Slow-scan television

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SSTV transmissions may include station call signs and RST reception reports, as well as New Year's greetings.

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 (black-and-white) or colour.

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 data transmission and thus sends still pictures, usually lasting from about ten seconds to a couple of minutes.

Since SSTV systems operate on voice frequencies, amateurs use it on shortwave, VHF and UHF radio.

History

The concept of SSTV was introduced by Copthorn Macdonald in 1957–1958^[1]. He developed the first SSTV system using an electrostatic monitor and a vidicon tube. Commercial systems started appearing in 1970. SSTV was used extensively during the early years of the NASA Apollo program to transmit images to Earth, and the first images from Apollo 11 on the Moon were SSTV.

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 a minute or so.

The modem would generate audio from pictures, and pictures from audio. The audio would be attached to a radio receiver and transmitter.

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 frequency modulation, in which every different value of brightness in the image gets a different audio frequency. In other words, the signal shifts up or down to designate brighter or darker pixels. 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)^[2]. 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). A parity bit follows, then a stop bit at 1200 Hz. For example, the bits corresponding the decimal number 26 imply that the mode is Martin M1, whereas the number 30 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 an 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 (e.g. 256, 320, 512, or 640 pixels per line). 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^[2]. The most common modes share many properties, like synchronization and frequencies. Their main difference is the image quality, which is proportional to the time taken to transfer the image.

There's also a proprietary mode family called AVT (Amiga Video Transceiver). It differs radically from the modes mentioned above, in that it has no horizontal synchronization pulse but a digital system preventing the slant. The ATV modes are mainly used in Japan.

Frequencies

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

Media

References

Notes

source: Wikipedia

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