SSTV Slow Scan TV

The concept of SSTV was introduced by Copthorne Macdonald [1] in 1957–1958.[2] He developed the first SSTV system using an electrostatic monitor and a vidicon tube. In those days it seemed sufficient to use 120 lines and about 120 pixels per line to transmit a black-and-white still picture within a 3 kHz phone channel. First live tests were performed on the 11 Meter ham band – which was later given to the CB service in the US. In the 1970s, two forms of paper printout receivers were invented by hams.

Early usage in space exploration

Astronaut Gordon Cooper, SSTV broadcast from Faith 7
SSTV was used to transmit images of the far side of the Moon from Luna 3.[3]

The first space television system was called Seliger-Tral-D and was used aboard Vostok. Vostok was based on an earlier videophone project which 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.

NASA slow scan image from the Moon.
The Apollo TV cameras used SSTV to transmit images from inside Apollo 7, Apollo 8, and Apollo 9, as well as the Apollo 11 Lunar Module television from the Moon. NASA had taken all the original tapes and erased them for use on subsequent missions; however, the Apollo 11 Tape Search and Restoration Team formed in 2003 tracked down the highest quality footage among the converted recordings of the first broadcast, pieced together the best footage, then contracted a specialist film restoration company to enhance the degraded black-and-white film and convert it into digital format for archival records.[4]
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.

Progression

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.

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).[5] 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.

Header

A calibration header is sent before the image. It consists of a 300-millisecond leader tone at 1900 Hz, a 10 ms break at 1200 Hz, another 300-millisecond leader tone at 1900 Hz, followed by a digital VIS (vertical interval signaling) code, identifying the transmission mode used. The VIS 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
Slow scan Test card
A transmission consists of horizontal lines, scanned from left to right. The color components are sent separately one line after another. The color encoding and order of transmission can vary between modes. Most modes use an RGB color model; some modes are black-and-white, with only one channel being sent; other 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 even though the horizontal resolution is often defined as 256 or 320 pixels, they can be sampled using any rate. The image aspect ratio is conventionally 4:3. Lines usually end in a 1200 Hz horizontal synchronization pulse of 5 milliseconds (after all color components of the line have been sent); in some modes, the synchronization pulse lies in the middle of the line.

Modes

Below is a table of some of the most common SSTV modes and their differences.[5] 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.

SSTV Slow Scan TV

Paul Leicester | MØFOX | Chesterfield UK | IO93HE | Yaesu FTDX1200