Slow-scan television

Lore & Background

The concept of SSTV was introduced by Copthorne Macdonald in 1957–58, who developed the first system using an electrostatic monitor and a vidicon tube, transmitting 120 lines at about 120 pixels per line within a 3 kHz telephone channel. First live tests were performed on the 11-meter ham band, later given to the CB service in the US. In the 1970s, two forms of paper printout receivers were invented by hams.

SSTV was used in early space exploration, including transmitting images of the far side of the Moon from Luna 3. The first space television system, Seliger-Tral-D, was used aboard Vostok, outputting 10 frames per second at 100 lines per frame. The Seliger system was tested during 1960 launches including Sputnik 5 (with dogs Belka and Strelka) and the 1961 flight of Yuri Gagarin on Vostok 1. Later Vostok missions used an improved 400-line system called Topaz, and a second-generation system (Krechet) was introduced after 1975. A similar SSTV concept was used on Faith 7 and early NASA Apollo missions, with Apollo cameras transmitting 10 frames per second at 320 lines.

Commercial systems appeared in the US in 1970 after the FCC legalized SSTV for advanced amateur operators in 1968. Early systems required specialized equipment including a scanner or camera, a modem generating audio tones between 1,200 and 2,300 Hz, and a cathode-ray tube with long-persistence phosphors. Modern systems since the early 1990s use a personal computer with special software and a sound card acting as a modem.

Reader's Guide

SSTV is significant as a method that allowed amateur radio operators to transmit still images over narrow bandwidths, using voice-frequency channels on HF, VHF, and UHF bands. Its legacy includes early space applications, such as transmitting the first images of the far side of the Moon from Luna 3 and supporting the Apollo program, where SSTV cameras were used on Apollo 7, 8, 9, and the Apollo 11 Lunar Module. The Apollo 11 tapes were later erased by NASA, but a restoration team formed in 2003 recovered and enhanced the best films. The modulation is analog frequency modulation, with brightness values represented by different audio frequencies, and color achieved by sending red, green, and blue components separately. A calibration header with a VIS code identifies the transmission mode. Common modes include Martin M1 (popular in Europe) and Scottie S1 (used mostly in the USA), with some black-and-white modes taking only 8 seconds. The AVT modes, designed for the Amiga computer, offer better noise resistance through synchronous transmission and interlace. SSTV systems have evolved from custom hardware to PC-based software, making the mode accessible to a wide community of amateur radio operators.

Did You Know?

• The Apollo 11 SSTV tapes were erased by NASA, but a restoration team formed in 2003 recovered the best films.
• The AVT modes, designed for the Amiga computer, can reconstruct full-frame images even if half the signal is lost.