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Slow-scan television

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[[Ima 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.

  • The Seliger system was tested during the 1960 launches of the Vostok capsule, including Sputnik 5, containing the space dogs Belka and Strelka, whose images are often mistaken for the dog Laika and the 1961 flight of Yuri Gagarin, the first man in space on Vostok 1.
  • Vostok 2 and thereafter used an improved 400-line television system referred to as Topaz.
  • A second generation system (Krechet, incorporating docking views, overlay of docking data, etc) was introduced after 1975.

A similar concept, also named SSTV, was used on Faith 7 as well as on the early years of the NASA Apollo program. The Faith 7 camera transmitted one frame every two seconds.

SSTV was used to transmit images from inside Apollo 7, Apollo 8, and Apollo 9, as well as the Apollo 11 Lunar Module television from the Moon, see Apollo TV camera.

  • The SSTV system used in NASA's early Apollo missions transferred ten frames per second with a resolution of 320 frame lines using less bandwidth than a normal TV transmission.
  • The early SSTV systems used by NASA differ significantly from the SSTV systems currently in use by amateur radio enthusiasts today.

Evolution

Commercial systems started appearing in the United States in 1970, after the FCC had legalized the use of SSTV for advanced level amateur radio operators in 1968.

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.

The modem would generate audio tones between 1200 and 2300 Hz from picture signals, and picture signals from received audio tones. The audio would be attached to a radio receiver and transmitter.

Current systems

A modern system, having gained ground since the early 1990s, uses a personal computer and special software in place of much of the custom equipment. The sound card of a PC, with special processing software, acts as a modem. The computer screen provides the output. A small digital camera or digital photos provide the input.

An SSTV signal begins with a digital mode identifier and then starts scanning between 1500 and 2300 Hz, with regular 1200 Hz clicks.

Modulation

SSTV uses analogue frequency modulation, in which every different value of brightness in the image gets a different audio frequency. In other words, the signal frequency shifts up or down to designate brighter or darker pixels, respectively. Color is achieved by sending the brightness of each color component (usually red, green and blue) separately. This signal can be fed into an SSB transmitter, which in part modulates the carrier wave.

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)[1]. 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.

VIS code

A digital VIS (vertical interval signaling) code can be sent before the image, identifying the transmission mode used. It consists of bits of 30 milliseconds in length. The code starts with a start bit at 1200 Hz, followed by 7 data bits (LSB first; 1100 Hz for 1, 1300 Hz for 0). An even parity bit follows, then a stop bit at 1200 Hz. For example, the bits corresponding the decimal numbers 44 or 32 imply that the mode is Martin M1, whereas the number 60 represents Scottie S1.

Scanlines

A transmission consists of horizontal lines, scanned from left to right. The RGB color components are sent separately one line after another in the order R, G, B. Some Robot modes use a YC color model, which consists of luminance (Y) and chrominance (R-Y and B-Y). The modulating frequency changes between 1500 and 2300 Hz, corresponding to the intensity (brightness) of the color component. The modulation is analogue, so there is not a defined number of pixels in each line; they can be sampled using any rate (though in practice, the image aspect ratio is conventionally 4:3). Lines end in a 1200 Hz horizontal synchronization pulse of 5 milliseconds (after all color components of the line have been sent).

Modes

Below is a table of some of the most common SSTV modes and their differences[1]. These modes share many properties, such as synchronization and/or frequencies and grey/color level correspondence. Their main difference is the image quality, which is proportional to the time taken to transfer the image and in the case of the AVT modes, related to synchronous data transmission methods and noise resistance conferred by the use of interlace.

Family Developer Name Color Time Lines
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
24 RGB 24 s 128×128
48w RGB 48 s 256×128
48h RGB 48 s 128×256
104 RGB 96 s 256×256
Martin Martin Emmerson M1 RGB 114 s 240¹
M2 RGB 58 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
S2 RGB 71 s 240¹
¹ Martin and Scottie modes actually send 256 scanlines, but the first 16 are usually grayscale.

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 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

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.

Frequencies

Using a receiver capable of demodulating single-sideband modulation, SSTV transmissions can be heard on the following frequencies:

Band Frequency Sideband
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

Media

A sample SSTV transmission
An image of a sunset sent as Martin M1.
Problems playing this file? See media help.

Interference

SSTV is very susceptible to interference, especially when transmitted over USB or LSB.

See also

References

Notes

  1. ^ a b Langner, John. ""SSTV Transmission Modes."". Retrieved May 8. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)

Modem software:

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