. These AVT modes differ radically from the other modes mentioned above, in that they have no per-line horizontal synchronization pulse but instead use the standard VIS vertical signal to identify the mode, followed by a frame-leading digital pulse train which pre-aligns the frame timing by counting first one way and then the other, allowing the pulse train to be locked in time at any single point out of 32 where it can be resolved or demodulated successfully, after which they send the actual image data, in a fully synchronous and typically interlaced mode.
Interlace, no dependence upon sync, and interline reconstruction gives the AVT modes a better noise resistance than any of the other SSTV modes. Full frame images can be reconstructed with reduced resolution even if as much as 1/2 of the received signal was lost in a solid block of interference or fade because of the interlace feature. For instance, first the odd lines are sent, then the even lines. If a block of odd lines are lost, the even lines remain, and a reasonable reconstruction of the odd lines can be created by a simple vertical interpolation, resulting in a full frame of lines where the even lines are unaffected, the good odd lines are present, and the bad odd lines have been replaced with an interpolation. This is a significant visual improvement over losing a non-recoverable contiguous block of lines in a non-interlaced transmission mode. Interlace is an optional mode variation, however without it, much of the noise resistance is sacrificed. Older computers sometimes needed to do this in order to make up for an inability to precisely match the synchronous timing of the frame over long periods.
The AVT modes are mainly used in Japan and the USA. There is a full set of them in terms of black and white, color, and scan line counts of 128 and 256. Color bars and greyscale bars may be optionally overlaid top and/or bottom, but the full frame is available for image data unless the operator chooses otherwise.
|80 meters||3845 kHz (3730 in Europe)||LSB|
|40 meters||7170 kHz (7043 in Europe)||LSB|
|20 meters||14230 kHz||USB|
|15 meters||21340 kHz||USB|
|10 meters||28680 kHz||USB|