science

Slow-scan television

Slow-scan television is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Slow-scan television rather than just read about it. In short: Slow-scan television (SSTV) is a picture transmission method, used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color. A literal term for SSTV is narrowband television.

Key takeaways

Slow-scan television belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Slow-scan television to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Slow-scan television from memory before moving on to harder problems.

Reference excerpt

Slow-scan television (SSTV) is a picture transmission method, used mainly by amateur radio operators, to transmit and receive static pictures via radio in monochrome or color. A literal term for SSTV is narrowband television. Analog broadcast television requires at least 6 MHz wide channels, because it transmits 25 or 30 picture frames per second (see ITU analog broadcast standards), but SSTV usually only takes up to a maximum of 3 kHz of bandwidth. It is a much slower method of still picture transmission, usually taking from about eight seconds to a couple of minutes, depending on the mode used, to transmit one image frame. Since SSTV systems operate on voice frequencies, amateurs use it on shortwave (also known as HF by amateur radio operators), VHF and UHF radio.

History

Concept The concept of SSTV was introduced by Copthorne Macdonald in 1957–58. He developed the first SSTV system using an electrostatic monitor and a vidicon tube. It was deemed sufficient to use 120 lines and about 120 pixels per line to transmit a black-and-white still picture within a 3 kHz telephone 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

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 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, with a resolution of 320 lines.

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 films among the converted recordings of the first broadcast, pieced together the best parts, then contracted a specialist film restoration company to enhance the degraded black-and-white film and convert it into digital format for archival records.

The SSTV system used in NASA's early Apollo missions transferred 10 frames per second with a resolution of 320 frame lines in order to use 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 1,200 and 2,300 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 Like the similar radiofax mode, SSTV is an analog signal. SSTV uses 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 signal. 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.

Header A calibration header is sent before the image. It consists of a 300-millisecond leader tone at 1,900 Hz, a 10 ms break at 1,200 Hz, another 300-millisecond leader tone at 1,900 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 1,200 Hz, followed by 7 data bits (LSB first; 1,100 Hz for 1, 1,300 Hz for 0). An even parity bit follows, then a stop bit at 1,200 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

… excerpt ends here. Continue reading the full article.

Illustrations

Slow-scan television: Slow-scan test card — Slow-scan test card

Worked examples

Example 1 — a first encounter with Slow-scan television

Start with the simplest possible case. Write down what Slow-scan television claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Slow-scan television before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Slow-scan television ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Slow-scan television

In research: Slow-scan television appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Slow-scan television in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Slow-scan television is common in secondary-school and first-year university syllabi. It links to neighbouring topics Amateur radio, Radio modulation modes, Television technology, so understanding it makes those chapters shorter.
In everyday life: Look for Slow-scan television outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Slow-scan television in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Slow-scan television means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Slow-scan television out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Slow-scan television in simple terms?

Why does Slow-scan television matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Slow-scan television?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Slow-scan television.